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LIPIDS IN FOOD AND THEIR ROLE

IN HUMAN HEALTH - 1993

An assessment of some nutritional and Technological Aspects of Fat and Cholesterol Modified Foods by Dr. G.S.Sidhu PhD. (now retired). Dr. Sidhu was previously the Senior Principal Research Scientist working in the Division of Food Science and Technology at the CSIRO.

NATURE OF FATS AND BODY LIPIDS

INTRODUCTION

Fats have physical, chemical and physiological attributes which make them important both in food technology and nutrition. The dietary fats are ingest and converted into human body lipids, some of which are essential to life, while others may accumulate in excessive amounts and cause problems in health and well being of the individual. To understand the role of fats in the diet some basic information about the chemical nature of fats and the metabolism of their component molecules is necessary. In these series of lecture Dr. Sidhu will try to explain the importance of fats in food and their role in human health.

PROPERTIES OF LIPIDS

The term "lipids" refers to a group of biochemical compounds which are extremely varied in nature but they possess the common property of being soluble in organic solvents such as benzene, ether, chloroform and mixtures of chloroform and methanol. Lipids are insoluble or sparingly insoluble in water and this property affects profoundly the way they are digested, absorbed, transported in the blood stream and metabolised in the body. These aspects will be illustrated during the course of these lectures. The lack of solubility in water also determines the processes used in food technology for extraction, purification and handling of lipids or foods containing lipids. It has become the practice to call the lipids as "hydrophobic" (water-hating) substances in contrast to materials that are soluble in water as "hydrophilic" (water-loving).

There are a number of groups of lipids usually arranged according to complexity. The first general group of lipids are called "simple lipids" and are esters containing only carbon, oxygen and hydrogen, and yield on hydrolysis in the presence of alkali only fatty acids and alcohol. The process of hydrolysis is called saponification and the common trihydric alcohol is glycerol esterified with three fatty acids. A distinction is made between fats and oils depending on their physical state at ordinary room temperatures, when fats are solids and oils are liquids (1).

FATTY ACIDS

Fatty acids are the main constituents of food fats, oils and depot fats in man and animals (2). The fatty acid molecules consist of carbon atoms linked to one another to form a linear chain varying in length from 4 to 26 carbons. In general, the carbon chain of fatty acids in oils and fats has an even number of carbon atoms. A good example of a fatty acid molecule is given in Fig. 1; it has a chain length of 18 carbon atoms and is called stearic acid.

Fig. 1 - Schematic illustration of a fatty acid with an 18-carbon chain

SATURATED, MONOUNSATURATED AND POLYUNSATURATED FATTY ACIDS

The methyl group at terminal end of chain consists of three hydrogen atoms linked to the carbon atom. However, the carbon atoms which form the inner links of the chain have only two bonds each where the hydrogen atoms can be attached, the other two being used to form the carbon-to-carbon bonds. If all the available bonds are occupied by hydrogen atoms, the molecule is called a saturated fatty acid (Fig. 1). If on the other hand, one hydrogen atom is missing from each of the two neighbouring carbons, the free bonds form a second carbon-to-carbon link, a double bond, and the fatty acid is said to be monounsaturated. The fatty acids are called polyunsaturated if two of more double bonds are formed between different carbon atoms. A typical example of a polyunsaturated fatty acid is given in Fig. 2.

Fig. 2- Schematic illustration of a polyunsaturated fatty acid molecule

NOMENCLATURE

The nomenclature of fatty acids is complex, and although standardised, it may still vary from one publication to another. The symbol(s) must indicate the chain length, degree of saturation or unsaturation, position of the double bond(s) and geometrical isomer(s) of the fatty acid.

The naming of saturated fatty acids does not present many problems as the main feature is simple the length of the chain. For example, stearic acid is a saturated fatty acid with a chain length of 18 carbons (Fig.1), and may be designated by the symbol C18:0 and according to systematic nomenclature it is called n-octadencanoic acid.

The terminology of monounsaturated and polyunsaturated fatty acids is more difficult, because in addition to chain length, the number of double bonds and their respective positions on the chain must also be indicated. Double bonds are indicated by the symbol ,  or n followed by one or more numbers giving the position on the carbon chain. The carbon atoms of the chain may be numbered from the carboxyl ( nomenclature) or from the methyl ( or n nomenclature) group. Taking linoleic acid as an example ( Fig. 2), the symbol indicates a carbon chain with two double bonds, the first one located on the 9th carbon counting from the carboxyl group end and the second one on the 12th carbon. The formula C18:26 could also describe linoleic acid, but the symbol 6 means that the first carbon with the double bond is number 6 counting from the methyl group. In this discussion the formula and nomenclature most commonly used in literature will be used: for example, stearic acid will be specified as C18:0 and linoleic acid as C18:6 and so forth.

Found in the plants and seeds of the family Malvaceae

The cyclopropene fatty acids, malvalic and sterculic acids (Table 4) contain 18 and 19 carbon atoms respectively. They have been reported in plants of families belonging to the Order Malvales. The oil from these plants gives a positive Halphen colour test (8). When contents of these plants, mainly seeds containing oil, are consumed by laying hens, the cyclopropene fatty acids exert their influence through increased permeability of membrane separating egg white from yolk. These fatty acids when consumed in daily doses of up to 50mg do not affect egg production. But a rapid onset of certain changes associated with pink-white disorder, viz. salmon or pinks colour of the white and yolk, is caused by diffusion of iron and protein during storage. The egg yolk becomes pasty during storage. There is also convergence of pH values of egg yolk and white from 6.2 and 9.0 respectively in normal eggs to 7.5 and 8.0 respectively in eggs showing a pink-white disorder (9,10). Subsequent work has established that cyclopropene fatty acids inhibit enzyme desaturase, which converts palmitic and stearic acids into palmitolic and oleic acids respectively (Tables 1 and 2). As a consequence the cyclopropene fatty acids cause elevated stearic acids levels, at the expense of oleic acids, in yolk, heart, plasma, liver and ovary fat of hens, and also in the body fat of hens and swine.

The pink-white disorder becomes important in keeping in view the modern trend of producing eggs from hens raised under free-range conditions. These birds are liable to pick up sees of weeds containing cyclopropene fatty acids.

In cows and buffaloes that are fed on cottonseed, the cyclopropene fatty acids produce hard butter because of the inhibition of desaturase enzyme. This butter contains higher levels of stearic acid than normal butter. Normally a small percentage of cyclopropene fatty acids escape from degradation by rumen microorganisms. These fatty acids have absorbed into the blood stream and cause enzyme inhibition. Hard butter may serve a useful purpose in preparing some confectionery and chocolates. For this purpose the cyclopropene fatty acids can be protected from degradation in the rumen by a process developed by CSIRO (10, 12).

CHAIN LENGTH AND MELTING POINT

For saturated fatty acids, chain length is the major factor determining the melting point. The longer the chain length, the higher is the meeting point. The following table illustrates this phenomenon.

Chain length also has an effect on the melting point of unsaturated fatty acids. The melting point of monounsaturated fatty acids rises from 10.5 to 13.5°C as the chain length in creases from C18:1 to C22:1.

PHYSICAL PROPERTIES MODIFIED BY UNSATURATION

The presence of a double bond of a carbon chain lowers the melting point compared with the corresponding saturated chain. In addition, the position of the double bond in the chain can have an effect on physical and biological properties. A second and third double bond in a chain of similar length would lower the melting point still further. The examples set out in Table 6 illustrate this point.

ISOMERISM

A double bond in a fatty acid carbon chain allows two possible arrangements around the point of unsaturation. This is called geometrical isomerism and two type of isomers, cis- and trans- are possible. In one case, the carbon atoms about the unsaturation bond may adopt a linear or relatively straight configuration to give the trans- isomer. In the other case, the carbon atoms take a relatively curved configuration to give a cis- isomer. This geometrical isomerism is illustrated in Fig. 3.

Fig. 3 - Schematic illustration of geometrical isomerism found in unsaturated fatty acids.
(Inspired by Spritz and Mishkel.)

At the point of unsaturation in unsaturated fatty acids, the carbon atoms with their attached hydrogen atoms may adopt two types of configuration; the cis isomers may be said to have a curved configuration, whereas the trans isomers have a linear configuration.

Vegetable oils contain only cis isomers but hydrogenation of oils in the production of margarines and shortenings may result in the formation of substantial quantities of  trans isomers. Hydrogenation is sometimes used as a resort to raise the melting point of the parent oil and to decrease its susceptibility to oxidation and rancidity. But this type of processing may result in the production of unhealthy products.

PEROXIDATION

Unsaturated fatty acids are susceptible to oxidation and peroxidation reactions. They can occur when the oil or fat is exposed to atmospheric oxygen or is inside the body and the fats are exposed to respiratory oxygen. These reactions are extremely complex and lead to the formation of various products, the first of these being free radicals which are unstable and cause a breakdown of the fatty acid molecule. Various rearrangements occur within the molecule and various new substances are formed, such as alcohols, ketones and aldehydes. These substances are responsible for the unpleasant odours and taste associated with oxidised fats and oils.

Autoxidation of fatty acids increases with the number of double bonds present in the molecule. The presence of natural or synthetic antioxidants is essential for the prevention of autoxidation of foods containing substantial amounts of polyunsaturated fatty acids. The polyunsaturated fatty acids present in the living cells are exposed to many peroxidation reactions. The free radicals formed during these reactions are similar to those formed during exposure to radioactivity (Fig. 4). The danger of autoxidation is present in human cells and must be constantly protected by natural antioxidant systems consisting of vitamins E and C, and carotene and selenium. Certain other substances act as pro-oxidants. These include copper, iron or their compounds and increase the autoxidation of fatty acids.

Peroxidation reactions associated with polyunsaturated fatty acids, therefore, pose a constant hazard to the food industry as well as to the living cells. Natural and synthetic antioxidants protect against these undesirable reactions. Hydrogenation of vegetable oils and fish oils diminish the risk of peroxidation of foods containing these oils but may increase other dangers.

Fig. 4  Sequence of events associated with the peroxidation of polyunsaturated fatty acids

TRIGLYCERIDES

Fats and oils do not contain any appreciable amount of free fatty acids. They exist in the form of triglycerides consisting of molecules of glycerol and three of fatty acids. Glycerol molecules contain three (3) carbon atoms and each of them is attached to a hydroxyl (-OH) group. These hydroxyl groups react with the carboxyl (-COOH) of three fatty acids to form a triacyl ester molecule. An example of this is given in Fig. 5.

Fig. 5 - Schematic illustration of triglyceride

It can be seen that the three positions (,  and ) on the glycerol molecule are occupied by three fatty acids (R1, R2 and R3). If two fatty acids form the esters the molecule is a diglyceride and if one fatty acid forms an ester it is called a monoglyceride. In diglyceride and monoglyceride molecules the unoccupied (-OH) groups retain their original properties.

Dietary oils and fats and depot fats in humans and animals consist of mixed triglycerides, meaning that the positions on the glycerol are occupied by different fatty acids. There are normally twenty fatty acids which form the natural fats and oils; this means there are a great number of chemically different triglycerides that can be found in edible fats and oils. For example, one could formulate at least 15 different triglycerides when three fatty acids such as palmitic, stearic and linoleic are used. Each of these 15 triglycerides would have different physical and biological properties.

The fatty acid distribution on the glycerol molecules in nature does not happen by chance. It is now known that the allocation of fatty acids to positions ,  or  follows a preference. For example, in milk fat, long chain saturated fatty acids tend to be located in the a position whereas the short chain fatty acids are found in the  position. Intermediate chain fatty acids are found mainly in the  position. Thus butyric acid (C4:0) in cow's milk fat is found mainly in the  position and stearic acid (C18:2) in the a position.

In human milk, unsaturated acids occupy mainly the  and  positions (Table 7).

In fats of animal origin the saturated fatty acids palmitic (C16:0) and stearic (C18:0) are found mainly in the  and  positions. However, the notable exception is pork fat where palmitic acid (C16:0) is primarily located in the  position. Unsaturated fatty acids in the depot fat of man and cattle are preferentially present in the  position while in pork fat they are found in high proportions in the  and  positions.

In vegetable oils, about 70% of linoleic acid (C18:26) content occupies the  position while the saturated fatty acids occupy the  and  positions. In coconut oil, which contains a high proportion of short and medium chain fatty acids, the distribution of different fatty acids resembles that of human milk fat.

The position of the particular fatty acids in the triglyceride gains significance during the digestion and absorption of oils and fats when mono-glycerides and diglycerides are formed. The short and medium chain fatty acids are hydrolysed from  and  positions and are absorbed into the blood and carried straight to the liver where they are oxidised. The long chain fatty acids, however, are incorporated into triglycerides after absorption and form chylomicrons. The position of the fatty acids is also of significance during the formation of phospholipids in the body, where the enzyme activity is specific for the different positions on the glycerol molecule (3). The polyunsaturated fatty acids are predominantly located on the  position of the phospholipid molecule. Similarly the polyunsaturated fatty acids are located mainly at the  position of the vegetable oils. They have a significant influence on the metabolism of trans fatty acids produced in hydrogenated vegetable oils.

PHOSPHOLIPIDS

1) Phosphoglycerides

The phosphoglycerides are found in animals and plants and their nomenclature is based on the name of particular molecules which are linked to the phosphoric acid molecule. Examples of some phosphoglycerides are depicted in Fig. 7.

Fig. 7 - Schematic illustration of some phosphoglycerides

Phosphoglycerides are the commonest of all phospholipids and are found in both animals and plants. The phosphatidylcholines are the lecithins found in soybean oil and are the common emulsifying agents used in the industrial preparation of many foods rich in fat or oil. In other phosphoglycerides, choline is replaced by serine, ethanolamine or inositol. In general the fatty acid R2 in position  is unsaturated and most of the time the fatty acid in the position  is saturated.

2) Sphingolipids

In these phospholipids, glycerol is replaced by sphingosine which contains a chain of 18 carbons on which are found one trans double bond, two hydroxyl groups and one amino group (-NH2). The phosphoric acid is esterified with the hydroxyl group of choline. The structure of sphingolipids is shown in Fig. 8.

Fig. 8 - Schematic illustration of a sphingomyelin molecule

Sphingomyelins are important constituents of the myelin sheath which surrounds nerve axons. The fatty acids R which are part of these molecules are long-chain fatty acids - 18 to 26 carbon atoms. Both the chain length and degree of saturation of the fatty acid R may alter the physical and physiological properties of these substances

Sphingomyelins are important components of the myelin sheath which surrounds the nerve axons. The difference with respect to sphingomyelins lies in the nature of fatty acids R which have a chain length varying from C16 to C24 carbon atoms. Lignoceric acids (C24:0) and nervonic acid (C24:2) are the common ones (4). The brain is rich in sphingolipids. The nature of the fats consumed may influence the ratio of the different fatty acids incorporated in the different sphingolipids of the body. The metabolic consequences of this influence are not well known at present.

3) Glycolipids

(i) Cerebrosides

The cerebrosides contain a sphingosine molecule joined with a fatty acid on one end and a sugar-like molecule, or more correctly, a galactose molecule joined at the other end (Fig. 9). The fatty acid is always a long chain, usually stearic acid (C18:0). The cerebrosides are important constituents of the myelin sheath.

(ii) Gangliosides

The gangliosides contain molecules of sphingosine joined with a complex carbohydrate on one end and a fatty acid on the other. The grey matter of the brain is rich in these lipids. The structure of gangliosides is shown in Fig. 10.

Fig. 9 - Schematic illustration of a cerebroside

Cerebrosides are constituents of the myelin sheath of the nerve axons. Stearic acid (C18:0) is the predominant fatty acid R

Fig. 10 - Schematic illustration of a ganglioside

Gangliosides are lipid compounds containing fatty acids, complex carbohydrates and sphingosine. The gray matter of the brain is rich in gangliosides. The nature of the constituent fatty acid R can be influenced by the type of fat consumed in the diet, but the physiological consequences of such a change are not well understood

CHOLESTEROL

What is Cholesterol?

Cholesterol is a yellowish-white fatty substance and is a component of all body cells of animals including humans. The group of lipids that are soluble in organic solvents and are converted to water-soluble substances upon hydrolysis with alkali are called saponifiable lipids. However, the extraction of animal tissues with organic solvents may yield an appreciable quantity of lipid material that is resistant to saponification. Such unsaponifiable lipids may include one or more of a variety of substances belonging to a group of crystalline alcohols known as sterols (Greek 'Stereos', solid). In the tissues of vertebrates, the principal sterol is the C27 alcohol cholesterol; its fundamental carbon skeleton is the cyclopentanoperhydrophenanthrene ring. It will be noted that cholesterol has a double bond in the 5,6 position and a hydroxyl group at position 3 (21). The illustrations of cholesterol molecule are given in Fig. 11 and Fig. 12.

Fig. 11 - Schematic illustration of the cholesterol molecule

The cholesterol molecule contains mainly carbon and hydrogen. The carbon atoms are linked to one another to form four rings designated as rings A, B, C and D. There is a hydroxyl group on carbon 3 (Ring A) which reacts with fatty acids to form cholesterol esters. Both cholesterol and cholesterol esters are practically insoluble in water and body fluids. Cholesterol and its fatty acid esters are essential for the proper functioning of body cells

Fig. 12 - The planar nature of the cholesterol molecule

As we understand it, chemical evolution preceded the biological evolution of living organisms as proposed by Darwin. Cholesterol and structurally related sterols of fungi and plants are, as far as we know, not universal in living organisms. We can, therefore, state with certainty that the sterol structure is not essential for life processes as a whole. The appearance of oxygen in the biosphere was essential for the biosynthetic pathway of sterols to develop. Vertebrates without exception synthesise cholesterol, and cholesterol synthesis in these animals is universally present. Perhaps it is not without surprise then that, the more advanced the organism, the more diverse is the role of the sterol molecule.

Fig. 13 - Some derivatives of cholesterol

Cholesterol is the precursor of many substances essential to life, such as bile acids, hormones and Vitamin D3. All body cells must be constantly supplied with cholesterol. This is ensured by a supply from the diet or by its biosynthesis in the liver

Cholesterol is an essential precursor of bile acids, corcosteroide and sex hormones and Vitamin D-derived hormones in all vertebrates (Fig. 13) (5).

Cholesterol in the Human Body

The average adult male weighing about 70kg, contains just about 0.2% or 145 g of cholesterol, out of which about 5.5% or 8 g is present in blood plasma. An average man in western countries would consume close to 0.45 g of cholesterol every day in addition to assorted amounts of related sterol. The average daily synthesis of cholesterol, working at only a fraction of full capacity, might be about 1.0 g, giving a total of 1.45 g. The average daily metabolic requirement should be no more than 350 mg, even less if the recycling efficiency is higher. Man's plight is actually even worse than this manifold excess suggests. Every cell of man's body not only contains cholesterol and has ready access to a large extracellular supply, but also, in addition, every cell can manufacture cholesterol (with the exception of red blood cells). In other words, every body cell must have 26 enzymes responsible for converting acetyl CoA to cholesterol. Such extravagance, on such a grand scale, is virtually unknown elsewhere in the mammalian world. As a result towards the later part of life, and sometimes very much earlier the body may contain more cholesterol than it should. The final irony of all this cholesterol imbalance is that it may lead to undesirable health consequences and the ultimate death of the average human being.

The information above gives a brief description of the lipids which are important in food and nutrition. On the Lipoproteins link I shall describe the transportation and metabolism of the lipids in the body in the form of lipoproteins. With many thanks for your interest.

REFERENCES

1. Fruton, J.S. and Simmonds, S. (1958). General Biochemistry (2nd Edition), Asia Publishing House, New Delhi

2. Brisson, G.J. (1981). Lipids in Human Nutrition, MTP Press Limited, Falcon House, Lancaster, England.

3. Vance, D.E. and Vance, J.E. eds. (1985). Biochemistry of Lipids and Membranes; The Benjamin/Cummings Publishing Company, Inc. Sydney

4. Karlson, P. (1971). Biochemie, (ed 2), Paris, Doin

5. Sabine, J.R. (1977). In: 'Cholesterol', Marcel Dekker Inc., New York, pp5-27

6. Breckenridge, W.C., Maria, L., and Kukis, A. (1969). Triglyceride structure of human milk. Can. J. Biochem. 47:761-769.

7. Breckenridge, W.C. (1978). Streospecific analysis of triglycerols. In: Kuksis, A. (ed). Handbook of lipid research, vol. 1. Fatty acids and glycerides, pp197-232. Plenum Press, New York.

8. Jamieson, G.S. (1943). Vegetable Fats and Oils. Reinhold Publishing Corp. New York, pp419-420

9. Shonstone, F.S. and Vickery, T.R. (1959). Substances in plants of the Order Malvales causing pink whites in stored eggs. Poultry Sci. 38: 1055-1070

10. Phelps, R.A., Shenstone, F.S., Memmerer, A.R. and Evans, J.R. (1965). Poultry Sci. 44: 358-394

11. Ashes, J.R, Welch, P. St.V., Gulatis, S.K., Scott, T.W. and Brown, G.H. (1992). J. Dairy Sci 75, 1090-1095

12. Ashes, J.R., Gulati, S.K. and Scott, T.W. (12995). In Animal Sciences Research and Developments. Moving Towards a New Century (Ivan, M. ed.) 75th Anniversayr Meeting of Canadian Society of Animal Scie4nce, July 9-12, Ottawa, Ontario, pp.177-188.