Protein-sparing effect in obese women fed low calorie diets

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Abstract
Six obese women were housed in a metabolic facility for 2 months. They were fed liquid diets which varied in nitrogen and energy content. During the first 15 days the women were fed a stabilization diet that contained 12 g N/day and during that period, energy intakes for each subject were adjusted to permit maintenance of an essentially constant weight (100% calories). During the next four metabolic periods of 12 days each, the liquid diets contained: 100% kcal, no N; 50% kcal, 12 g N; 25% kcal, no N; 25% kcal, 3 g N). Endogenous nitrogen excretion was determined as total nitrogen loss per day at the end of the 2nd metabolic period, when no protein and 100% kcal were fed. A mean endogenous nitrogen loss of 3.01 ± 0.25 g N/day was found. When the subjects were fed a protein-free diet containing only 25% of the calories required for weight maintenance, the mean endogenous loss was not significantly different, i.e., 2.86 ± 0.27 g N/day. Nitrogen balance was achieved when 12 g N and 50% of the calories required for weight maintenance were fed, but mean negative nitrogen balance occurred when 3.5 g N and 25% of the necessary calories were fed. For the experiment as a whole, urinary excretion of creatinine and uric acid fell progressively during the 63 days. When the diet contained no protein, however, urinary acid decreased and serum uric acid increased. The obese women minimized body protein breakdown on diets that provided as little as 25% of the energy needed for weight maintenance.