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1.6 BIOLOGICAL SCIENCES
Although it is not known iflinolenic acid (18:3w3) is essential its derivatives are important (Tinoco et a/. 1979). Eicosapentaenoic acid (20:5w3) is the precursor of the triene prostaglandins (Gryglewski et a/. 1979) and when incorporated into platelet lipids may influence bleeding time (Sanders et al. 1980). Docosahexaenoic acid (22:0013) is a major component of human brain and retinal lipids and is found in its highest concentrations in the phosphoglycerides of synaptic membranes and rod outer segments implying that it has a role in neural transmission. Radiotracer experiments in vitro suggest that man can convert 18:3w3 to 20:5w3 and 22:0013 (De Gomez Dumm & Brenner, 1975; Aerberhard et al. 1978) but this capacity may be limited (Dyerberg et a/. 1980). Consequently dietary 20: 5w3 and 22: 0013 may be important (Crawford & Sinclair, 1972), the only significant sources being fish, fish oil and offal (Shepherd et al. 1978; Southgate & Paul, 1978). Indeed, vegans whose diets are devoid of 20:5w3 and 22:0013 have very much lower proportions of these fatty acids in their plasma choline phosphoglycerides than do omnivores (Sanders eta/. 1977). It was suggested that the high value for the ratio, 18:2w6:18:3w3 in the vegan diets suppressed the transformation of 18:3w3 to 20: 5w3 and 22:6w3 and instead favoured the conversion of 18:2w6 to 20:4w6 and 22:4w6. This being so then a dietary supplement of 18: 3w3 should reduce the value for 18:2w6:18:3w3 and lead to an increase in the proportions of 20:5w3 and 22:0013 in this lipid fraction. In order to test this hypothesis, vegan and omnivore subjects were given a supplement of linseed oil, rich in 18:3w3 for 2 weeks and changes in their blood lipids were monitored. So that a comparison could be made between dietary linolenate and its long-chain derivatives, the effect of a fish oil supplement, rich in 20:5w3 and 22: 0013, was also studied in the omnivores.
Sanders, T., Younger, K.: The Effect of Dietary Supplements of w3 Polyunsaturated Fatty Acids on the Fatty Acid Composition of Platelets and Plasma Choline Phosphoglycerides. British Journal of Nutrition