Protein-fat interaction on serum cholesterol level, fatty acid desaturation and eicosanoid production in rats.

The combined effects of dietary protein (casein or soybean protein) and fat (palm olein or mold oil) on several lipid parameters were studied in rats. The fatty acid composition of the dietary fats was made comparable except for the proportions of polyunsaturated fatty acids; mold oil contributed gamma-linolenic acid (GLA) at the expense of a portion of the linoleic acid in palm olein. When animals were fed casein rather than soybean protein, serum cholesterol levels were higher irrespective of the fat source, but it took a longer time to produce a significant difference when the dietary fat was mold oil. Soybean protein increased fecal steroid excretion, and mold oil tended to stimulate the excretion of neutral steroids. The ratio of arachidonate to linoleate in phosphatidylcholine from plasma, liver and thoracic aorta was markedly higher in the casein than in the soybean protein groups. Mold oil predictably improved a reduction of arachidonate by vegetable protein. The aortic production of prostacyclin was higher with mold oil than with palm olein irrespective of the protein source, although there was a trend toward a higher production with casein. No protein-fat interaction was observed on the concentration of plasma thromboxane B2. Thus GLA effectively modified metabolic consequences of dietary protein.