A Lot More Physical Activity Needed to Prevent Weight Gain
Abstract & Commentary
Synopsis: A prospective study found that women gained weight at similar rates even though their physical activity levels varied substantially. Only women with normal BMI at baseline and higher levels of activity who maintained that activity level also maintained normal BMI.
Source: Lee IM, et al. Physical activity and weight gain prevention. JAMA 2010;303:1173-1179.
The women's health study was a prospective cohort study involving almost 40,000 women. The women completed health questionnaires from 1992 to 2004. When that study ended, more than 33,000 women continued in an observational follow-up study until 2007. Detailed data on physical activity were collected, which allowed calculation of energy expenditure per week. These calculations allowed women to be classified into three groups based on energy expenditure: < 7.5, 7.5-21, or > 21 metabolic equivalent (MET) hours per week. These are equivalent to < 150 min/week, 150-420 min/week, and > 420 min/week of moderate-intensity physical activity.
The women reported their body weight in the questionnaires along with several potential confounders of activity and weight: race, educational attainment, height, smoking status, menopausal status, postmenopausal hormone use, diabetes, hypertension, alcohol intake, and diet. Statistical analyses were carried out on physical activity and weight trends. Repeated measures linear regression was used to test correlations. All of the confounders were examined statistically.
At baseline, the average age of the women was 54.2 years. The mean follow-up time in the study was 13.1 years. Data for this study was analyzed at intervals of approximately 3 years. At baseline, the distribution of women between the least, intermediate, and most active groups was 49.5%, 28.8%, and 21.7%. After 14 years, the proportions were 34.2%, 30.3%, and 35.5%. On average, women gained weight during the study, with the average weight increasing from 70.2 kg at baseline to 72.8 kg at the end. All three groups showed similar weight gain patterns. Those in the least active group gained 0.12 kg more weight per three-year interval compared to those in the most active group (P = 0.002). The intermediate activity level group had 0.11 kg more weight gain per three-year interval compared to those in the most active group (P = 0.003). The weight gain in the least and intermediate activity groups did not differ significantly. The two less active groups were significantly more likely to have meaningful weight gain in each three-year interval. Meaningful weight gain was defined as 2.3 kg (5 lb).
The overall association between activity and weight gain was significantly correlated to age and BMI, but not smoking status or menopausal status. When these associations were examined by activity groups, only BMI was significant. Only women who started the study with a BMI of 25 or less (normal) were unlikely to have meaningful weight gain of 2.3 kg (P < 0.001). No other group showed this correlation. Approximately 4,500 women started the study with a normal BMI and remained at normal BMI throughout the study without gaining 2.3 kg weight. The mean activity level of these women over the duration of the study was 21.5 MET hours per week, or 60 minutes per day of moderate- intensity activity.
The authors draw two main conclusions from their data. The first is that once people are overweight, physical activity at the level carried out by these participants was not associated with less weight gain. Prevention of overweight is crucial and requires much further research. The second conclusion is that 60 minutes per day of moderate-intensity physical activity is necessary to maintain normal BMI and prevent weight gain.
Nothing further needs to be said about the growing problem with obesity and the challenges of helping people lose weight. Each new weight loss strategy runs into problems with the difficulties many people have maintaining a healthy weight. With all of the attention these issues are receiving, relatively little research has examined weight gain prevention strategies. The old adage that "an ounce of prevention is worth a pound of cure" would appear to be remarkably apt in this area. Yet much remains unclear about how much physical activity is needed to prevent weight gain.
For example, the most recent guidelines suggest people need at least 150 min/week of moderate-intensity exercise for health benefits.1,2 Whether this helps prevent weight gain is unclear. The Institute of Medicine recommends 420 min/week (an hour per day) of moderate exercise to prevent becoming overweight.3
Moderate-intensity physical activity is defined in various ways as it varies by individual. Common rules of thumb are moderate activities allow someone to carry on a conversation, or the heart rate reaches 50%-70% of resting. According to the Centers for Disease Control and Prevention, moderate-intensity activities include walking briskly, bicycling (slower than 10 mph), water aerobics, doubles tennis, ballroom dancing, and general gardening.4 Jogging, running, faster bicycling, hiking uphill, and singles tennis are categorized as vigorous activities.
This prospective study adds important insight to help people understand what is necessary to maintain healthy weight over many years. Although the overall weight gain found in this study may appear small, averaging 2.6 kg over 13 years, this can have significant health impact. The study's findings point to the importance of preventing weight gain in the first place. They also point to the not-insignificant quantity of physical activity necessary to prevent weight gain. An hour per day of moderate-intensity activity will appear challenging to many people, although the benefits are increasingly being shown to be very significant.
The study has some important limitations. The average age of the women was older (54.1 years, with a standard deviation of 6.8 years). The findings may not be directly applicable to women of all ages, nor to men. The women were consuming "a usual U.S. diet," and thus factors related to special diets were not taken into account. The study used self-reported physical activity and weight, which have limitations, although these are often the only methods available in large, long-term studies.
The findings suggest that an important caveat must be added to recent guidelines on physical activities. These recommend 150 min/week of moderate activity for health benefits.1,2 While this is important for overall health, prevention of weight gain requires more physical activity, on the order of 420 min/week (or an hour a day). Given the failure of many weight-loss strategies to achieve long-term weight-loss maintenance, preventing weight gain in the first place is crucial. This supports giving high priority to the physical activity levels of children and adolescents and instilling healthy habits early in life.
1. Physical Activity Guidelines Committee. Physical Activity Guidelines Advisory Committee Report. Washington, DC: Department of Health and Human Services; 2008.
2. Haskell WL, et al. Physical activity and public health: Updated recommendation for adults from the American College of Sports Medicine and the American Heart Association. Circulation 2007;116:1081-1093.
3. Institute of Medicine. Dietary Reference Intake for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, DC: National Academies Press; 2002.
4. Centers for Disease Control and Prevention. Physical activity. Available at: www.cdc.gov/physicalactivity. Accessed April 16, 2010.