EFFECTS OF VARYING THE TYPE OF SATURATED FATTY-ACID IN THE RAT DIET UPON SERUM-LIPID LEVELS AND SPLEEN LYMPHOCYTE FUNCTIONS

To obtain further information about the effects of specific dietary sa turated fatty acids, weanling male rats were fed for 6 weeks on low fa t (7.7% by weight) or high fat (17.8% by weight) diets which differed according to the principal fatty acids present. The diets were rich in caprylic and capric acids (medium chain triacylglycerols; MCT), lauri c acid, palmitic acid at the sn-1(3) position, palmitic acid at the sn -2 position or stearic acid. The total proportions of saturated (42-46 %), monounsaturated (36%), n-6 polyunsaturated (15%) and n-3 polyunsat urated (2.2%) fatty acids were the same in all diets. Serum cholestero l concentrations were not different among rats fed the different diets , except that the concentration in the serum of rats fed the high fat diet with palmitic acid in the sn-2 position was high. This was reflec ted in higher HDL and LDL cholesterol concentrations in the serum of a nimals fed this diet. Triacylglycerol (TAG) concentrations tended to b e higher in the serum of rats fed the low fat diets compared with thos e fed the high fat diets. They were lowest in the serum of MCT-fed rat s irrespective of the level of fat in the diet and were highest in the serum of rats fed the low fat diet rich in stearic acid. These differ ences were due to lower chylomicron and VLDL TAG concentrations in the serum of MCT-fed rats and higher chylomicron and VLDL TAG concentrati ons in the serum of low fat, stearic acid-fed animals. The fatty acid compositions of the serum and of spleen lymphocytes were influenced by that of the diet fed. The ex vivo proliferation of lymphocytes from t he spleens of rats fed the high fat diet rich in palmitic acid at the sn-2 position was greater than that of lymphocytes from animals fed th e other diets. Natural killer (NK) cell activity tended to be lower fo r spleen lymphocytes from rats fed high fat diets than for those fed l ow fat diets irrespective of the principal saturated fatty acid presen t. NK cell activity was highest for spleen lymphocytes from animals fe d the diets rich in palmitic acid and was lowest for those from animal s fed the high fat diet rich in stearic acid. Spleen lymphocytes from the latter animals had the lowest proportion of CD16(+) cells, a marke r for NK cells. Thus, this study shows that the type of saturated fatt y acid present in the diet not only has subtle effects upon blood lipi d and lipoprotein levels but can significantly affect lymphocyte funct ions. Spleen lymphocyte NK cell activity is decreased as the fat conte nt of the diet increases. NK cell number and activity are reduced by a high fat diet rich in stearic acid. Spleen lymphocyte proliferation i s enhanced by palmitic acid-rich diets, particularly if palmitic acid is in the sn-2 position of dietary TAG. (C) 1997 Elsevier Science B.V.