Women using depot medroxyprogesterone acetate (DMPA), commonly known as the birth control shot, gained an average of 11 pounds and increased their body fat by 3.4 percent over three years, according to researchers at the University of Texas Medical Branch (UTMB).
However, women who switched to nonhormonal contraception began to slowly lose the weight and fat mass they gained – nearly four pounds over two years, while those who used oral contraception after the shots gained an average of four additional pounds in the same time span. The amount of weight gain was dependent on the length of time DMPA was used, as the rate of weight gain slowed over time.
The study, which appears in the March 4 issue of the American Journal of Obstetrics and Gynecology , is one of the most comprehensive studies of its kind.
DMPA is an injected contraceptive administered to patients every three months. More than two million American women use DMPA, including approximately 400,000 teens. DMPA is relatively inexpensive compared to some other forms of birth control, has a low failure rate and doesn't need to be administered daily, which contributes to the contraceptive's popularity.
"Women and their doctors should factor in this new data when choosing the most appropriate birth control method," said lead author Abbey Berenson, M.D., professor in the Department of Obstetrics and Gynecology and director of the Center for Interdisciplinary Research in Women's Health at UTMB.
"One concern is DMPA's link to increased abdominal fat, a known component of metabolic syndrome, which increases the risk of cardiovascular disease, stroke and diabetes," said Berenson.
The study followed 703 women in two age categories, 16- to 24-years-old, and 25- to 33-years-old, using DMPA, oral (desogestrel) or nonhormonal (bilateral tubal ligation, condom or abstinence) contraception for three years. DMPA users who discontinued this method and selected another form of birth control were followed for up to two additional years. Throughout the course of the study, researchers compared changes in body weight and composition and took into account the influence of age, race, caloric intake and exercise, among other factors.
When researchers compared all three groups, DMPA users were more than twice as likely as women using nonhormonal or oral birth control to become obese over the next three years. "The findings are worrisome; however, more research is needed to determine if DMPA use directly contributes to obesity-related conditions and puts patients' overall health at risk," said Berenson.
Women using oral contraception did not gain more weight than those using a nonhormonal form of birth control. However, the study found that their body fat increased slightly while their lean body mass (muscle) decreased. Researchers said this was less likely among those women who exercised regularly and consumed a healthy diet that included increased protein intake.
The study will enable physicians to counsel women accurately about the body changes associated with widely used forms of contraception and also shed light on how weight gain might be reversed, said Berenson.
According to Berenson, the mechanism by which DMPA causes an increase in weight gain and fat mass is not known, and no connection was found between DMPA use and caloric intake, fat consumption or amount of exercise on body mass changes. The findings seem to argue against the theory that weight gain could be due to the drug's perceived effects on increased caloric intake and decreased energy expenditure, but ongoing research is needed to confirm or discount varying possible explanations, she said.
UTMB researchers are conducting follow-up studies to determine which subset of women is most likely to gain weight on DMPA. Berenson noted that in ongoing research, preliminary data has shown that approximately 25 percent of women on DMPA experience significant and potentially dangerous body composition changes.