The prevalence of obesity in the Pima Indian population is extremely high. Since obesity represents a major risk factor for insulin resistance and the development of NIDDM we have focused our effort in seaching for the possible causes of obesity in this population including metabolic and genetic factors. We have identified four known familial metabolic parameters predicting body weight gain: a low metabolic rate, a high respiratory quotient, insulin sensitivity, and a low spontaneous physical activity. Also Pima Indians have a low level of sympathetic nervous activity. Recent data suggest that Pima Indians as a population have a low level of physical activity even at age five. We know that regarding the causes of the variability of these metabolic parameters: 1) age is not a major determinant of sedentary energy expenditure, but is associated with lowering of physical activity in free-living conditions; 2) metabolic rate is regulated to maintain a genetically determined body temperature but Pima Indians have lower sleeping body temperatures than Caucasians; and 3) Pima Indians have a lower sympathetic nervous system activity than Caucasians, and a blunted increase in response to an oral glucose load despite larger increases in plasma insulin levels. This blunted response may represent another feature of the obesity/insulin resistance syndrome. Since a low fat oxidation is a risk factor for body weight gain, we are studying the mechanisms regulating fat oxidation. Skeletal muscle lipoprotein lipase activity and muscle fiber types account for part of the inter-individual variability in energy metabolism. Surprisingly we could not find any relationship between sympathetic nervous activity and substrate oxidation has been found. Preliminary data in a Pima Indian population in nothern Mexico indicate that despite a potential genetic predisposition, a traditional lifstyle may protect against the development of obesity and NIDDM.
