The role of dietary proteins in atherosclerosis

Abstract

A series of experiments were carried out to investigate the role of dietary proteins in atherosclerosis with particular emphasis on the involvement of dietary protein in the immunological induction of atheroma and in cholesterol metabolism. Experiments 1 and 3 failed to produce complicated arterial lesions in rabbits by the combined effects of cholesterol feeding and serum antibody production to dietary protein. Clearly, the magnitude of the immune response to dietary protein was insufficient to mimic the effect of cholesterol feeding coupled with the immunological injury induced by serum sickness or graft rejection. The most significant observation to emerge from this part of the study was the development of immune tolerance to a given dietary protein in weanling rabbits induced by exposing their dams to the particular protein during the pre-weaning period. It was not possible to identify the nature of the tolerizing factor nor the critical stage of exposure i.e. in utero or during lactation.Several experiments were designed to investigate the hypocholesterolaemic effect of vegetable proteins. The possible role of plant saponins as the effective agents in soya bean induced hypocholesterolaemia was investigated. The increased excretion of faecal sterols due to saponin was not associated with a reduction in serum cholesterol. Furthermore, in studies with rats and hamsters fed hypercholesterolaemic diets (12 cholesterol + 0.252 cholic acid) there was no evidence that the dietary fibre (cellulose) reduced the serum cholesterol and no evidence to suggest that the addition of saponin to the diets altered the result. It was therefore concluded that whereas saponin may reduce the absorption of cholesterol, it does not alter the effect of dietary protein or fibre on serum cholesterol. It was also found that at nitrogen intakes of near maintenance, the hypocholesterolaemic effect of soya was still evident, although somewhat reduced. The nutritive value of methionine supplemented soya bean and cow's milk protein were compared in young rabbits. There was no evidence that nitrogen digestibility, nitrogen balance or the utilization of absorbed nitrogen differed between the two protein sources. From the above studies it may be concluded that dietary protein may exert a significant effect on the development of atherosclerosis partly through its effect on cholesterol metabolism and partly through its possible immunological injury.

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