The mechanisms of nitrogen sparing in fasting supplemented by protein and carbohydrate.

Estimated rates of total body protein synthesis breakdown, and amino nitrogen flux were determined in five obese females using [15N]glycine. The subjects were fed a baseline diet of 1.5 g protein/kg ideal BW and 1602 +/- 146 Cal total for at least 3 days, followed by 3 weeks where the sole caloric intake was a mixed diet of 0.8 g meat protein and 0.7 g carbohydrate/kg ideal BW and approximately 437 +/- 44 Cal. Amino nitrogen flux, total body protein synthesis, and breakdown values were 289 +/- 56, 198 +/- 37, and 202 +/- 47 g protein/day (187 +/- 24, 129 +/- 19, and 131 +/- 21 g protein/g creatinine) initially and fell significantly (P less than 0.01) to 192 +/- 30 (P less than 0.005), 138 +/- 27 (P less than 0.01), and 146 +/- 25 (P less than 0.01) g protein/day [125 +/- 13 (P less than 0.001), 90 +/- 14 (P less than 0.005), and 95 +/- 11 (P less than 0.005) g protein/g creatinine] by the final 60 h of the third week. Nitrogen balance remained significantly negative (P less than 0.01) during each week of the mixed diet and overall (--3.3 +/- 0.9 g N/day; P less than 0.01). Thus, total body protein synthesis and the net balance between synthesis and catabolism are not maintained with this diet.