Quantitative effect of an isoenergetic exchange of fat for carbohydrate on dietary protein utilization in healthy young men

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Abstract
The quantitative effect on protein utilization of an isoenergetic exchange of dietary fat for carbohydrate was studied in 10 healthy young men. Milk protein (0.57 g/kg body weight per day, a safe level of protein intake as recommended by FAO/WHO) was given for two 21-day experimental periods with two ratios of carbohydrate to fat calories: diet A, a ratio supplying an equal proportion of energy from carbohydrate and from fat, generally representative of the usual Western diet, and diet B, a ratio supplying twice as much energy from carbohydrate as from fat, chosen in view of recommendations to reduce the amount of fat in the diet. Total calorie intake for each subject was constant over the two experimental periods. During each period, five subjects showed a consistent gain and five a loss of body weight. Urinary creatinine excretion was essentially unchanged. Nitrogen (N) balance and dietary protein utilization were significantly (P < 0.05) improved on diet B. The protein-sparing effect was greatest in those subjects who were on marginal energy and protein intakes and who were losing weight. The additional carbohydrate in diet B resulted in significant reductions in fasting serum urea N (P < 0.001), total urinary N (P < 0.01), and urinary urea N (P < 0.001); fasting plasma insulin values were unchanged. Whether or not the longer-term effects of the change in dietary carbohydrate:fat ratio on N metabolism are mediated solely by the action of insulin remains to be determined.