Studies on the origin of milk fat. 2. The secretion of dietary long-chain fatty acids in milk fat by ruminants

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Abstract
Tritium-labeled stearic acid was given orally, either as free acid or as triglyceride, to lactating goats and to a cow, and the appearance of radioactivity in milk fat and its distribution in milk constituents were studied. Measurable activity was found in the milk fat 4 hours after administration of labeled fat, the maximum specific activity occurring within 24 hours. Activity was still detectable in milk fat secreted up to 37 days after administration. Up to 59% of the ingested activity ultimately appeared in the milk fat. High specific activities were found in the long-chain acids, both saturated and unsaturated, of milk glycerides and phospho-lipids. Much lower specific activities were found in the steam-volatile water-soluble and -insoluble fatty acids of milk fat, and in milk glyceride glycerol, lactose, cholesterol, casein and in body water. Results suggest that the contribution of dietary fat to milk fat was not more than about 25%, and that the characteristic short-chain acids of the milk fat of ruminants do not arise mainly by degradation of long-chain acids.