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Contraceptive pill 'does not cause weight gain'

January 25, 2006 By Michael Hopkin This article courtesy of Nature News.

Fattening fears are an 'urban myth', say medics.

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The contraceptive pill does not cause women to put on weight, say researchers who have surveyed data from more than 40 studies.

"The word on the street is that if you take the pill, you're gonna get fat," says reproductive health expert David Grimes of Family Health International in Research Triangle Park, North Carolina. "This attitude is rampant around the world."

But these fears are unfounded, Grimes and his colleagues argue. They compiled the results of 44 studies carried out over the past few years that examined the effects of contraceptive pills and patches. As they report in The Cochrane Database of Systematic Reviews1, they found no evidence that beginning to use the pill leads to any jump in weight gain.

There are many brands of the pill on the market, but all contain a synthetic version of the hormone progesterone, usually together with synthetic oestrogen, which stop the body's natural production of these hormones and prevent ovulation.

The most logical explanation is that all of us, men and women, gain weight with age.
David Grimes
Family Health International, Research Triangle Park, North Carolina
Some doctors believe that these hormonal changes can also stimulate increased appetite, causing women to put on weight. And many women believe that they piled on the pounds after starting the pill. But Grimes and his colleagues' data suggest that this supposed 'cause and effect' is merely anecdotal, and that patterns of weight gain among new pill users are no different to those seen in the population at large.

"The most logical explanation is that all of us, men and women, gain weight with age," says Grimes. The average American, for example, gains about one pound (0.45 kilograms) every year, he says. But most people seek out something to blame for this other than their personal behaviour, he adds.

Appetizing results

Of the 44 studies collated by Grimes' group, only three compared the effects of the pill with those of a placebo, the ultimate test of whether the contraceptives cause any discernible change. Such studies are difficult to carry out, in part because of the ethical implications involved in prescribing a placebo to women who do not want to conceive.

These studies showed no difference in weight gain between those on the pill and those not. Nor was there an effect on appetite.

To add to the weight of their survey, the researchers looked at studies that compared different brands of the pill. These also showed no evidence of any specific combination of hormones causing weight gain, Grimes says. "If there were one pill that 'made you fat', you would see it stand out in the results; but you don't," Grimes says.

From preconceptions to conception

The authors concede that the debate is likely to continue, because the perception of weight gain is quite firmly entrenched in the minds of both pill users and doctors. A trawl of online health services turns up sites such as www.netdoctor.co.uk, for example, which states that new users can expect to put on about three or four pounds through increased appetite or water retention. The site does add that most weight gain is nevertheless associated with lifestyle, exercise and diet.

Grimes argues that it is important to get this new information out, because preconceptions about contraceptives may be deterring women from using birth control.

Doctors should be encouraging women to avoid weight gain in general, he says, rather than attaching it to advice about contraception. "One-third of Americans are obese," he points out. "We need to counsel them as a matter of course."

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References

  1. Gallo M. F., Lopez L. M., Grimes D. A., Schulz K. F. & Helmerhorst F. M.Cochrane Database Systematic Rev., doi: 10.1002/14651858.CD003987.pub2. (2006).

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