Much of the discussion in this chapier is concerned with the positive contributions made by dietary lipids, especially fatty acids, to nutrition and health. It is equally important to consider whether dietary lipids can also have adverse or toxic effects.
Of the fatty acids, those containing a cyclopropene ring (Section 2.1.4) have been considered toxic as a result of their ability to inhibit the A9-desaturase. One result of this is to alter membrane permeability as seen in 'pink-white disease'. If cyclopropene fatty acids are present in the diet of laying hens, the permeability of the membrane surrounding the yolk is increased, allowing release of pigments into the yolk. Rats given diets containing 5% of dietary energy as sterculic acid (see Table 2.4) died within a few weeks and at the 2% level, the reproductive performance of females was completely inhibited. Cottonseed oil is the only important oil in the human diet that contains cyclopropene fatty acids. However, their concentration in the natural oil is low (0.6-1.2%) and is reduced still further to harmless levels (0.1-0.5%) by processing. There has been no evidence that consumption of cottonseed oil in manufactured products has had any adverse nutritional effects.
The long-chain monoenoic acid, erucic acid (13c-22:1n-9; see Table 2.2) is present in high concentration (up to 45% of total fatty acids) in the seed oil of older varieties of rape (Brassica napus). When young rats were given diets containing more than 5% of dietary energy as high erucic rapeseed oil, their heart muscle became infiltrated with tri-acylglycerols. After about a week on the diet, the hearts contained three to four times as much lipid as normal hearts and although, with continued feeding, the size of the fat deposits (referred to as a lipidosis) decreased, other pathological changes were noticeable. These included the formation of fibrous tissue in the heart muscle. The biochemistry of the heart muscle was also affected. Mitochon-drial oxidation of substrates, such as glutamate, was reduced and the rate of ATP biosynthesis was impaired. The degradation of triacylglycerols that contain erucic acid is somewhat slower than with fatty acids of normal chain length and this may have contributed to the accumulation of lipid deposits.
Despite lack of evidence for harmful effects in man, an extensive breeding programme was undertaken to replace older varieties of rape with new 'zero-erucic' varieties (see Table 3.2). The use of low erucic varieties in manufactured products is now mandatory in most industrial countries, but high erucic varieties continue to be used in countries such as China.
High concentrations of long-chain monoenoic fatty acids also occur in some fish oils and therefore contribute to the diets of people consuming these fish, as well as to diets containing certain fat spreads that incorporate hardened fish oils (see Table 3.1). The nutritional and toxicological consequences of long-term consumption of marine long-chain monoenes have been less extensively studied than have those of high erucic acid-containing rapeseed oil.
Concerns about possible toxic effects of fatty acids with irans-unsaturation began with the publication of results of experiments with pigs given diets containing hydrogenated vegetable fat for 8 months. They had more extensive arterial disease than those given otherwise equivalent diets devoid of irans-unsaturated fatty acids. Subsequently, numerous animal feeding trials, epidemiological studies of human populations and controlled dietary experiments with human subjects have been reported. Some, but not all, epidemiological studies have shown an apparent statistically significant correlation between intakes of certain types of irans-unsaturated fatty acids and increased risk of coronary heart disease. Critical evaluation of all such studies leads to the conclusion that an association between irans-unsaturated fatty acid consumption and coronary heart disease has not been demonstrated; nor is there any reliable evidence linking irans-unsaturated fatty acid intake with cancer or other chronic diseases.
A theoretical basis for some of the claimed effects of irans-unsaturated fatty acids on heart disease has been discussed in terms of their influence on plasma lipoproteins and on components of the blood-clotting system. While there is no conclusive evidence for the latter, there is an effect of trans-unsaturated fatty acids in raising LDL-cholesterol and decreasing HDL-cholesterol concentrations in the blood. This is further discussed in Section 5.4.2.
High concentrations of trans-unsaturated fatty acids may adversely affect the metabolism of the parent essential fatty acids, linoleic and a-linolenic acids to their long-chain metabolites by inhibiting desaturase activity or competing with normal substrates for desaturases (Section 188.8.131.52). In this way, they may raise the dietary requirement for essential fatty acids as explained later. This kind of 'nutritional imbalance' must be distinguished, however, from direct and specific toxic effects. It is worth noting that the hydrogenated fats used in many experiments designed to investigate potential adverse effects of dietary trans-unsaturated fatty acids, contain many unusual cis as well as trans isomers and their potential toxicity has been less extensively researched.
Lipid peroxides may be formed from poly-unsaturated fatty acids when they are in contact with oxygen. As described in Sections 2.3.4 and 4.1.3, the reaction is accelerated in the presence of catalysts such as transition metal ions or haem compounds and by heating. When lipid hydroperoxides are ingested, they are rapidly metabolized in the mucosal cells of the small intestine to various oxyacids that are then rapidly oxidized to carbon dioxide. High concentrations of lipid peroxides in the gut may damage the mucosa and potentiate the growth of tumours. However, there is little evidence for the absorption of unchanged hydroperoxides nor for their incorporation into tissue lipids. Hydroperoxyalkenals, lower molecular weight breakdown products of lipid hydroperoxides, are absorbed and may be toxic. Rats given diets enriched in these compounds manifested increased liver weight, increased concentrations of mal-ondialdehyde, peroxides and other carbonyl compounds and decreased concentrations of a-tocopherol and linoleic acid in tissues. The toxicity of peroxidized cholesterol has been referred to in Section 4.1.3.
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