PROTEIN-METABOLISM IN OBESE SUBJECTS DURING A VERY-LOW-ENERGY DIET

We postulated that the return to nitrogen equilibrium after 3 wk of a negative balance during a very-low-calorie diet (VLCD) providing low-quality protein in obese subjects was due to availability of endogenously originating amino acids from a "pool" that, when depleted, would result in worsening balance. This should be reflected in altered kinetics of protein metabolism with the requirement for increased breakdown to maintain synthesis constant. Seven female obese subjects [body mass index (BMI) = 34.4 +/- 1.8 kg/m2] were given a 1.7-MJ/d all-protein diet (16.8 g N) derived from hydrolyzed gelatin (supplemented with tryptophan and methionine) that provides 18% of its amino acids as essential, a multivitamin-mineral supplement, and 16 mmol KCl for 42 d. At baseline (7-d isocaloric diet), and weeks 4 and 6 of VLCD, amino nitrogen flux rate was calculated from the N-15 abundance in urinary urea using the oral N-15-glycine method and rates of synthesis (S) and breakdown (B) inferred from N flux. Whole-body N flux did not change from baseline to weeks 4 and 6 (39.5 +/- 2.0 vs 37.4 +/- 2.0 vs 39.2 +/- 1.9 g N/d). By contrast, S and B decreased at weeks 4 and 6 with S decreasing more so that net protein synthesis (S-B) was less positive at week 4 than at baseline (2.2 +/- 0.2 and 0.9 +/- 0.3 g N/d; P < 0.05) and became negative at week 6 (-0.9 +/- 0.2 g N/d; P < 0.05). Concurrently, N equilibrium was achieved by week 4 but returned to negative balance by week 6. A decrease in net protein synthesis may reflect insufficient exogenous amino acid when such diets are used alone for a prolonged period of time and explain the return to negative N balance with net depletion of available endogenous sources.