Effects of amino acid supplementation of dietary proteins on serum cholesterol and fatty acids in rats

A 120 d feeding study with adult rats was conducted to investigate the effe cts of amino acid supplementation of casein and gelatin (based on differenc es in the amino acid profiles of the two protein sources) on blood lipid pa rameters (total cholesterol, HDL-cholesterol and fatty acid composition of phospholipids). The concentrations of total cholesterol and HDL-cholesterol were significantly (P < 0.05) higher in rats fed on unsupplemented casein diet compared with those fed on the unsupplemented gelatin diet. Supplement ation of casein with glycine or glycine+arginine (amino acids present in la rge amounts in gelatin) significantly (P < 0.05) reduced concentrations of total serum cholesterol. However, supplementation of gelatin with glutamic acid, methionine, phenylalanine or tyrosine or a mixture of these four amin o acids (present in large amounts in casein) had no effect on total serum c holesterol. Supplementation of gelatin with a mixture of glutamic acid, met hionine, phenylalanine and tyrosine significantly (P < 0.05) reduced the co ncentration of stearic acid (18: 0) and elevated the concentration of palmi toleic (16:1n-7) and oleic (18: 1n-9) acids in blood, suggesting increased formation of these monounsaturates from their precursor fatty acids by prom otion of the activity of Delta 9 desaturase by the mixture of the supplemen tal amino acids. However, supplementation of gelatin with a mixture of the four amino acids significantly (P < 0.05) increased the concentration of li noleic acid (18: 2n-6) and decreased concentrations of arachidonic acid (20 : 4n-6) and other C-20 and C-22 n-6 long-chain polyunsaturated fatty acid ( n-6 LC-PUFA) metabolites of linoleic acid, suggesting a slowing of the lino leic acid to n-6 LC-PUFA pathway. (C) 2000 Elsevier Science Inc.