Background A number of epidemiologic studies have shown an association between low levels of physical activity and elevated weight gain, but most reports of physical activity have been made by self-report. Recently, we directly measured physical activity using three-way accelerometers in adult female rhesus monkeys and showed that sedentary monkeys gained significantly more weight than active monkeys (Sullivan et al, Am J Physiol Regul Integ Comp Physiol 2006;291:R633-42). An eightfold variation in daily physical activity was found between the least and most active monkeys, and the physical activity level was very stable over time.
Purpose Over adolescent development, human studies have shown a dramatic decrease in average physical activity level. In this study, we sought to determine if individual differences in physical activity already exist prepubertally and whether an individual's prepubertal level of physical activity predicts their postpubertal level of physical activity.
Methods Forty-three rhesus monkeys were studied over a 1-year period between 2005 and 2006. Monkeys were living in seminatural conditions in large outdoor troops at the Oregon National Primate Research Center. Each year the monkeys were brought indoors for a 10-day period in the summer and each monkey had physical activity monitored one summer by a three-way accelerometer (set to record activity counts each minute) that was attached to a loose-fitting metal collar. During the 10-day period of study, monkeys were housed individually in single cages.
Results There was a 10.5-fold difference in physical activity level among monkeys. Over the 1-year study period there was a 2.6-fold decrease in physical activity. There was a significant correlation (r = .519, < .001) in activity levels measured when monkeys were 2 years old (ie, prepubertal) and 3 years old (ie, postpubertal).
Conclusion We conclude that like humans, monkeys experience a decrease in physical activity as they transition through puberty; however, prepubertal physical activity level is a strong predictor of postpubertal physical activity level. These findings suggest that sedentary adolescents will remain sedentary in adulthood and that these individuals will be at risk of obesity.
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