Metabolic and hormonal factors as predictors of nitrogen retention in obese men consuming very low calorie diets.

The ability to conserve body protein during very low calorie diets in ten obese men was observed to correlate with plasma free amino acid concentrations, urinary N tau-methylhistidine: creatinine ratios, resting oxygen consumption, and serum triiodothyronine levels. A diet consisting of only protein, 1.3 g/kg ideal body weight/24 h, was given for 40 days. Cumulative nitrogen deficit ranged from -64 g to -227 g. Nitrogen balance on days 36 to 40 ranged from + 1.37 g/24 h to -3.30 g/24h. Nitrogen balance during this period had a significant direct correlation with pre-diet concentrations of branched-chain amino acids (r = 0.69 to 0.89), methionine (r = 0.85), histidine (r = 0.66), alanine (r = 0.73), arginine (r = 0.70), ornithine (r = 0.66), total essential (r = 0.87, and nonessential (r = 0.68) amino acids, with initial serum levels of triiodothyronine (r = 0.66) and with the fall in triiodothyronine over the 40 days (r = 0.79). Initial resting oxygen consumption was directly correlated (r = 0.78) with final nitrogen balance and inversely with total nitrogen loss (r = -0.81). On day 0, triiodothyronine levels also correlated positively (r ranging from 0.71 to 0.93) with plasma concentrations of several essential and nonessential amino acids. These correlations suggest that individuals who ultimately will or will not achieve nitrogen equilibrium during very low calorie diets can be identified prior to dieting. These data are consistent with the hypothesis that individuals with higher initial protein flux and triiodothyronine levels are better able to adjust the balance between synthesis and degradation to attain nitrogen equilibrium during hypocaloric dieting.