Thursday June 9 2011
Any effect of the pill on weight is likely to be small
“The popular belief that taking the Pill makes you pile on the pounds has been exposed as a myth by an expert in women’s health,” reported the Daily Express.
The news story is based on a study that followed over 1,400 women of fertile age for 15 or 25 years to find out if the combined contraceptive pill had any influence on body weight. Researchers found no association between changes in weight and pill use, and that getting older was the only factor associated with weight gain.
This study has some strengths but also several limitations. The major problem is that the researchers did not measure the women’s weight but relied on the women themselves to give accurate measurements through a postal questionnaire every five years. As such, the results may be subject to error or bias. Also, though the study looked at other factors that may influence weight, such as exercise and having children, it did not look at the women’s diets, a major influence. These limitations, and the fact that 50% of the women dropped out of the study, mean the findings should be viewed with caution. Even so, the findings, which are also supported by a recent review of existing evidence, suggest that any changes in weight due to taking the pill are likely to be small.
Where did the story come from?
The study was carried out by researchers from Gothenburg University, Sweden, and was funded by the Goteborg Medical Society, Hjalmar Svenssons Fund and a national government grant. The study was published in the peer-reviewed medical journal Human Reproduction.
The study was generally reported accurately by the Daily Express, although the paper made a mistake in reporting that 1,749 women took part in the survey, when in fact 1,436 women were invited to take part.
What kind of research was this?
This prospective cohort study aimed to determine whether using the contraceptive pill influenced long-term weight change in women. The researchers also investigated how much weight women put on during their fertile years. Although this type of study is often carried out to look at possible associations between events (in this case, contraceptive use) and outcomes (in this case, weight), it cannot prove that one thing causes the other.
The researchers point out that the fear of putting on weight may put young women off taking the pill, with one survey finding that 73% of women of all ages believed that weight gain was a disadvantage of taking the pill. They warn that improper use of the pill may increase the risk of unintended pregnancies. They argue that although there has been little research into weight gain and the pill, the little evidence there is suggests there is no link.
What did the research involve?
In 1981, a random sample of 656 women living in the city of Gothenburg, Sweden, was invited to participate in the study. Of these, 594 (91%) responded. A second group of 780 women was also invited to take part in 1991, of whom 641 (82%) responded.
The women were 19 years old when they enrolled and are referred to by the year of their birth (the first group is called the ’62 cohort and the second the ’72 cohort). The women all received a questionnaire with 40 questions about contraceptive use, height, weight, smoking, reproductive health and exercise. All the women who returned the questionnaire were sent the same questionnaire every fifth year until 2006. The questionnaire was completed and returned on all six of these occasions by 286 women in the ’62 cohort (44%) and on all four occasions by 375 (48%) of the ’72 cohort. At the final update, the ’62 cohort were 44 years of age and the ’72 cohort were 34.
The researchers grouped the two different cohorts together and used validated statistical methods to analyse the possible association between weight change and use of the contraceptive pill. They looked at other risk factors separately, testing their analysis for the association between weight change and other factors such as having children, smoking and exercise.
What were the basic results?
The researchers found that women in the ’62 cohort gained an average of 10.6kg (95% confidence interval 9.4 to 11.8) between the ages of 19 and 44. Meanwhile, women in the ’72 cohort gained on average 7.7kg between 19 and 34 with corresponding increases in Body Mass Index (BMI). Generally, women from the ’72-cohort were heavier at each five-year questionnaire than women of the same age from the ’62-cohort.
The researchers found that:
- There was no significant difference in weight increase between women who used the contraceptive pill and those who did not.
- There was no significant association between weight increase and duration of pill use.
- There was no correlation between weight change and the number of children the women had or the amount of exercise they did.
- The only factor associated with weight gain was age, with an average gain of 0.45kg annually.
- The only factor associated with weight reduction was smoking, with smokers decreasing their weight by 1.64kg over the whole 15 years.
How did the researchers interpret the results?
The researchers say their study calls into question the idea that the contraceptive pill influences long-term weight gain, but that further studies are needed. They say it is important when giving contraceptive counselling to inform women that weight gain is a natural development over the course of their life.
This study had some positive features in that it was a prospective study that following women over time and used appropriate analysis. It also had some limitations, which affect how the results can be interpreted:
- The researchers relied on information about weight and other factors from postal questionnaires filled in by the women. Though they did evaluate the questionnaire’s validity and reliability in a sub-sample of 30 women, their reliance on self-reporting (for weight in particular) may affect the reliability of the findings as the women may have given inaccurate information.
- While they asked about other factors that could influence weight change (such as number of children, smoking and exercise), they only considered a limited number of these in the full analysis and did not adjust their main findings for any of these factors. The research questionnaire did not look at the women’s diets, a major influence on weight.
- This was a long study. Partly because of this, it had a high drop-out rate, with less than 50% of women in either cohort completing it. A high drop-out rate in any study inevitably affects the reliability of the findings.
Finally, many young women worry that using the pill will cause them to gain weight in the short term, but this study only reported on long-term weight changes.
These limitations, and a dropout rate of 50%, mean the findings should be viewed with caution. However, the findings from this study, as well as those of a 2008 Cochrane systematic review, suggest that any changes in weight due to taking the pill are likely to be small.