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 < 0.01) to 192 ± 30 (P < 0.005), 138 ± 27 (P < 0.01), and 146 ± 25 (P < 0.01) g protein/day [125 ± 13 (P < 0.001), 90 ± 14 (P < 0.005), and 95 ± 11 (P < 0.005) g protein/g creatinine] by the final 60 h of the third week. Nitrogen balance remained significantly negative (P < 0.01) during each week of the mixed diet and overall (–3.3 ± 0.9 g N/day; P < 0.01). Thus, total body protein synthesis and the net balance between synthesis and catabolism are not maintained with this diet.