Effects of dietary protein type on oxidized cholesterol-induced alterationin age-related modulation of lipid metabolism and indices of immune function in rats

Exogenous oxidized cholesterol disturbs both lipid metabolism and immune fu nctions. Therefore, it may perturb these modulations with ageing. Effects o f the dietary protein type on oxidized cholesterol-induced modulations of a ge-related changes in lipid metabolism and immune function was examined usi ng differently aged (4 weeks versus 8 months) male Sprague-Dawley rats when casein, soybean protein or milk whey protein isolate (WPI) was the dietary protein source, respectively. The rats were given one of the three protein s in diet containing 0.2% oxidized cholesterols mixture. Soybean protein, a s compared with the other two proteins, significantly lowered both the seru m thiobarbituric acid reactive substances value and cholesterol, whereas it elevated the ratio of high density lipoprotein-cholesterol/cholesterol in young rats, but not in adult. Moreover, soybean protein, but not casein and WPI, suppressed the elevation of Delta 6 desaturation indices of phospholi pids in both liver and spleen, particularly in young. On the other hand, WP I, compared to the other two proteins, inhibited the leukotriene B4 product ion of spleen, irrespective of age. Soybean protein reduced the ratio of CD 4(+)/CD8(+) T-cells in splenic lymphocytes. Therefore, the levels of immuno globulin (Ig)A, IgE and IgG in serum were lowered in rats given soybean pro tein in both age groups except for IgA in adult, although these observation s were not shown in rats given other proteins. Thus, various perturbations of lipid metabolism and immune function caused by oxidized cholesterol were modified depending on the type of dietary protein. The moderation by soybe an protein on the change of lipid metabolism seems to be susceptible in you ng rats whose homeostatic ability is immature. These observations may be ex erted through both the promotion of oxidized cholesterol excretion to feces and the change of hormonal release, while WPI may suppress the disturbance of immune function by oxidized cholesterol in both ages. This alleviation may be associated with a large amount of lactoglobulin in WPI. These result s thus showed a possibility that oxidized cholesterol-induced perturbations of age-related changes of lipid metabolism and immune function can be mode rated by both the selection and combination of dietary protein. (C) 2000 El sevier Science B.V. All rights reserved.