Whole body protein turnover, studied with 15N-glycine, and muscle protein breakdown in mildly obese subjects during a protein-sparing diet and a brief total fast

Abstract

A modification of the Picou and Taylor-Roberts Model was used to estimate rates of total body protein synthesis (S), breakdown (C), and amino nitrogen (N) flux (Q) in the metabolic N pool of five obese females. The subjects were fed egg white albumin at 1.5/kg ideal body weight (IBW) and total calories at 1.2 times the basal energy expenditure (fat:carbohydrate = 30%:50%) as a formula diet (period 1, 1 wk). This was followed by 3 wk during which the nonprotein calories were omitted (period 2, protein-sparing modified fast [PSMF]) and a 1-wk total fast (period 3). Estimates of body protein turnover and skeletal protein breakdown were made during the last 60 and 48 hr, respectively, of each period. Q, S, and C were 223 ± 22, 154 ± 22, and 150 ± 22 g protein 24 hr , respectively, for period 1. These values were unchanged at the end of period 2. Total fasting decreased Q and S by 36% and 27%, respectively ( p < 0.001), but C remained unchanged. Skeletal protein breakdown, as estimated by urinary Nτ-methylhistidine excretion, was 108 ± 47 μmole in period 1, 79 ± 51 μmole in period 2 ( p < 0.01), and 100 ± 49 μmole in period 3, representing 16 ± 5%, 12 ± 5% ( p < 0.01), and 16 ± 4% of whole body breakdown. N balance was unchanged in period 1 (−0.4 ± 1.2 g N) and the final week of period 2 (−0.4 ± 1.5 g N), but was −5.8 ± 0.6 g N in period 3. These data indicate that weight reduction with a PSMF is associated with a maintenance of total body protein turnover parameters and N balance but a reduction in skeletal protein breakdown, whereas a total fast causes a marked reduction in whole body protein synthesis and amino N flux with little change in the rate of total body and skeletal protein breakdown, resulting in a negative N balance. The minimization of N losses that develops after prolonged starvation is achieved at rates of whole body and skeletal protein breakdown similar to those found when the diet is adequate, suggesting that endogenous fat-derived fuels are as effective as exogenous energy in limiting protein catabolism. However, protein intake is necessary to maintain whole body protein synthesis under these conditions.