Combined effects of dietary protein and fat on lipid metabolism in rats.

The combined effects of dietary protein (casein, whey protein, or soy protein) and fat (safflower oil, perilla seed oil, or their mixed oil; linoleic acid/alpha-linolenic acid = 5.0) on several lipid parameters were studied in rats. Serum cholesterol and triglyceride levels were lower in perilla seed oil groups irrespective of the protein sources. Liver triglyceride level, and fecal excretion of steroids were influenced mainly by protein sources, particularly isolated soy protein lowered liver triglyceride level, and increased the fecal steroid excretion. The fatty acid composition of liver phosphatidylcholine was influenced by dietary fat sources. The proportion of arachidonic acid was significantly lower in rats fed perilla seed oil than in those fed safflower oil or mixed oil, while the proportion of eicosapentaenoic acid was higher in rats fed perilla seed oil than in those fed safflower oil. Arachidonic acid/linoleic acid ratio was significantly lower in perilla seed oil groups, and tended to be higher in casein groups. Eicosapentaenoic acid/arachidonic acid ratio was influenced by both dietary protein and fat, and protein-fat interaction was observed. Eicosapentaenoic acid/arachidonic acid ratio was higher in perilla seed oil groups, and isolated soy protein in perilla seed oil groups lowered the ratio in comparison with other protein sources in perilla seed oil groups.