Fat-like substances lipids are components that take part in vital processes in the human body. There are several groups that perform leading functions of the body, such as the formation of hormonal levels or metabolism. In this article we will explain in detail what it is and what its role is in life processes.

Lipids are organic compounds that include fats and other fat-like substances. They actively participate in the process of cell structure and are part of membranes. They affect the permeability of cell membranes, as well as enzymatic activity. They influence the creation of intercellular connections and various chemical processes in the body. Insoluble in water, but they dissolve in organic solvents (such as gasoline or chloroform). In addition, there are types that are fat soluble.

This substance can be of plant or animal origin. If we are talking about plants, then most of them are in nuts and seeds. Of animal origin are mainly located in the subcutaneous tissue, nervous and brain.

Classification of lipids

Lipids are present in almost all tissues of the body and in the blood. There are several classifications, below we present the most common, based on the characteristics of structure and composition. According to their structure, they are divided into 3 large groups, which are subdivided into smaller ones.

The first group is simple. They include oxygen, hydrogen and carbon. They are divided into the following types:

1. Fatty alcohols. Substances containing from 1 to 3 hydroxyl groups.
2. Fatty acid. Found in various oils and fats.
3. Fatty aldehydes. The molecule contains 12 carbon atoms.
4. Triglycerides. These are precisely the fats that are deposited in the subcutaneous tissues.
5. Sphingosine bases. They are located in the plasma, lungs, liver and kidneys, and are found in nerve tissues.
6. Waxes. These are esters of fatty acids and high molecular weight alcohols.
7. Saturated hydrocarbons. They have exclusively single bonds, with the carbon atoms in a state of hybridization.

The second group is complex. They, like simple ones, include oxygen, hydrogen and carbon. But, besides them, they also contain various additional components. In turn, they are divided into 2 subgroups: polar and neutral.

Polar ones include:

1. Glycolipids. They appear after combining carbohydrates with lipids.
2. Phospholipids. These are esters of fatty acids, as well as polyhydric alcohols.
3. Sphingolipids. They are derivatives of aliphatic amino alcohols.

Neutral ones include:

1. Acylglycerides. Includes monoglycerides and diglycerides.
2. N-acetylethanolamides. They are ethanolamides of fatty acids.
3. Ceramides. They contain fatty acids combined with sphingosine.
4. Sterol esters. They represent complex cyclic alcohols of high molecular weight. They contain fatty acids.

The third group is oxylipids. The substances appear as a result of oxygenation of polyunsaturated fatty acids. In turn, they are divided into 2 types:

1. Cyclooxygenase pathway.
2. Lipoxygenase pathway.

Importance for membrane cells

increase

The cell membrane is what separates the cell from the environment around it. In addition to protection, it performs a fairly large number of functions necessary for normal life. The importance of lipids in the membrane cannot be overestimated.

In the cell wall, the substance forms a double layer. This helps cells interact normally with their environment. Therefore, there are no problems with controlling and regulating metabolism. Membrane lipids maintain the shape of the cell.

Part of a bacterial cell

An integral part of the cell structure are bacterial lipids. As a rule, they contain waxes or phospholipids. But the amount of the substance directly varies between 5-40%. The content depends on the type of bacterium, for example, the diphtheria bacillus contains about 5%, but the tuberculosis pathogen already contains more than 30%.

A bacterial cell is different in that the substances in it are associated with other components, for example, proteins or polysaccharides. In bacteria they have many more varieties and perform many tasks:

• energy storage;
• participate in metabolic processes;
• are a component of membranes;
• cell resistance to acids depends on them;
• components of antigens.

What functions do they perform in the body?

Lipids are a component of almost all tissues of the human body. There are different subspecies, each of which is responsible for a specific function. Next, we will dwell in more detail on the importance of the substance for life:

1. Energy function. They tend to disintegrate and in the process a lot of energy appears. The body's cells need it to support processes such as air flow, substance formation, growth and respiration.
2. Backup function. In the body, fats are stored in reserve; they are what make up the fatty layer of the skin. If hunger sets in, the body uses these reserves.
3. Thermal insulation function. The fat layer conducts heat poorly, and therefore it is much easier for the body to maintain temperature.
4. Structural function. This applies to cell membranes because the substance is a permanent component of them.
5. Enzymatic function. One of the secondary functions. They help cells form enzymes and help with the absorption of certain microelements coming from outside.
6. Transport function. The side effect lies in the ability of some types of lipids to transport substances.
7. Signal function. It is also secondary and simply supports some body processes.
8. Regulatory function. This is another mechanism that has a secondary meaning. By themselves, they are almost not involved in the regulation of various processes, but are a component of substances that directly affect them.

Thus, we can say with confidence that the functional importance of lipids for the body is difficult to overestimate. Therefore, it is important that their level is always normal. Many biological and biochemical processes in the body are tied to them.

What is lipid metabolism

Lipid metabolism is a process of physiological or biochemical nature that occurs in cells. Let's look at them in more detail:

1. Triacyglycerol metabolism.
2. Phospholipid metabolism. They are distributed unevenly. There are many of them in the liver and plasma (up to 50%). The half-life is 1-200 days, depending on the type.
3. Cholesterol exchange. It is formed in the liver and comes with food. Excess is eliminated naturally.
4. Catabolism of fatty acids. Occurs during β-oxidation; α- or ω-oxidation is less commonly involved.
5. Included in the metabolic processes of the gastrointestinal tract. Namely, the breakdown, digestion and absorption of these substances coming from food. Digestion begins in the stomach with the help of an enzyme called lipase. Next, pancreatic juice and bile come into action in the intestines. The cause of failures may be a violation of the secretion of the gallbladder or pancreas.
6. Lipogenesis. Simply put - the synthesis of fatty acids. Occurs in the liver or adipose tissue.
7. This includes the transport of various fats from the intestines.
8. Lipolysis. Catabolism, which occurs with the participation of lipase and provokes the breakdown of fats.
9. Synthesis of ketone bodies. Acetoacetyl-CoA gives rise to their formation.
10. Interconversion of fatty acids. From fatty acids found in the liver, acids characteristic of the body are formed.

Lipids are an important substance that affects almost all areas of life. The most common triglycerides and cholesterol in the human diet. Triglycerides are an excellent source of energy; it is this type that forms the fat layer of the body. Cholesterol affects the body’s metabolic processes, as well as the formation of hormonal levels. It is important that the content is always within the normal range, neither exceeding nor underestimating it. An adult needs to consume 70-140 g of lipids.

LIPIDS - this is a heterogeneous group of natural compounds, completely or almost completely insoluble in water, but soluble in organic solvents and in each other, yielding high molecular weight fatty acids upon hydrolysis.

In a living organism, lipids perform various functions.

Biological functions of lipids:

1) Structural

Structural lipids form complex complexes with proteins and carbohydrates, from which the membranes of cells and cellular structures are built, and participate in a variety of processes occurring in the cell.

2) Spare (energy)

Reserve lipids (mainly fats) are the body's energy reserve and participate in metabolic processes. In plants they accumulate mainly in fruits and seeds, in animals and fish - in subcutaneous fatty tissues and tissues surrounding internal organs, as well as liver, brain and nervous tissues. Their content depends on many factors (type, age, nutrition, etc.) and in some cases accounts for 95-97% of all secreted lipids.

Calorie content of carbohydrates and proteins: ~ 4 kcal/gram.

Caloric content of fat: ~ 9 kcal/gram.

The advantage of fat as an energy reserve, unlike carbohydrates, is its hydrophobicity - it is not associated with water. This ensures compactness of fat reserves - they are stored in anhydrous form, occupying a small volume. The average person's supply of pure triacylglycerols is approximately 13 kg. These reserves could be enough for 40 days of fasting under conditions of moderate physical activity. For comparison: the total glycogen reserves in the body are approximately 400 g; when fasting, this amount is not enough even for one day.

3) Protective

Subcutaneous adipose tissue protects animals from cooling, and internal organs from mechanical damage.

The formation of fat reserves in the body of humans and some animals is considered as an adaptation to irregular nutrition and living in a cold environment. Animals that hibernate for a long time (bears, marmots) and are adapted to living in cold conditions (walruses, seals) have a particularly large reserve of fat. The fetus has virtually no fat and appears only before birth.

A special group in terms of their functions in a living organism are the protective lipids of plants - waxes and their derivatives, covering the surface of leaves, seeds and fruits.

4) An important component of food raw materials

Lipids are an important component of food, largely determining its nutritional value and taste. The role of lipids in various food technology processes is extremely important. Spoilage of grain and its processed products during storage (rancidity) is primarily associated with changes in its lipid complex. Lipids isolated from a number of plants and animals are the main raw materials for obtaining the most important food and technical products (vegetable oil, animal fats, including butter, margarine, glycerin, fatty acids, etc.).

2 Classification of lipids

There is no generally accepted classification of lipids.

It is most appropriate to classify lipids depending on their chemical nature, biological functions, and also in relation to certain reagents, for example, alkalis.

Based on their chemical composition, lipids are usually divided into two groups: simple and complex.

Simple lipids – esters of fatty acids and alcohols. These include fats , waxes And steroids .

Fats – esters of glycerol and higher fatty acids.

Waxes – esters of higher alcohols of the aliphatic series (with a long carbohydrate chain of 16-30 C atoms) and higher fatty acids.

Steroids – esters of polycyclic alcohols and higher fatty acids.

Complex lipids – in addition to fatty acids and alcohols, they contain other components of various chemical natures. These include phospholipids and glycolipids .

Phospholipids - these are complex lipids in which one of the alcohol groups is associated not with FA, but with phosphoric acid (phosphoric acid can be connected to an additional compound). Depending on which alcohol is included in the phospholipids, they are divided into glycerophospholipids (contain the alcohol glycerol) and sphingophospholipids (contain the alcohol sphingosine).

Glycolipids – these are complex lipids in which one of the alcohol groups is associated not with FA, but with a carbohydrate component. Depending on which carbohydrate component is part of the glycolipids, they are divided into cerebrosides (they contain a monosaccharide, disaccharide or a small neutral homooligosaccharide as a carbohydrate component) and gangliosides (they contain an acidic heterooligosaccharide as a carbohydrate component).

Sometimes into an independent group of lipids ( minor lipids ) secrete fat-soluble pigments, sterols, and fat-soluble vitamins. Some of these compounds can be classified as simple (neutral) lipids, others - complex.

According to another classification, lipids, depending on their relationship to alkalis, are divided into two large groups: saponifiable and unsaponifiable. The group of saponified lipids includes simple and complex lipids, which, when interacting with alkalis, hydrolyze to form salts of high molecular weight acids, called “soaps”. The group of unsaponifiable lipids includes compounds that are not subject to alkaline hydrolysis (sterols, fat-soluble vitamins, ethers, etc.).

According to their functions in a living organism, lipids are divided into structural, storage and protective.

Structural lipids are mainly phospholipids.

Storage lipids are mainly fats.

Protective lipids of plants - waxes and their derivatives, covering the surface of leaves, seeds and fruits, animals - fats.

FATS

The chemical name of fats is acylglycerols. These are esters of glycerol and higher fatty acids. "Acyl" means "fatty acid residue".

Depending on the number of acyl radicals, fats are divided into mono-, di- and triglycerides. If the molecule contains 1 fatty acid radical, then the fat is called MONOACYLGLYCEROL. If the molecule contains 2 fatty acid radicals, then the fat is called DIACYLGLYCEROL. In the human and animal body, TRIACYLGLYCEROLS predominate (contain three fatty acid radicals).

The three hydroxyls of glycerol can be esterified either with only one acid, such as palmitic or oleic, or with two or three different acids:

Natural fats contain mainly mixed triglycerides, including residues of various acids.

Since the alcohol in all natural fats is the same - glycerol, the differences observed between fats are due solely to the composition of fatty acids.

Over four hundred carboxylic acids of various structures have been found in fats. However, most of them are present only in small quantities.

The acids contained in natural fats are monocarboxylic acids, built from unbranched carbon chains containing an even number of carbon atoms. Acids containing an odd number of carbon atoms, having a branched carbon chain, or containing cyclic moieties are present in small quantities. The exceptions are isovaleric acid and a number of cyclic acids contained in some very rare fats.

The most common acids in fats contain 12 to 18 carbon atoms and are often called fatty acids. Many fats contain small amounts of low molecular weight acids (C 2 -C 10). Acids with more than 24 carbon atoms are present in waxes.

The glycerides of the most common fats contain significant quantities of unsaturated acids containing 1-3 double bonds: oleic, linoleic and linolenic. Arachidonic acid containing four double bonds is present in animal fats; acids with five, six or more double bonds are found in fats of fish and marine animals. Most unsaturated acids of lipids have a cis configuration, their double bonds are isolated or separated by a methylene (-CH 2 -) group.

Of all the unsaturated acids contained in natural fats, oleic acid is the most common. In many fats, oleic acid makes up more than half of the total mass of acids, and only a few fats contain less than 10%. Two other unsaturated acids - linoleic and linolenic acid - are also very widespread, although they are present in much smaller quantities than oleic acid. Linoleic and linolenic acids are found in noticeable quantities in vegetable oils; For animal organisms they are essential acids.

Of the saturated acids, palmitic acid is almost as widespread as oleic acid. It is present in all fats, with some containing 15-50% of the total acid content. Stearic and myristic acids are widely used. Stearic acid is found in large quantities (25% or more) only in the storage fats of some mammals (for example, in sheep fat) and in the fats of some tropical plants, such as cocoa butter.

It is advisable to divide the acids contained in fats into two categories: major and minor acids. The main acids of fat are acids whose content in fat exceeds 10%.

Physical properties of fats

As a rule, fats do not withstand distillation and decompose even if they are distilled under reduced pressure.

The melting point, and therefore the consistency of fats, depends on the structure of the acids that make up them. Solid fats, i.e. fats that melt at a relatively high temperature, consist predominantly of glycerides of saturated acids (stearic, palmitic), and oils that melt at a lower temperature and are thick liquids contain significant amounts of glycerides of unsaturated acids (oleic , linoleic, linolenic).

Since natural fats are complex mixtures of mixed glycerides, they do not melt at a certain temperature, but in a certain temperature range, and they are first softened. To characterize fats, it is usually used solidification temperature, which does not coincide with the melting point - it is slightly lower. Some natural fats are solids; others are liquids (oils). The solidification temperature varies widely: -27 °C for linseed oil, -18 °C for sunflower oil, 19-24 °C for cow lard and 30-38 °C for beef lard.

The solidification temperature of fat is determined by the nature of its constituent acids: the higher the content of saturated acids, the higher it is.

Fats are soluble in ether, polyhalogen derivatives, carbon disulfide, aromatic hydrocarbons (benzene, toluene) and gasoline. Solid fats are poorly soluble in petroleum ether; insoluble in cold alcohol. Fats are insoluble in water, but they can form emulsions that are stabilized in the presence of surfactants (emulsifiers) such as proteins, soaps and some sulfonic acids, mainly in a slightly alkaline environment. Milk is a natural fat emulsion stabilized by proteins.

Chemical properties of fats

Fats undergo all the chemical reactions characteristic of esters, but their chemical behavior has a number of features associated with the structure of fatty acids and glycerol.

Among the chemical reactions involving fats, several types of transformations are distinguished.

Organic substances. General characteristics. Lipids

Organic matter are complex carbon-containing compounds. These include proteins, fats, carbohydrates, enzymes, hormones, vitamins and products of their transformations present in living organisms.

The name “organic compounds” appeared at an early stage in the development of chemistry and speaks for itself: scientists of that era believed that living beings consist of special organic compounds.

Among all the chemical elements carbon most closely related to living organisms. More than a million different molecules built on its basis are known. Of interest is the unique ability of carbon atoms to form covalent bonds with each other, forming long chains, complex rings and other structures.

Most organic compounds in nature are formed as a result of the process of photosynthesis - from carbon dioxide and water with the participation of solar radiation energy in chlorophyll-containing organisms.

Low molecular weight organic compounds got their name due to their low molecular weight. These include amino acids, lipids, organic acids, vitamins, coenzymes (vitamin derivatives that determine enzyme activity) and others.

Low molecular weight organic compounds make up 0.1 - 0.5% of the cell mass.

High molecular weight organic compounds (biopolymers)

A macromolecule consisting of monomers is calledpolymer(from Greek poly - "a lot of"). Consequently, a polymer is a multi-link chain in which a link is some relatively simple substance.

Polymers- These are molecules consisting of repeating structural units - monomers.

The properties of biopolymers depend on the number and variety of monomer units that form the polymer. If you combine 2 types of monomers together A And B, then it is possible to obtain a variety of polymers, the structure and properties of which will depend on the number, ratio and order of alternation of monomers in the chains.

Let's say there are 16 units in paraffin. You won’t repeat methylene - methylene - methylene 16 times... For such a long word there is a simplification - “hexadecane”. What if there are a thousand units in a molecule? We speak in simplified terms poly- "a lot of". For example, let's take a thousand links ethylene, connect, we get something familiar to everyone polyethylene.

Homopolymers (or regular) are built from monomers of the same type (for example, glycogen, starch and cellulose consist of molecules glucose).

Heteropolymers(or irregular) are built from different monomers (for example, proteins consisting of 20 amino acids and nucleic acids built from 8 nucleotides).

Each of the monomers determines some property of the polymer. For example, A- high strength, B- electrical conductivity. By alternating them in different ways, you can get a huge number of polymers with different properties. This principle underlies the diversity of life on our planet.

Lipids, their structure, properties and functions

Lipids- these are esters of the trihydric alcohol glycerol and higher fatty acids. Each of them contains an acidic COOH residue; it, losing a hydrogen atom, combines with glycerol, and a carbon chain is connected to the residue. Lipids are low molecular weight hydrophobic organic compounds.

« Bold"Acids are called because some high-molecular members of this group are part of fats. General formula of fatty acids: CH 3 - (CH 2) p - COOH. Most fatty acids contain an even number of carbon atoms (from 14 to 22).

Fatty acids are synthesized from cholesterol in the liver, then enter the duodenum with bile, where they promote the digestion of fats, emulsifying them, thereby stimulating their absorption.

Lipids include fats, waxes, steroids, phospholipids, terpenes, glycolipids, and lipoproteins.

Lipids are usually divided into fats and oils depending on whether they remain solid at 20°C (fats) or have a liquid consistency at this temperature (oils).

Pure fat is always white, and pure oil is always colorless. The yellow, orange and brown color of the oil is due to the presence of carotene or similar compounds. Olive oil sometimes has a greenish tint: it contains a little chlorophyll.

Fats have a high boiling point. This makes it convenient to fry food in fats. They do not evaporate from a hot frying pan; they begin to burn only at a temperature of 200 - 300 0 C.

Neutral fats(triglycerides) are compounds of high molecular weight fatty acids and trihydric alcohol glycerol. In the cytoplasm of cells, triglycerides are deposited in the form of fat droplets.

Excess fat can cause fatty degeneration. The main sign of fatty degeneration is an enlargement and thickening of the liver due to the accumulation of fat in hepatocytes (liver cells).

Waxes- plastic substances with water-repellent properties. In insects, they serve as material for the construction of honeycombs. A waxy coating on the surface of leaves, stems, and fruits protects plants from mechanical damage and ultraviolet radiation and plays an important role in regulating water balance.

Phospholipids- representatives of the class of fat-like substances, which are esters of glycerol and fatty acids, containing a phosphoric acid residue.

They form the basis of all biological membranes. In their structure, phospholipids are similar to fats, but in their molecule one or two fatty acid residues are replaced by a phosphoric acid residue.

Glycolipids- substances formed as a result of the combination of carbohydrates and lipids. The carbohydrate components of glycolipid molecules are polar, and this determines their role: like phospholipids, glycolipids are part of cell membranes.

TO fat-like substances (lipoids) include precursors and derivatives of simple and complex lipids: cholesterol, bile acids, fat-soluble vitamins, steroid hormones, glycerin and others.

General properties of lipids:

1) have high energy intensity;
2) have a density lower than that of water;
3) have a favorable boiling point;
4) high-calorie substances.

Variety lipids	Role in plants and animals
Fats and oils	1. Serve as an energy depot. 2. Storage (oils usually accumulate in plants). 3. In vertebrates, fats are deposited under the skin and serve for thermal insulation; in whales they also contribute to buoyancy. 4. Source of metabolic water in animals living in the desert.
Wax	Mainly used as a water-repellent coating: 1) forms an additional protective layer on the cuticle of the epidermis of some plant organs, for example leaves, fruits and seeds (mainly in xerophytes); 2) covers skin, wool and feathers; 3) is part of the exoskeleton of insects. Bees use wax to build honeycombs.
Phospholipids	Membrane components.
Steroids	Bile acids, such as cholic acid, are part of bile. Bile salts help emulsify and solubilize lipids during digestion. With a lack of vitamin D, rickets develops. Cardiac glycosides, such as digitalis glycosides, are used for heart disease.
Terpenes	Substances on which the aroma of plant essential oils depends, for example menthol in mint, camphor. Gibberellins are plant growth substances. Phyton is part of chlorophyll. Carotenoids are photosynthetic pigments.
Lipoproteins	Membranes are made of lipoproteins.
Glycolipids	Components of cell membranes, especially in the myelin sheath of nerve fibers and on the surface of nerve cells, as well as components of chloroplast membranes.

General functions of lipids

Function	Explanation
Energy	When 1 g of triglycerides is broken down, 38.9 kJ of energy is released
Structural	Phospholipids and glycolipids are involved in the formation of cell membranes
Storage	Fats and oils are the most important reserve substances. Fats are stored in the adipose tissue cells of animals and serve as a source of energy during hibernation, migration or hunger. Plant seed oils provide energy to future seedlings
Metabolic water source	When 1 g of fat is oxidized, 1.1 g of water is formed
Protective	Layers of fat provide cushioning for animal organs, and subcutaneous fatty tissue creates a heat-insulating layer. Wax serves as a water-repellent coating for plants
Regulatory	Steroid hormones regulate fundamental processes in animal organisms - growth, differentiation, reproduction, adaptation, etc.
Catalytic	Fat-soluble vitamins A, D, E, K are cofactors of enzymes, and although they themselves do not have catalytic activity, without them enzymes cannot perform their functions

Lipids- substances that are very heterogeneous in their chemical structure, characterized by varying solubility in organic solvents and, as a rule, insoluble in water. They play an important role in life processes. Being one of the main components of biological membranes, lipids affect their permeability, participate in the transmission of nerve impulses, and the creation of intercellular contacts.

Other functions of lipids are the formation of an energy reserve, the creation of protective water-repellent and thermally insulating covers in animals and plants, and the protection of organs and tissues from mechanical stress.

CLASSIFICATION OF LIPIDS

Depending on their chemical composition, lipids are divided into several classes.

1. Simple lipids include substances whose molecules consist only of fatty acid (or aldehyde) residues and alcohols. These include
   • fats (triglycerides and other neutral glycerides)
   • waxes
2. Complex lipids
   • orthophosphoric acid derivatives (phospholipids)
   • lipids containing sugar residues (glycolipids)
   • sterols
   • steroids

In this section, lipid chemistry will be discussed only to the extent necessary to understand lipid metabolism.

If animal or plant tissue is treated with one or more (usually sequentially) organic solvents, such as chloroform, benzene or petroleum ether, some of the material goes into solution. The components of such a soluble fraction (extract) are called lipids. The lipid fraction contains substances of various types, most of which are presented in the diagram. Note that due to the heterogeneity of the components included in the lipid fraction, the term “lipid fraction” cannot be considered as a structural characteristic; it is only a working laboratory name for the fraction obtained during the extraction of biological material with low-polarity solvents. However, most lipids share some common structural features that give them important biological properties and similar solubility.

Fatty acid

Fatty acids - aliphatic carboxylic acids - can be found in the body in a free state (trace amounts in cells and tissues) or act as building blocks for most classes of lipids. Over 70 different fatty acids have been isolated from the cells and tissues of living organisms.

Fatty acids found in natural lipids contain an even number of carbon atoms and have predominantly straight carbon chains. Below are the formulas for the most commonly found naturally occurring fatty acids.

Natural fatty acids, although somewhat arbitrarily, can be divided into three groups:

• saturated fatty acids [show]
• monounsaturated fatty acids [show]

  Monounsaturated (with one double bond) fatty acids:

• polyunsaturated fatty acids [show]

  Polyunsaturated (with two or more double bonds) fatty acids:

In addition to these main three groups, there is also a group of so-called unusual natural fatty acids [show] .

Fatty acids that make up the lipids of animals and higher plants have many common properties. As already noted, almost all natural fatty acids contain an even number of carbon atoms, most often 16 or 18. Unsaturated fatty acids in animals and humans involved in the construction of lipids usually contain a double bond between the 9th and 10th carbons; additional double bonds, such as usually occur in the area between the 10th carbon and the methyl end of the chain. The counting starts from the carboxyl group: the C-atom closest to the COOH group is designated as α, the one next to it is designated as β, and the terminal carbon atom in the hydrocarbon radical is designated as ω.

The peculiarity of the double bonds of natural unsaturated fatty acids is that they are always separated by two simple bonds, that is, there is always at least one methylene group between them (-CH=CH-CH 2 -CH=CH-). Such double bonds are referred to as “isolated.” Natural unsaturated fatty acids have a cis configuration and trans configurations are extremely rare. It is believed that in unsaturated fatty acids with several double bonds, the cis configuration gives the hydrocarbon chain a bent and shortened appearance, which makes biological sense (especially considering that many lipids are part of membranes). In microbial cells, unsaturated fatty acids usually contain one double bond.

Long chain fatty acids are practically insoluble in water. Their sodium and potassium salts (soaps) form micelles in water. In the latter, the negatively charged carboxyl groups of fatty acids face the aqueous phase, and the nonpolar hydrocarbon chains are hidden inside the micellar structure. Such micelles have a total negative charge and remain suspended in solution due to mutual repulsion (Fig. 95).

Neutral fats (or glycerides)

Neutral fats are esters of glycerol and fatty acids. If all three hydroxyl groups of glycerol are esterified with fatty acids, then such a compound is called a triglyceride (triacylglycerol), if two are esterified, a diglyceride (diacylglycerol) and, finally, if one group is esterified, a monoglyceride (monoacylglycerol).

Neutral fats are found in the body either in the form of protoplasmic fat, which is a structural component of cells, or in the form of reserve fat. The role of these two forms of fat in the body is not the same. Protoplasmic fat has a constant chemical composition and is contained in tissues in a certain amount, which does not change even with morbid obesity, while the amount of reserve fat undergoes large fluctuations.

The bulk of natural neutral fats are triglycerides. The fatty acids in triglycerides can be saturated or unsaturated. The most common fatty acids are palmitic, stearic and oleic acids. If all three acid radicals belong to the same fatty acid, then such triglycerides are called simple (for example, tripalmitin, tristearin, triolein, etc.), but if they belong to different fatty acids, then they are mixed. The names of mixed triglycerides are derived from the fatty acids they contain; in this case, the numbers 1, 2 and 3 indicate the connection of the fatty acid residue with the corresponding alcohol group in the glycerol molecule (for example, 1-oleo-2-palmitostearin).

The fatty acids that make up triglycerides practically determine their physicochemical properties. Thus, the melting point of triglycerides increases with increasing number and length of saturated fatty acid residues. In contrast, the higher the content of unsaturated or short-chain fatty acids, the lower the melting point. Animal fats (lard) usually contain a significant amount of saturated fatty acids (palmitic, stearic, etc.), due to which they are solid at room temperature. Fats, which contain many mono- and polyunsaturated acids, are liquid at ordinary temperatures and are called oils. Thus, in hemp oil, 95% of all fatty acids are oleic, linoleic and linolenic acids, and only 5% are stearic and palmitic acids. Note that human fat, which melts at 15°C (it is liquid at body temperature), contains 70% oleic acid.

Glycerides are capable of entering into all chemical reactions characteristic of esters. The most important reaction is the saponification reaction, which results in the formation of glycerol and fatty acids from triglycerides. Saponification of fat can occur either through enzymatic hydrolysis or through the action of acids or alkalis.

Alkaline breakdown of fat under the action of caustic soda or caustic potassium is carried out during the industrial production of soap. Let us remember that soap is sodium or potassium salts of higher fatty acids.

The following indicators are often used to characterize natural fats:

1. iodine number - the number of grams of iodine that, under certain conditions, is bound to 100 g of fat; this number characterizes the degree of unsaturation of fatty acids present in fats, the iodine number of beef fat is 32-47, lamb fat 35-46, pork fat 46-66;
2. acid number - the number of milligrams of potassium hydroxide required to neutralize 1 g of fat. This number indicates the amount of free fatty acids present in the fat;
3. saponification number - the number of milligrams of potassium hydroxide used to neutralize all fatty acids (both those included in triglycerides and free ones) contained in 1 g of fat. This number depends on the relative molecular weight of the fatty acids that make up the fat. The saponification number for the main animal fats (beef, lamb, pork) is almost the same.

Waxes are esters of higher fatty acids and higher monohydric or dihydric alcohols with the number of carbon atoms from 20 to 70. Their general formulas are presented in the diagram, where R, R" and R" are possible radicals.

Waxes can be part of the fat covering the skin, wool, and feathers. In plants, 80% of all lipids that form a film on the surface of leaves and trunks are waxes. Waxes are also known to be normal metabolites of certain microorganisms.

Natural waxes (for example, beeswax, spermaceti, lanolin) usually contain, in addition to the esters mentioned, a certain amount of free higher fatty acids, alcohols and hydrocarbons with a number of carbon atoms of 21-35.

Phospholipids

This class of complex lipids includes glycerophospholipids and sphingolipids.

Glycerophospholipids are derivatives of phosphatidic acid: they contain glycerol, fatty acids, phosphoric acid and usually nitrogen-containing compounds. The general formula of glycerophospholipids is presented in the diagram, where R 1 and R 2 are radicals of higher fatty acids, and R 3 is a radical of a nitrogenous compound.

A characteristic feature of all glycerophospholipids is that one part of their molecule (radicals R 1 and R 2) exhibits pronounced hydrophobicity, while the other part is hydrophilic due to the negative charge of the phosphoric acid residue and the positive charge of the R 3 radical.

Of all lipids, glycerophospholipids have the most pronounced polar properties. When glycerophospholipids are placed in water, only a small part of them passes into the true solution, while the bulk of the “dissolved” lipid is found in aqueous systems in the form of micelles. There are several groups (subclasses) of glycerophospholipids.

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Unlike triglycerides, in the phosphatidylcholine molecule, one of the three hydroxyl groups of glycerol is associated not with fatty acid, but with phosphoric acid. In addition, phosphoric acid, in turn, is connected by an ester bond to the nitrogenous base [HO-CH 2 -CH 2 -N+=(CH 3) 3 ] - choline. Thus, the phosphatidylcholine molecule contains glycerol, higher fatty acids, phosphoric acid and choline

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The main difference between phosphatidylcholines and phosphatidylethanolamines is that the latter contain the nitrogenous base ethanolamine (HO-CH 2 -CH 2 -NH 3 +) instead of choline.

Of the glycerophospholipids in the body of animals and higher plants, phosphatidylcholines and phosphatidylethanolamines are found in the largest quantities.

• These two groups of glycerophospholipids are metabolically related to each other and are the main lipid components of cell membranes. [show] .

  

  Phosphatidylserines

  In the phosphatidylserine molecule, the nitrogenous compound is the amino acid residue serine.

• Phosphatidylserines are much less widespread than phosphatidylcholines and phosphatidylethanolamines, and their importance is determined mainly by the fact that they participate in the synthesis of phosphatidylethanolamines. [show] .

  

  Plasmalogens (acetal phosphatides)

  They differ from the glycerophospholipids discussed above in that instead of one higher fatty acid residue, they contain a fatty acid aldehyde residue, which is linked to the hydroxyl group of glycerol by an unsaturated ester bond:

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Thus, plasmalogen, upon hydrolysis, breaks down into glycerol, higher fatty acid aldehyde, fatty acid, phosphoric acid, choline or ethanolamine.

The R3 radical in this group of glycerophospholipids is the six-carbon sugar alcohol - inositol:

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Phosphatidylinositols are quite widespread in nature. They are found in animals, plants and microbes. In animals, they are found in the brain, liver and lungs.

It should be noted that free phosphatidic acid occurs in nature, although in relatively small quantities compared to other glycerophospholipids.

Among the fatty acids that make up glycerophospholipids, both saturated and unsaturated fatty acids are found (usually stearic, palmitic, oleic and linoleic).

It has also been established that most phosphatidylcholines and phosphatidylethanolamines contain one saturated higher fatty acid, esterified in position 1 (at the 1st carbon atom of glycerol), and one unsaturated higher fatty acid, esterified in position 2. Hydrolysis of phosphatidylcholines and phosphatidylethanolamines with the participation of special enzymes contained , for example, in cobra venom, which belong to phospholipases A 2, leads to the cleavage of unsaturated fatty acids and the formation of lysophosphatidylcholines or lysophosphatidylethanolamines, which have a strong hemolytic effect.

Sphingolipids

Glycolipids

Complex lipids containing carbohydrate groups in the molecule (usually a D-galactose residue). Glycolipids play an essential role in the functioning of biological membranes. They are found primarily in brain tissue, but are also found in blood cells and other tissues. There are three main groups of glycolipids:

• cerebrosides
• sulfatides
• gangliosides

Cerebrosides contain neither phosphoric acid nor choline. They contain a hexose (usually D-galactose), which is linked by an ester bond to the hydroxyl group of the amino alcohol sphingosine. In addition, Cerebroside contains a fatty acid. Among these fatty acids, the most common are lignoceric, nervonic and cerebronic acids, i.e. fatty acids having 24 carbon atoms. The structure of cerebrosides can be represented by a diagram. Cerebrosides can also be classified as sphingolipids, since they contain the alcohol sphingosine.

The most studied representatives of cerebrosides are nervon, containing nervonic acid, cerebron, which includes cerebronic acid, and kerazin, containing lignocyric acid. The content of cerebrosides is especially high in the membranes of nerve cells (in the myelin sheath).

Sulfatides differ from cerebrosides in that they contain a sulfuric acid residue in the molecule. In other words, the sulfatide is a cerebroside sulfate in which the sulfate is esterified at the third carbon atom of the hexose. In the mammalian brain, sulfatides, like n cerebrosides, are found in the white matter. However, their content in the brain is much lower than that of cerebrosides.

When hydrolyzing gangliosides, one can detect higher fatty acid, sphingosine alcohol, D-glucose and D-galactose, as well as amino sugar derivatives: N-acetylglucosamine and N-acetylneuraminic acid. The latter is synthesized in the body from glucosamine.

Structurally, gangliosides are largely similar to cerebrosides, the only difference being that instead of a single galactose residue they contain a complex oligosaccharide. One of the simplest gangliosides is hematoside, isolated from the stroma of erythrocytes (scheme)

Unlike cerebrosides and sulfatides, gangliosides are found predominantly in the gray matter of the brain and are concentrated in the plasma membranes of nerve and glial cells.

All the lipids discussed above are usually called saponified, since their hydrolysis produces soaps. However, there are lipids that do not hydrolyze to release fatty acids. These lipids include steroids.

Steroids are compounds widespread in nature. They are derivatives of a core containing three fused cyclohexane rings and one cyclopentane ring. Steroids include numerous substances of a hormonal nature, as well as cholesterol, bile acids and other compounds.

In the human body, the first place among steroids is occupied by sterols. The most important representative of sterols is cholesterol:

It contains an alcohol hydroxyl group at C3 and a branched aliphatic chain of eight carbon atoms at C17. The hydroxyl group at C 3 can be esterified with a higher fatty acid; in this case, cholesterol esters (cholesterides) are formed:

Cholesterol plays a role as a key intermediate in the synthesis of many other compounds. The plasma membranes of many animal cells are rich in cholesterol; it is found in significantly less quantity in mitochondrial membranes and in the endoplasmic reticulum. Note that there is no cholesterol in plants. Plants have other sterols, collectively known as phytosterols.

It is usually believed that fats in the human body act as energy (calorie) suppliers. But this is not entirely correct. Of course, a significant part of fat is consumed as energy material. Moreover, fat serves as a source of energy in the body, either through direct use, or potentially in the form of reserves in adipose tissue. However, to a certain extent, fats are a plastic material, since they are part of cellular components (in the form of complexes with proteins - lipoproteins), in particular, membranes, i.e. are an essential nutritional factor. In addition, body fat provides insulation by accumulating in the subcutaneous layer and around certain organs. In addition, fats act as food solvents for fat-soluble vitamins and serve as a source of essential polyunsaturated fatty acids (linolenic, arachidonic).

With prolonged restriction of fats in the diet, disturbances in the physiological state of the body are observed: the activity of the central nervous system is disrupted, the immune system is weakened and life expectancy is reduced. However, excessive consumption of saturated fats leads to disruption of cholesterol metabolism, increased blood clotting properties, kidney and liver diseases, and contributes to the development of atherosclerosis and obesity with all the ensuing consequences.

The definition of lipids given in the literature is ambiguous. Fats (more correctly termed lipids) are organic compounds that are soluble in a number of organic solvents and insoluble in water. The main components of fats are trigcerides and lipoid substances, which include phospholipids, sterols, waxes, etc. In food technology, the term “fat” is used, which means the sum of substances extracted by organic solvents. When fat is almost completely extracted from food products, the term “fat” is equivalent to the term “lipids”.

It seems more preferable to define lipids as natural derivatives of fatty acids and related compounds that are part of all living cells and are extracted from organisms and tissues with non-polar solvents.

According to Blore's classification, lipids are divided into three groups:

Simple,

Complex,

Precursors and derivatives of lipids.

Simple lipids. Simple lipids are esters of fatty acids with various alcohols. These include, for example, fats and waxes.

Fats (triglycerides). Fats (triglycerides) are esters of fatty acids with glycerol. If they are in a liquid state, they are called oils. The composition of triglycerides includes glycerol (about 9%) and fatty acids with different lengths of hydrocarbon chains and degrees of saturation, the structure of which determines the properties of triglycerides.

Animal and vegetable fats have different physical properties and composition. Animal fats are solid substances that contain large amounts of saturated fatty acids that have a high melting point. Vegetable fats are usually liquid substances containing mainly unsaturated fatty acids with a low melting point. The source of vegetable fats is mainly vegetable oils (99.9% fat), nuts (53–65%), oat (6.1%) and buckwheat (3.3%) cereals. The source of animal fats is pork lard (90–92% fat), butter (72–82%), fatty pork (49%), sausages (20–40%), sour cream (30%), cheeses (15–30% ).

The main component of lipids are fatty acids. Naturally occurring trigcerides contain at least two different fatty acids.

1-Palmitoyl-2,3-distearoylgicerine

The chemical, biological and physical properties of fats are determined by the triglycerides included in its composition and, first of all, by the chain length and the degree of saturation of fatty acids. The composition of fats consists mainly of unbranched fatty acids containing an even number of carbon atoms (4–26), both saturated and mono- and polyunsaturated acids.

Saturated fatty acids (palmitic, stearic, etc.) are used by the body as a whole as energy material. Palmitic and stearic acids are found in all animal and vegetable fats. The largest amount of saturated fatty acids is found in animal fats: for example, in beef and pork fat - 25% palmitic, 20% and 13% stearic acids, respectively, in butter - 7% stearic, 25% palmitic and 8% myristic acids. They can be partially synthesized in the body from carbohydrates (and even from proteins).

Unsaturated fatty acids vary in their degree of “unsaturation.” Monounsaturated fatty acids contain one hydrogen-unsaturated bond between carbon atoms, polyunsaturated fatty acids contain several bonds (2–6). The most common monounsaturated fatty acids include oleic acid, which is abundant in olive oil (65%), margarines (43–47%), pork and beef fat, butter and goose meat (11–16%).

Most fatty acids that make up triglycerides contain 20 carbon atoms per molecule. There are 18 carbon atoms in the molecules of oleic, linoleic, and linolenic acid and they are dehydro derivatives of stearic acid, cis-isomers.

The most common saturated fatty acids in triglycerides are: stearic (C 17 H 35 COOH), palmitic (C 15 H 31 COOH), myristic (C 13 H 27 COOH), arachidic (C 19 H 39 COOH), lauric (C 11 H 23 COUN).

Of particular importance are polyunsaturated fatty acids, such as linoleic, linolenic and arachidonic acids, which are part of cell membranes and other structural elements of tissues and perform a number of important functions in the body, including ensuring normal growth and metabolism, vascular elasticity, etc. Most polyunsaturated acids cannot be synthesized in the human body and therefore these acids are essential, just as some amino acids and vitamins are essential. On the other hand, these acids, mainly linoleic and arachidonic, serve as precursors of hormone-like substances - prostaglandins, prevent the deposition of cholesterol in the walls of blood vessels (promote its removal from the body), and increase the elasticity of the walls of blood vessels. It should be noted that these functions are performed only by cis-isomers of unsaturated acids.

Saturated fatty acids perform mainly an energy function in the body and their excess in the diet often leads to impaired fat metabolism and increased blood cholesterol levels.

The composition of fats synthesized in different parts of the same body is different. Thus, in pigs, the outer layers of subcutaneous fat are more unsaturated than the inner ones. The acid composition of human fats is close to the composition of rendered beef lard.

Waxes. Waxes are esters of fatty acids with monohydric alcohols. Waxes are the historical name for products of different composition and origin, mostly natural, whose properties are close to beeswax. Most natural waxes contain esters of monobasic saturated carboxylic acids of normal structure and sterols with 12–46 carbon atoms per molecule. Such waxes are similar in chemical properties to fats (triglycerides), but are saponified only in an alkaline environment. Waxes differ from fats in that instead of glycerol, they contain sterols or higher aliphatic alcohols with an even number of carbon atoms (16–36). Vegetable waxes also contain paraffinic hydrocarbons.

Waxes are widely distributed in nature. In plants, they cover leaves, stems, and fruits with a thin layer, protecting them from wetting with water, drying out, and the action of microorganisms. The wax content in grains and fruits is low. The shells of sunflower seeds contain up to 0.2% of waxes by weight of the shell, soybean seeds - 0.01%, rice - 0.05%.

Complex lipids. Complex lipids are esters of fatty acids with alcohols, additionally containing other groups.

Phospholipids. The most important representatives of complex lipids are phospholipids. These are lipids containing, in addition to fatty acids and alcohol, a phosphoric acid residue. They contain nitrogenous bases (most often choline + OH - or ethanolamine HO-CH 2 -CH 2 -NH 2), amino acid residues and other components. Depending on the alcohol included in the molecule, a phospholipid is either a glycerophospholipid (glycerol acts as an alcohol) or a sphingophospholipid, which includes sphingosine. Phospholipid molecules contain non-polar hydrophobic hydrocarbon radicals - “tails” and a polar hydrophilic “head” (residues of phosphoric acid and nitrogenous base), which determines the ability of phospholipids to form biological membranes. As part of cell membranes, phospholipids play an essential role in their permeability and metabolism between cells and the intracellular space.

The most common group of phospholipids is phosphoglycerides. They contain glycerin, fatty acids, phosphoric acid and amino alcohols (for example, choline in lecithin, ethanolamine in cephalin). The amino alcohol included in the phospholipid determines the biological effect of the phospholipid. For example, lecithin is a glyceride esterified with two, usually different fatty acids (for example, stearic and oleic) and containing a phosphocholine group, which, when saponified, gives inorganic phosphate and a quaternary base - choline.

Lecithin exhibits a lipotropic effect, i.e. helps remove cholesterol from the body. Lecithin and choline prevent fatty liver and these drugs are used to prevent liver diseases. Choline, in addition, is part of the nervous tissue, in particular in the brain tissue. Acetylcholine plays an important role in the transmission of nerve impulses. In the human body, choline can be formed from serine, but choline biosynthesis is limited and choline must be supplied additionally from food. Thus, choline, like polyunsaturated fatty acids and a number of amino acids, is an essential nutrient.

Phospholipids in food products differ in their chemical composition and biological effects. The latter, as already mentioned, largely depends on the nature of the amino alcohol included in their composition. The foods found mainly include lecithin, which contains choline, an amino alcohol, and cephalin, which contains ethanolamine.

Phospholipids contained in food products promote better absorption of fats. Thus, the fat in milk is in a finely dispersed state, largely due to milk phospholipids. Milk fat is considered one of the most easily digestible fats. The largest amount of phospholipids is found in eggs (3.4%), relatively high (0.3–0.9%) in grains and legumes and unrefined oils. When storing unrefined vegetable oil, phospholipids precipitate. When refining vegetable oils, the content of phospholipids in them is reduced to 0.2–0.3%. It is believed that the optimal content of phospholipids in food should be 5–10 g per day.

In addition to phospholipids, complex lipids include g lycolipids(glycosphingolipids) containing a fatty acid, sphingosine and a carbohydrate component. Glycolipids are present in noticeable quantities in plant products (lipids from wheat, oats, corn, sunflower). They are also found in animals and microorganisms. Glycolipids perform structural functions, participate in the construction of membranes, and play an important role in the formation of wheat gluten proteins, which determine the baking properties of flour. Complex lipids are also sulfolipids and aminolipids. Lipoproteins also fall into this category.

Precursors and derivatives of lipids. This group includes fatty acids, glycerol, steroids and other alcohols, fatty acid aldehydes and ketone bodies, hydrocarbons, fat-soluble vitamins and hormones.

Sterols (sterols). Sterols (sterols) are alicyclic natural alcohols (monohydric secondary alcohols of the series, containing a hydroxyl group at the carbon atom in position 3 and methyl groups at the C 10 and C 13 atoms), related to steroids. Sterols are a component of the unsaponifiable fraction of animal and plant lipids. There are animal sterols (zoosterols), plant sterols (phytosterols) and fungal sterols (mycosterols). The main sterol in higher animals is cholesterol, and the main sterol in plants is b-sitosterol. Cholesterol is found in the tissues of all animals and is absent, or present in small quantities, in plants. Phytosterols, unlike cholesterol, are not absorbed by the body.

Sterols, along with lipids and phospholipids, are the main structural components of cell membranes. They are believed to affect cellular metabolism. Sterols carry out their functions in the body in the form of complexes with proteins (lipoproteins) and esters of higher fatty acids, being their carriers to all organs and tissues through the bloodstream system. Cholesterol is also involved in the metabolism of bile acids and hormones. Up to 80% of cholesterol in the human body is synthesized in the liver and other tissues. The cholesterol content in eggs reaches 0.57%, and in cheeses – 0.28–1.61%. Butter contains about 0.20%, and meat – 0.06–0.10%. It is believed that the daily intake of cholesterol from food should not exceed 0.5 g. Otherwise, the level of its content in the blood increases, which means the risk of the occurrence and development of atherosclerosis increases.

The importance of lipids. When discussing groups of lipids, mention was made of their various functions in the body. Summarizing the above, we can distinguish the following functions of lipids in a living organism.

Lipids, being part of cell walls, perform a plastic function in the body and are called structural. They are part of the cell membrane and participate in a variety of processes occurring in the cell.

Moreover, as already mentioned, lipids can serve as a source of energy in the body, either through direct use, or potentially in the form of reserves in adipose tissue. While body fat is composed primarily of glycerides, brain and spinal tissue contain complex structural units made of protein, cholesterol, and phospholipids such as lecithin.

The lipids found in special “fat” cells are called storage lipids and consist mainly of triglycerides. These lipids are an accumulator of chemical energy and are used when there is a lack of food. Lipids have a high calorie content: 1 g is 9 kcal - this is 2 times higher than the calorie content of proteins and carbohydrates. Most all plant species also contain storage lipids, mainly in the seeds. Lipids help the plant to withstand the adverse effects of the external environment, for example, low temperatures, i.e. perform a protective function.

In plants, lipids accumulate mainly in seeds and fruits, and their content depends on the variety, location and growing conditions. In animals and fish, lipids are concentrated in the subcutaneous, brain and nervous tissues and tissues surrounding important organs (heart, kidneys). The lipid content of animals is determined by the species, feed composition, housing conditions, etc.

The composition of food products includes so-called “invisible” fats (in meat, fish and milk) and “visible” - vegetable oils and animal fats specially added to food. In foods, lipids are contained in the form of individual fat cells, from where they are easily extracted by most organic solvents (often called “free lipids”) or are part of almost all vital cells. In the latter case, they are more tightly bound in cells (so-called tightly bound lipids). Lipid quantification methods take these features into account.

In addition to the fact that lipids are necessary in nutrition as energy and structural material, they participate in the metabolism of other nutrients, for example, they contribute to the absorption of vitamins A and D, and animal fats are a source of these vitamins. The only source of vitamin E and b-carotene is vegetable fats.

None of the fats, taken separately, can fully meet the body's needs for fatty substances. The recommended caloric content of lipids in the diet is 30–35%, which in weight units (on average 102 g) is slightly higher than the amount of proteins. Of the indicated 102 g, it is recommended to consume 45–50 g directly in the form of fat. When working in the cold, the amount of fat in the diet should be increased, since fat is involved in the processes of thermoregulation of the body. This increase should come from a quota of carbohydrates, not proteins, since proteins are necessary for the proper processing of fats.

It is recommended to consume animal and vegetable fats in combination. The optimal ratio is 70% animal and 30% vegetable fats. This ratio ensures that the body receives the necessary amounts of polyunsaturated and saturated acids. As you age, it is recommended to reduce your consumption of animal fats.