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The effect of two types of protein, supplementary methionine and dietary cholesterol on plasma and liver cholesterol levels Craig, Rosemary Eleanor Anne


High serum cholesterol levels have been identified as an important risk factor for atherosclerosis in humans. The purpose of this study was to investigate the effect of protein quality and quantity and dietary cholesterol supplementation on plasma and liver cholesterol levels, using a rat model. Male Wistar rats, weighing approximately 150 g, were fed ad libitum, with food intakes recorded daily and body weights weekly. At the end of six weeks, plasma and liver samples were collected and analyzed enzymatically for cholesterol content. Dietary protein was fed at levels of 10%, 15% and 20%, using peanut protein, peanut protein with methionine and casein. Methionine was added to one group of peanut protein diets to increase the amount of this amino acid to the same level as is found in a casein diet of equivalent protein quantity. Cholesterol was fed to three diet, groups: 20% peanut protein, peanut protein with methionine, and casein. Rats were also fed a laboratory chow diet to evaluate the effect of time on their tissue cholesterol levels. The results reveal no significant effect of protein quality on plasma cholesterol and only at 15% and 20% protein levels does liver cholesterol increase with improvements in protein quality. Time and the amount of protein in the diet, whether casein or peanut protein, had no effect on plasma and liver cholesterol levels although a trend revealed by Pearson correlation analysis showed a negative correlation between plasma cholesterol and peanut protein, and a positive correlation between plasma and liver cholesterol and casein. The effect of protein on tissue cholesterol appeared to be a minor one in comparison with the effect of dietary cholesterol, although accumulation of cholesterol in the livers of rats fed diets containing cholesterol could be decreased by dietary methionine.

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