The commonly held belief that oral contraceptives cause weight gain appears to be false, say health researchers at Oregon Health & Science University. Their findings have been published in the journal Human Reproduction.
"A simple Google search will reveal that contraceptives and the possibility that they may cause weight gain is a very highly debated topic," said Alison Edelman, lead author of the study. "This is an extremely important question as concern about weight gain is 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."
The study itself was conducted with a group of rhesus macaque monkeys over a one year period. Rhesus monkeys were used in this study because their reproductive system is nearly identical to humans. However, unlike human studies, more variables can be controlled and measured - such as exact food intake - to provide more accurate data.
At the beginning of the study, half the animals were obese and half were normal weight. During the treatment period, the animals received doses of oral contraceptives, adjusted to the weight of the animals so that it mimicked dosage in humans.
The 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 percent) and percent of body fat (12 percent) due to an increase in basal metabolic rate. No changes were seen in food intake, activity or lean muscle mass for either group.
"This study suggests that worries about weight gain with pill use appear to be based more on fiction than on fact," said co-researcher Judy Cameron. She speculated that the most likely reason why this belief continues to exist is that the weight gain that seems to occur with age is being attributed to these medications. "We realize that research in nonhuman primates cannot entirely dismiss the connection between contraceptives and weight gain in humans, but it strongly suggests that women should not be as worried as they previously were."
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Source: Oregon Health & Science University