Metabolizable energy intake during long-term calorie restriction in rhesus monkeys

Abstract

Calorie restriction (CR) is a dietary intervention shown to increase maximum life-span. The aim of this study was to compare the metabolizable energy of the pelleted semi-purified diet with estimated energy intake from food weight. Energy density of diet, urine and feces were measured by bomb calorimetry in rhesus monkeys (23-29 years old) on CR (CR, n=11) and control (C, n=9). Food moisture was measured to be 2-fold higher (9+/-1%) than indicated on the label (approximately 5%). The measured gross energy of diet was 4.4 kcal/g dry weight of CR and 4.5 kcal/g dry weight of C diets. In a two-day trial, food intake (mean+/-SD) was 112+/-20 g and 136+/-26 g of dry mass/d in the CR and C monkeys, respectively (p=0.003). The fraction of the diet absorbed (CR=0.91; C=0.95) was different (p<0.001) between CR and C monkeys. Using these coefficients, the metabolizable energy intake averaged over 6 months was 450+/-53 and 534+/-97 kcal/d in CR and C monkeys, respectively (Diff=16%; p=0.03). These values were compared with energy expenditure (EE), as measured annually by indirect calorimetry (490+/-61 kcal/d in CR and 532+/-62 kcal/d in C monkeys). Adjusted for changes in body composition (2+/-10 kcal/d in CR and -7+/-12 kcal/d in C), energy balance was not different from zero in CR (-42+/-42 kcal/d) and C (9+/-61 kcal/d) monkeys. Use of diet weight is a reasonable estimate of the level of CR when food waste is assessed.

Figure 1. Comparison of excretory loss and energy intake in CR and C monkeys

Analysis of covariance (ANCOVA) showed that the CR monkeys had a greater excretory energy loss per unit gross energy intake (F_1,17 =16; p<0.001) compared to C monkeys. Treatment was used as a factor with excretory losses as the dependent variable and gross energy intake as the covariate in the model. There was no significant difference in the slopes (F_1,16=1; p=0.3),.

Figure 2. Energy Balance

Each bar represents the mean ± S.D. of the energy equivalents. The food intake was calculated using 6-month average values of food intake and the coefficients of metabolizable energy for CR and C monkeys. Total energy expenditure (TEE) was measured using indirect calorimetry once during the 6-month period. Body composition (BC) changes were calculated as the difference in the fat and lean mass over 2 consecutive DXA measurements made 6 months apart. Energy balance calculated as- Energy balance= Metabolizable energy (ME intake) – [energy expenditure (TEE) + change in body composition (BC changes)].