Friday March 26 2010
Researchers say women need an hour of exercise a day
“An hour of daily exercise is ‘needed to stay slim’,” reported the BBC. It said that the recommended 30 minutes of physical activity a day may not be enough to stop weight gain.
This news story is based on research that followed 34,000 American women over 13 years to see whether there was an association between the weight gain normally associated with age and how much exercise the women did. The researchers estimate that women need to do at least an hour of exercise a day to prevent putting on weight.
This was a large study conducted over a long period of time. However, it has several limitations, including the fact that the women provided their weight and exercise levels by questionnaire, raising the possibility that bias was introduced.
This study suggested that an hour a day of moderate exercise was needed to prevent weight gain. However, the study did not also monitor the women’s diet over time, and this was likely to vary between slimmer and overweight women.
Further research is needed to assess how both diet and exercise affect weight gain over a long period, to develop guidelines for helping people to avoid putting on weight as they age.
Where did the story come from?
This research was carried out by Dr I-Min Lee and colleagues from the Brigham and Women’s Hospital, Harvard Medical school. The study was funded by the National Institutes of Health. The paper was published in the peer-reviewed The Journal of the American Medical Association.
What kind of research was this?
The aim of this prospective cohort study was to investigate the relationship between different amounts of physical activity and long-term weight changes in American women who were eating a normal diet.
The researchers say that clearer guidelines are needed so that people know how much physical activity they need to do to keep slim.
What did the research involve?
This study used data from 39,876 women who had participated in the Women’s Health Study and who agreed to continue in an observational follow-up study afterwards. The Women’s Health Study was a randomised trial that ran from 1992 to 2004, comparing low-dose aspirin or vitamin E against placebo for preventing cardiovascular disease and cancer. The original study excluded women who had cardiovascular diseases, cancer or other major chronic illnesses at the start.
During the 13-year follow-up study, women completed two questionnaires in the first year and then a questionnaire annually thereafter asking them about their physical activity and their weight.
The researchers were interested in the women’s weight gain over 13 years and the amount of physical activity they were engaged in over the same time.
For their analysis of weight gain, the researchers excluded women who developed cardiovascular disease or cancer within the 13 years of the study, because these diseases can influence weight. They also excluded women who had missing data on weight or levels of physical activity. In total, 34,079 women were followed up with an average age of 54 at the start of the study.
The women’s levels of physical activity were assessed at the start of the study by asking them how much they had done each week, on average, for the past year. Different activities were categorised as low intensity or high intensity. Low-intensity activities included yoga, swimming and tennis, while high-intensity activities included aerobics, cycling and running. As the activities were of different intensities the researchers used a measurement unit called a metabolic equivalent (MET) to standardise how much energy each activity would burn.
An MET takes into account the weight of the person and is a method of expressing the energy expenditure of physical activities in a way that they can be compared between people of different weight. The researchers worked out the number of METs per activity and the number of METs each women used each week. The women’s physical activity data was updated through questionnaires in the 3rd, 6th, 8th, 10th and 12th year of the study.
One MET of a task is equivalent to the energy expended during quiet sitting, whereas jogging, for example, is valued at seven METs.
The women were grouped into three physical activity levels at each assessment.
- LOW: Those who engaged in up to 7.5 MET hours a week (equivalent to up to 150 minutes of moderate-intensity physical activity).
- MEDIUM: 7.5 up to 21 MET hours a week.
- HIGH: 21 or more MET hours a week (equivalent to more than 420 minutes per week of moderate intensity activity.
Women reported their weight during the 13-year follow-up.
Information on factors that can influence weight were also collected, including ethnicity, educational level, height, smoking status, menopausal status, post-menopausal hormone use, diabetes, hypertension, alcohol intake and diet measured by a 131-item food frequency questionnaire at the start of the study. The main analysis was also adjusted to take into account the women’s age, weight at the start of the study, height and time interval between weight assessments. Other factors that could have influenced the results were adjusted for in a second analysis.
What were the basic results?
Women of a lower weight at the beginning of the study were associated with higher activity levels. More-active women were also more likely to have had postgraduate education, used post-menopausal hormones and to have been healthier (according to their medical history profiles).
Over the 12 years of the study, the women’s average weight rose 2.6kg, from 70.2kg to 72.8kg.
Over a three-year period, women in the medium-activity group gained 0.11kg, and women in the low-activity group gained 0.12kg more in weight than the women in the high activity group.
Age, menopausal status and body mass index (BMI) affected the rate of weight gain. The trend of increasing weight gain with lower levels of activity appeared only in women with a BMI lower than 25, and the magnitude of gain was larger in the least active pre-menopausal women compared with post-menopausal women.
The researchers also looked at the likelihood of women gaining 2.3kg over an average interval of 2.88 years. They found that for women with a BMI over 25 at the start of the study, the intensity of physical activity did not affect how likely they were to put on this weight. However, women with a BMI of less than 25 were more likely to avoid this weight gain with greater levels of physical activity.
How did the researchers interpret the results?
The researchers suggested that the rate of weight gain, 2.6kg over 13 years, was small but enough to affect health adversely. They suggest that for women consuming a normal diet, “sustained moderate-intensity physical activity for approximately 60 minutes per day is needed to prevent weight gain”.
This study estimated that women should do an hour of moderate physical activity a day to avoid weight gain over time. It also suggested that physical activity is more effective in preventing subsequent weight gain in slimmer women than women who are overweight.
Although this study followed a large number of women, it also has several limitations that the researchers themselves highlight:
- Women self-reported their physical activity levels and weight. This could lead to inaccuracies in both the amount of exercise they performed and their weight.
- The researchers asked the women about their diet once, at the start of the study. However, it is possible the women’s diet changed over the 12 years of the study and that women who did a lot of exercise are more likely to eat a healthier diet.
- These women were American, and therefore their diet may have differed from British women.
- This study followed women only and so the results may not apply to men.
The research suggests that preventing weight gain associated with age is possible with frequent exercise. The levels of exercise that the researchers estimate are needed to prevent weight gain are greater than the amount that is officially recommended to maintain a healthy heart (at least five 30-minute sessions of moderate exercise a week).
In addition, this study did not look at diet, a major determinant of weight and fitness. Differences in diet between slim and overweight individuals over the follow-up period may have contributed to the observation that overweight women did not appear to benefit to the same degree as slimmer women from increased exercise.
Although this study followed a large number of women, further research is needed that takes into account diet over the long term to assess the right balance of diet and exercise to avoid putting on weight.