The effect of overnight fasting on the dietary protein-dependent change in the fatty acid composition of tissue lipids was studied in rats fed with casein or soybean protein (20%) diets containing 5 or 2% corn oil.The activity of the J6-desaturase of liver microsomes, a key enzymeof linoleate metabolism to arachidonate, was depressed significantly by overnight fasting, and the protein effect disappeared, irrespective of the level of dietary fat.The proportion of linoleate in liver phosphatidylcholine was decreased, whereas that of arachidonate was increased after overnight fasting in rats fed with a low fat diet, resulting in an elevation of the linoleate desaturation index.Although the effect of fasting became obscure on a high fat diet, the protein effects were maintained even after fasting.Asimilar trend was also observed in various lipid fractions.Thus, the effect of dietary protein on the polyunsaturated fatty acid profile was not modulated by overnight fasting, particularly when a minimal amount of linoleic acid was supplied.Dietary protein influences not only the tissue lipid level1'2) but also the metabolism oflinoleic acid.3'4)There is evidence that casein (CAS)
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Koba et al. (1991) studied this question.