Origin of milk cholesterol in the rabbit and guinea pig

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Abstract
Milk cholesterol may be derived from 2 sources: by transfer from the blood directly or by mammary gland synthesis. To study this problem, cholesterol-4-14C was given in the diet to 7 pregnant female guinea pigs to establish an isotopic steady state. This isotope was also administered by gavage in a single dose to 4 guinea pigs immediately after parturition and to 4 rabbits during pregnancy. The diet of these animals was cholesterol-free. A comparison of milk and serum radioactivities indicated that milk cholesterol originated in part by rapid transfer from the plasma cholesterol (28-45%), and in part by mammary gland synthesis in the lactating guinea pigs. In the rabbit, however, milk cholesterol appeared to be derived principally from the plasma cholesterol. Four female guinea pigs were fed a diet containing cholesterol-4-14C and different amounts of nonradioactive cholesterol until an isotopic steady state was achieved. After parturition, the serum, milk, mammary gland, and liver were analyzed. As judged from the ratio of specific activity of cholesterol in milk to that in the diet and blood, 50-70% of milk cholesterol appeared to originate from the blood and about 20% of milk cholesterol was of dietary origin. In these cholesterol-fed animals, milk and mammary gland generally had similar specific activities as usually did the liver and serum. Dietary cholesterol increased the cholesterol content of milk and also increased the proportion of the milk cholesterol derived from the plasma, presumably as a result of increased transfer of cholesterol from the blood to the milk.

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