DECREASED RATIO OF FAT TO CARBOHYDRATE OXIDATION INCREASING AGE IN PIMA-INDIANS

Background: Some metabolic changes related to age may increase the pre valence of obesity. Previous studies have shown that a low relative me tabolic rate and a low ratio of fat to carbohydrate utilization are pr edictors of body weight gain. However, a possible relationship between age and energy substrate utilization (respiratory quotient; RQ = VCO2 /VO2 has not been reported. Objective: To determine whether RQ increas es and therefore fat oxidation decreases with age in Pima Indian men, independent of body fat and energy balance. Methods: We analyzed longi tudinal data collected in seven non-diabetic Pima Indian men (31 +/- 6 years, 167 +/- 8 cm, 111.0 +/- 23.7 kg and 41 +/- 9% fat at baseline) who had repeated measurements of 24-hour RQ 7 years apart. On both ad missions, subjects were fed a weight maintenance diet (50% carbohydrat e, 30% fat and 20% protein) for 3 days before spending 1 day within a respiratory chamber for measurements of 24-hour energy expenditure, ba sal metabolic rate, sleeping metabolic rate and 24-hour RQ. Paired t-t est was used to determine differences between the first and last measu rement of 24-hour RQ. Cross-sectional data in 131 Pima Indian men (28 +/- 9 years, 171 +/- 6 cm, 94.5 +/- 24.4 kg, and 32 +/- 9% fat) were a lso analyzed to determine the relationship between 24-hour RQ and age. Multiple regression analysis was used to adjust 24-hour RQ for differ ences in energy balance (intake/expenditure in %) and percent body fat and metabolic rate for differences in body size and composition. Resu lts: Over a 7-year period, mean unadjusted and adjusted 24-hour RQ inc reased (p < 0.01). Cross-sectional data analysis showed that both the unadjusted (r = 0.19, p < 0.03) and adjusted (r = 0.19, p < 0.03) 24-h our RQ correlated with increasing age while adjusted BMR (r = -0.21, p < 0.02) correlated inversely with age. Conclusions: Despite a higher body fat content, older individuals utilize less fat than their younge r counterparts. Reduced fat utilization and decreased BMR with age may both contribute to increasing obesity in older individuals.