Fat is a heterogeneous mixture of lipids, predominantly triglycerides but also includes phospholipids, glycolipids and sterols. Triacylglycerols (triglycerides) are the principal dietary lipids which are stored in fat stores in humans and consist of esters of fatty acids, both saturated and unsaturated and glycerol. Phospholipids contain phosphoric acid as a mono or diester. Glycolipids contain between one and four linked monosaccharide residues joined through a glycosyl linkage to a lipid ( diacylglycerol or sphingosine) Cholesterol, cholesterol ester and phospholipids are important in the structure of cell membranes, mitochondria, lysosomes and the endoplasmic reticulum. Lipids are important in providing insulation against the cold.
The principal dietary sources of fat are dairy products, meat, margarine and other fats, biscuits, cakes and pastries. Plant storage fats are present in nuts, cereal grains and fruits such as the avocado. Other dietary lipids include cooking fats, salad oils and mayonnaise. Eggs are a source of lipid, predominantly saturated and monounsaturated fatty acids, lipoprotein, triacylglycerols, cholesterol and phospholipids. Many foods contain structural fats, phospholipids and glycolipids, cholesterol and plant sterols. Eating brain as a food provides animal sphingolipids to the diet. Dairy products contain milk fat globule membranes. Green leafy vegetables contain galactolipids and there are membrane lipids in cereal, grains, vegetables and fruit.
Major lipid categories
Saturated fatty acids
Most saturated fatty acids are straight-chain structures with an even number of carbon atoms. Saturated fatty acids have a basic formula CH3(CH2)nCOOH where n can be any even number from 2 upwards. Almost without exception dietary fatty acids are formed as even numbers of carbons in an unbranched chain. In naturally occurring lipids the number of carbons in the chain ranges from two to more than 30, the most common fatty acid being palmitic, n = 14 and stearic acid, n = 16.
Unsaturated fatty acids
Unsaturated fatty acids contain double bonds, and are of nutritional and biological importance. Removal of hydrogen bonds results in ethylenic double bonds which are unsaturated. The hydrogen atoms on either side of the double bond in the fatty acid molecules are of cis geometrical configuration or trans configuration These are stereoisomers in that the two forms differ in the arrangement of their atoms in space; cis- means that the hydrogens are on the same side (the most common configuration for fatty acids in nature), whereas in trans the hydrogens are on the opposite side. This results in differing physical properties and response to enzymatic attack.
Monoenoic unsaturated fatty acids These contain one unsaturated double bond. The more common have an even number of carbon atoms with chain length of 16–22 carbons and a double bond in the cis – form, often in the delta 9 (D 9 )position. A double bond causes restriction in the movement of the acyl chain at that point. The cis configuration introduces a kink into the average molecular shape. This means that the cis form is less stable thermodynamically and has a lower melting point than the trans form.
Polyunsaturated fatty acids These are derived from monoenoic fatty acids, the position of the second double bond being dictated by the synthetic processes. Mammalian enzymes may only remove hydrogen atoms between an existing double bond and the carboxyl group. Further desaturation has to be preceded by chain elongation. Unsaturated fats with one or more cis double bond are more common in natural lipids than are trans. The cis bond affects the linearity of the chain of methylene groups in that the molecule folds back on itself. Polyunsaturated fats are susceptible to oxidation but are protected by natural antioxidants, e.g. vitamin E.
Nomenclature of polyunsaturated fatty acids
There are several nomenclatures for the polyunsaturated fatty acids. The nomenclature may be based on the saturated parent acid, number of carbon atoms and position of the double bonds. The differences are dictated by whether the numbering is taken from the methyl or carboxyl (COOH) end. Numbering from the carboxyl end is the chemical nomenclature and called the Geneva system. Numbering is from the carbon 1 carboxyl group
A short-hand system indicates the number of double bonds: stearic acid C 18:0 and oleic or elaidic acid C 18:1. The position of the double bond is given by cis-9-, 18:1 (oleic acid), or trans-9, 18:1 (elaidic acid).
An important aspect of unsaturated fatty acids is the opportunity for isomerism, which may be either positional or geometric. Positional isomerism occurs when double bonds are located at different positions in the carbon chain. A 16-carbon mono unsaturated fatty acid may have positional isomeric forms with double bonds at C7 and C9. These are called D 7 and D 9, the position of unsaturation is numbered with reference to the first of the pair of carbon atoms between the double bond.
Linoleic acid can be written as cis (D -9 ), cis(D -12–18 :2) or (cis, cis)9,12-octadecadienoic acid, showing that it is an 18-carbon fatty acid with cis double bonds 9 and 12 carbons from the carboxyl end.
The alternative nutritional or biological system uses the prefix n — or historically v . In this system the numbering is from the terminal methyl end. The main dietary unsaturated fatty acids families are n-3, n-6, n-7, n-9. This numbering system is determined by the position of the first double bond from the methyl carbon atom (Figure 12.3 ). This double bond determines the number of double bonds which can be inserted.
Essential fatty acids
The essential fatty acids and their longer-chain molecular products are necessary for the maintenance of growth, good health and reproduction. They are important in biological membranes and affect the permeability of the membrane to water and sugars and metal ions, and also eicosanoid synthesis. All essential fatty acids are polyunsaturated fats.. Essential fatty acid activity depends on the presence of a cis-9, cis-12 methylene-interrupted double bond system. If the double bond is converted from cis into trans this essential biological activity disappears.
The process of desaturation and elongation is important in the tissue synthesis of some polyunsaturated fats. The parent unsaturated fatty acids are extended by alternate desaturation, i.e. the introduction of a double bond and chain-lengthening reactions. There are limited enzymes involved in these reactions and fatty acids from each family compete for the enzymes. Linoleic acid and a -linolenic acid are the preferred substrates. Arachidonic acid, the main product of the elongation and desaturation of linoleic acid, has essential fatty acid activity but is only essential when insufficient of its precursor linoleic acid is available.
Humans and other animals are unable to insert double bonds into fatty acids at carbon position 12 and 15 towards the methyl end of the fatty acid chain therefore linoleic and a -linolenic acids cannot be synthesised and are therefore essential. desaturate fatty acids. There are three distinct non-interconvertible families of fatty acid. Polyunsaturation is undertaken by three desaturases, D 4, 5 and 6 which introduce double bonds between carbon atoms 4–5, 5–6 and 6–7. The fatty acids which have a double bond at n-3, n-6, n-7 and n-9 cannot be interconverted in animal tissues.
The parent substrates for desaturation are oleic, linoleic and a -linolenic acids. They give rise to a series of fatty acid families: The essential fatty acids all belong to the n-3 and n-6 groups, but not n-7 or n-9.
• The n-3 family, originates from a -linolenic acid n-3; a -linolenic acid: cis, cis, cis-9, 12,15 (C-18:3) is the parent fatty acid. a -Linolenic acid (C-18:3, n-3) can add methylene groups to increase the chain length to eicosapentaenoic acid.
• The n-6 family, originates from linoleic acid n6; linoleic acid: cis, cis-9,12 (C-18:2) is the parent fatty acid, with the n-6 numbering from the methyl end. Linoleic acid can be extended to gamma ( g ) -linolenic and arachidonic acids (C-20:4, n-6)
• The n-9 family, originates from oleic acid n-9; a non-essential fatty acid family. Oleic acid (cis-9,C-18:1) is the parent fatty acid. Oleic acid can increase the chain length to become eicosatrienoic acid.
Deficiency of essential fatty acids in adults is rare but has been seen in children fed virtually fat-free diets. The skin abnormalities were those previously seen in experimental animals with dermatosis of the skin, increased water permeability, increased sebum secretion and decreased epithelial hypoplasia. The n-3 fatty acids (parent a -linolenic acid) are important components of brain and retinal lipid tissue. These fatty acids cannot be synthesised in the animal and their dietary provision may be particularly important in early life for brain development.
Trans fatty acids
Fatty acids with trans bonds may also be monounsaturated or polyunsaturated, with cis and trans bonds within the same molecules. The biohydrogenation of fats in the rumen of ruminants can result in the production of trans fatty acids. Trans fatty acids, in addition to occurring naturally, can be formed during the partial hydrogenation of a cis unsaturated fatty acid.
Conjugated fatty acids
Conjugated linoleic acid is a collective term for a mix of geometric and positional isomers of octadecadienoic acid ( 18 : 2 ) where the double bonds are conjugated rather than methylene separated as in linoleic acid.
Recommended intake of fatty acids
Dietary expert committees have recommended a reduction of the fat content of the diet to 30–35% of energy and that unsaturated fatty acids should be increased and saturated fatty acids should provide only 10% of food energy. A Report of the British Nutrition Foundation Task Force in 1992 suggests that this is achievable by eating fewer meat products, dairy products and baked foods and by eating more oily fish (herring, mackerel, sardine and salmon) fruit, vegetable and wholemeal cereals. Saturated fat should be replaced by starch in the form of bread, cereals, fruit and vegetables.
The properties of trans fatty acids and their contribution to the diet are not fully understood, though the UK COMA committee suggested that they should not contribute more than 2% of total fat intake.
Intakes of essential fatty acids in Great Britain have been suggested to be 10 g/kg of food /day ( 2% energy intake ) of n-6 fatty acids and 1-2 g / day of n-3 fatty acids. The main daily dietary sources of n-6 fatty acids are vegetables, fruits and nuts (3.0 g), cereal products (2.6 g) and vegetable oils (2.4 g). The main sources of n-3 fatty acids are vegetables (0.4 g), meat and meat products (0.3 g), cereal products (0.3 g) and fat spreads (0.3 g). Most adult western diets provide 15–18 g of essential fatty acids and in general , healthy individuals have body reserves of 1000–2000 g in their adipose tissue.
Human milk is rich in linoleic acid (C18 : 2), and is important for the development of infants and their brain structure . The linoleic acid content of milk lipids varies in amount (3–12%), being dependent upon maternal dietary intake and possibly smoking habit. a -Linolenic acid (C 18:3) contributes 0.4 % of human milk and docosahexaenoic acid ( DHA (22 : 6)) contributes 0.2%of human milk. Infant formula feeds do not always meet these requirements.
Pregnant and lactating women
Their requirements are similar to other adult recommendations.
n-6/n-3 Polyunsaturated fatty acid ratios
In human milk the ratio of n-6/n-3 polyunsaturated fatty acids is of the order of 11:1.
Monounsaturated fatty acids
An intake of total monounsaturated fats 15% (oleic acid 12%) of total energy intake has been recommended for adults.
The other lipids eg cholesterol are adequately synthesised by the body and there is no dietary requirements of these.
1. Dietary fat is a heterogeneous mixture of lipids, predominantly triglycerides but also includes phospholipids and sterols.
2. Fatty acids have a basic formula CH3 (CH2)n-COOH where n can be any even number from 2 upwards. Unsaturated fatty acids are important nutritionally and biologically. There are several nomenclatures for the unsaturated fatty acids which may be based on the saturated parent acid, the number of carbon atoms and position of the double bonds. The differences are dictated by whether the numbering is taken from the methyl or carboxyl end.
The four main dietary unsaturated fatty acid families are n-3, n-6, n-7 and n-9. This numbering system is determined by the position of the first double bond from the methyl carbon atom.
The essential fatty acids and their longer-chain molecular products are necessary for the maintenance of growth, good health and reproduction. The essential fatty acids all belong to the n-3 and n-6 groups, linoleic acid (n-6) and a -linolenic acid (n-3). The fatty acids which have a double bond at, n-3, n-6, cannot be synthesised by humans and are therefore essential in the diet.
Glycerol (1,2,3-propanetriol) is the alcohol present in the natural triester glycerides, phospholipids and waxes.
6. The functions of lipids are structural, storage and metabolic, although individual lipids may have several different roles.
7. Structural lipids are important at surfaces and in membranes, functioning as barriers between one environment and another. Such barriers are able to exclude water and other molecules.
8. . Food fats are the storage fats of animals and plants. The high-energy densities of triacylglycerols make them ideal as long-term fuel sources. The fatty acid composition of storage lipids is very variable and depends on the composition of the diet.
9. Cholesterol is a structural component of cell walls and membranes, precursor of bile acids, of adrenal and gonadal hormones and vitamin D. It can also accumulate in atheromatous lesions of arterial walls. Cholesterol belongs to the family of steroidal alcohols containing between 27 and 30 carbon atoms and exists as the free sterol or as esters with fatty acids.
10. The more common sterols are found in higher animals and plants, with the exception of ergosterol, found in yeasts. Sterols are also found in fungi, algae and marine invertebrates.
11. The fat content of the diet should be 30–35% of the dietary energy content. Unsaturated fatty acids should replace saturated fatty acids and provide 10% of food energy. A ratio of unsaturated to saturated fatty acids should be more than 0.45, even approaching 1.0; a mixture of n-6 and n-3 polyunsaturated fats and mono-oleic acid should be a target. Human milk is rich in linoleic acid (C18:2, n-6), and is important for the development of infants.