EARLIER experiments by Thomas, Culbertson and Beard1 have shown that feeding liberal allowances of highly unsaturated oils to steer calves over a period of 260 days was without effect on the mean unsaturation of the body fat. It has similarly been shown by Shorland and co-workers2–5 that the main dietary fatty constituent of pasture-fed animals—linolenic acid—although present in non-ruminants in quantity, appears only in traces in the depot fats of ruminants. It has been suggested by one of us that differences in fatty acid composition between the fats of ruminants and non-ruminants might be attributed to modification of the dietary fat in the rumen2,5. Reiser6 found that linseed oil emulsions incubated with rumen contents showed a diminution in linolenic acid content with a corresponding increase in linoleic acid, indicating the hydrogenation of linolenic acid to linoleic acid. Hartman, Shorland and McDonald7 have suggested that such hydrogenation would explain the presence of appreciable amounts (3.5–11.2 per cent) of trans acids in ruminant fats and their absence in the fats of non-ruminants.
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