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

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 (∼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.