Alison Edelman, M.D, a physician and researcher in the Department of Obstetrics and Gynecology at OHSU and lead author of the study, agrees that there is much debate on this subject and that the research thus far has been insufficient to demonstrate whether or not oral contraceptives cause weight gain or loss
This is not a trivial question to debate as one of the main reasons why women may avoid or discontinue birth control – which in turn places them at greater risk for an unplanned pregnancy – is concern about weight gain. Her study was not based on humans, but on our near neighbours the rhesus macaque monkeys at the OHSU Oregon National Primate Research Center. Their reproductive system is almost identical to that of humans and they were studied for a year.
At the beginning of the study, like their human counterparts it seems, half the animals were obese and half were normal weight. During the treatment period, animals received doses of oral contraceptives, adjusted to the weight of the animals so that it mimicked dosage in humans. Researchers tracked weight, food intake, activity levels, body fat and lean muscle mass. At the study’s conclusion, the normal weight group remained weight stable whereas the obese group lost a significant amount of weight (8.5%) and percent of body fat (12%) due to an increase in basal metabolic rate. No changes were seen in food intake, activity or lean muscle mass for either group.
So the conclusion seems to be that weight gain on the Pill is one of those urban myths that has just kept being repeated and does not bear up to scientific scrutiny. In fact, if you are overweight and keep your diet stable Edelman believes you may see a weight loss with pill use. So one reason for not using oral contraceptives is busted, but the research did not examine potential side effects of synthetic hormone use such as increased health risks. For which the Macaque monkeys are presumably very grateful.