Male, weanling Sprague-Dawley rats were fed isocaloric diets containing graded levels of protein or amino acid mixtures. Food intakes and weight gains were recorded daily or every other day. Both short-term and long-term (64 days) experiments were carried out. Linear regressions of food intake versus time and weight gain versus time were used to establish daily weight gains and food intakes. The four-parameter mathematical model for physiological responses was used to predict daily food intake, daily weight gain, daily food intake per 100 g weight and efficiency of food conversion (daily weight gain/daily food intake) as functions of dietary protein concentration. The changes in the four parameters generated from the four-parameter model were examined as a function of time. Several aspects of food intake, weight gain and efficiency were shown to be functions of concentration of dietary protein.
The previous finding (Am J Clin Nutr 1981;34:2376-81) that the serum level of high-density lipoprotein (HDL) cholesterol is positively correlated with the serum concentration of zinc (r = +0.81; p less than 0.01) led us to evaluate further the zinc-HDL relationship as affected by dietary cholesterol. The two diets, one control diet containing no cholesterol and the other containing 1% cholesterol, were isocalorically formulated with an equal but adequate level of zinc. Cholesterol feeding produced a significant decrease in the serum level of HDL cholesterol at 8th wk of dietary treatment and a significant increase in very low density lipoprotein cholesterol at 4th and 8th wk. At the same intervals of dietary treatment, significant decreases in serum zinc levels were observed in cholesterol-fed rats; no changes were noted in the serum levels of other related elements such as copper, calcium, and magnesium. Linear regression analysis of the 44 pairs of serum HDL and zinc values revealed a significant positive correlation (r = +0.57; p less than 0.01) between the two parameters. The rather selective lowering of serum zinc due to cholesterol feeding and the observation of the positive serum zinc-HDL relationship observed in the present and previous studies warrant further investigation into the role of zinc in cholesterol and high-density lipoprotein metabolism.
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