On the lipotropic action of protein.

The lipotropic action of protein, which was first recognized in 1935 (1, 2), has been the subject of many investigations (3-6). These have centered largely around the question of whether the lipotropic activity of a protein can be attributed entirely to it,s content of cystine and methionine. Evidence has been presented on both sides of the controversy that has arisen (6-ll), and, although some factors of a general nature that affect the lipotropic activity of a diet have been elucidated (6, 12, 13), methionine is the only amino acid that has been shown to cause a reduction in the deposition of fat in the livers of animals fed diets deficient in choline (11). Attempts to evaluate the information available on this subject are complicated by the variety of experimental conditions (the differences in the age and weight of the rats, the amount and type of dietary protein, the amount and type of dietary fat, the rate of growth of the animals, the method of feeding, and the nutritive value of the entire diet) that have been used, and the claims for a lipotropic action of proteins beyond that of the methionine contained in them still appear to rest on rather tenuous grounds. When it became apparent that protein (14) and the amino acids, threonine (14, 15) and glycine (16), were effective in reducing the deposition of fat in the livers of young rats fed low protein diets containing choline, the idea was entertained that these results might be linked to the earlier observations on the lipotropic action of protein. Since much of the evidence for a lipotropic action of proteins, other than that of the methionine contained in them, had been obtained from studies in which diets relatively high in methionine or other lipotropic factors had been used, it appeared that this secondary effect of protein on fat deposition in the liver might be most clearly demonstrated in animals fed dew protein diets containing close to the required amount of choline or choline precursors.