Decreased ratio of fat to carbohydrate oxidation with increasing age in Pima Indians.

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Abstract
Some metabolic changes related to age may increase the prevalence of obesity. Previous studies have shown that a low relative metabolic rate and a low ratio of fat to carbohydrate utilization are predictors of body weight gain. However, a possible relationship between age and energy substrate utilization (respiratory quotient; RQ = VCO2/VO2) has not been reported. To determine whether RQ increases and therefore fat oxidation decreases with age in Pima Indian men, independent of body fat and energy balance. We analyzed longitudinal 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 admissions, subjects were fed a weight maintenance diet (50% carbohydrate, 30% fat and 20% protein) for 3 days before spending 1 day within a respiratory chamber for measurements of 24-hour energy expenditure, basal metabolic rate, sleeping metabolic rate and 24-hour RQ. Paired t-test was used to determine differences between the first and last measurement 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 also analyzed to determine the relationship between 24-hour RQ and age. Multiple regression analysis was used to adjust 24-hour RQ for differences in energy balance (intake/expenditure in %) and percent body fat and metabolic rate for differences in body size and composition. Over a 7-year period, mean unadjusted and adjusted 24-hour RQ increased (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-hour RQ correlated with increasing age while adjusted BMR (r = −0.21, p < 0.02) correlated inversely with age. Despite a higher body fat content, older individuals utilize less fat than their younger counterparts. Reduced fat utilization and decreased BMR with age may both contribute to increasing obesity in older individuals.