Combined oral contraceptives and body weight: do oral contraceptives cause weight gain? A primate model.

BACKGROUND The aim of this study was to determine if oral contraceptive (OC) use affects body weight, body composition and metabolism in primates. METHODS Reproductive-age female rhesus monkeys of normal and obese BMI were studied to document baseline weight stability, then treated continuously with an OC (dosed to achieve equivalent human serum levels for a 30 µg ethinyl estradiol/150 µg levonorgestrel preparation) for 237 days. Monkeys were monitored for changes in body weight, levels of physical activity (measured by a triaxial Actical accelerometer), food/caloric intake, percent body fat (dual energy X-ray absorptiometry, DEXA) and metabolism (24 h metabolic rate and serum metabolic substrate and hormone concentrations). RESULTS All 10 monkeys completed the study protocol with no adverse events. While body weight (-0.73% change) and percent body fat (-1.78% change) of the normal BMI group did not significantly decrease from baseline, obese monkeys showed a significant decrease in body weight (-8.58% change, P < 0.01) and percent body fat (-12.13% change P = 0.02) with OC treatment. In both the obese (P = 0.03) and the normal BMI (P = 0.01) groups, there was a significant increase in basal metabolic rate with OC use. No changes were seen in food intake, activity level or % lean muscle mass with OC use for either BMI-based group. CONCLUSIONS Overall, OC use appears to cause a slight increase in basal metabolic rate in female monkeys, leading to a decrease in body weight and percent body fat in obese individuals.