Lipids are important constituents of the diet, although the proportion of energy provided by dietary lipids varies widely between species, between different human societies and between individuals. Quantitatively, the most important lipids in the human diet are the triacylglycerols that are provided mainly by the adipose tissue of farm animals, the oils of fish and the seed oils of plants. Their fatty acids may be predominantly saturated and monounsaturated, as in ruminant fats, coconut and palm oils, di- and triunsaturated as in many seed oils or highly polyunsaturated as in many marine oils. Their physical form and chemical composition is usually modified during food manufacture and processing, for example in the hydrogenation employed in the manufacture of margarine to reduce the degree of unsaturation.
Lipids confer palatability to the diet through the textural properties they give to foods and through low molecular weight flavour and odour compounds produced by enzymic oxidation, heating and fermentation. Modern industrial techniques now allow the production of dietary fats with reduced or even zero energy value and of different textures. Triacylglycerols provide a concentrated form of dietary energy and act as vehicles for the fat-soluble vitamins. They also provide the fatty acids required for cellular structures such as biological membranes. Whereas the human body has the capacity to synthesize for itself saturated and monounsaturated fatty acids and cholesterol, it is unable to synthesize linoleic and a-linolenic acids (essential fatty acids) and vitamins A, E and K. The amounts required daily in the diet can be assessed, albeit rather imprecisely. Requirements vary between individuals and depend on the rate of growth, age and physiological states such as puberty, pregnancy and lactation.
Linoleic and a-linolenic acids are converted in many tissues into two series of longer chain, more highly polyunsaturated fatty acids, classified as the n-6 and n-3 families, respectively. These are important in contributing to the structural integrity of membranes and in acting as precursors for a range of oxygenated fatty acids, the eicosanoids. The latter are produced locally, have short half-lives and exert powerful physiological activities such as muscle contraction and platelet aggregation at extremely low concentrations. Although many human tissues contain the enzymes for converting the parent fatty acids of the n-6 and n-3 families (linoleic and a-linolenic) into longer chain, more highly polyunsaturated fatty acids by a series of desaturations and elongations, the measured activities of these enzyme systems are often very weak. There is uncertainty, therefore, as to whether essential fatty acid requirements can always be satisfied by dietary consumption of the parent fatty acids alone, or whether there may sometimes be a specific requirement for the long-chain polyunsaturates. This additional requirement may be particularly acute in times of high demand, for example during brain development, especially in premature infants. There is continuing debate about the potential requirements for long-chain polyunsaturates (Box 4.1) which can only be resolved by further research.
The metabolism of essential fatty acids is influenced by the amounts and types of other fatty acids in the diet as a result of competition for the elon-gases and desaturases. Consequently, the ratio between the different families of unsaturated fatty acids in the dietary lipids may be as important in nutrition as the absolute amounts of dietary linoleic and a-linolenic acids.
Lipids influence immunity in several ways. Some glycolipids are components of cell-surface antigens. Dietary fat enhances immunity in malnourished people through its energy contribution. Individual lipids have more specific roles through effects on membrane structure, eicosanoid production, peroxidation and gene expression.
Lipids may influence processes leading to cancer development by affecting membrane properties, cell signalling pathways, gene expression and immune function. Some dietary lipids may exacerbate carcinogenesis, others retard or prevent it. Liposomes may be used to target drugs in cancer therapy (see also Section 6.5.13).
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