In this large cohort of middle-aged and older women followed for 13 years, there was an overall weight gain over time. Compared with women who engaged in the equivalent of ≥420 min/week (60 min/day) of moderate-intensity physical activity, those carrying out 150-<420 min/week of such activity, as well as those less active (<150 min/week), gained significantly more weight, with no difference in weight gain between these two lesser active groups. The two lesser active groups also were significantly more likely to gain ≥2.3 kg (5 lb) over a 3-year period than the most active group. There was an interaction of these associations with BMI, such that physical activity was inversely related to weight gain only among normal weight women; there was no relation among heavier women. Normal weight women who successfully maintained their normal weight and gained <2.3 kg throughout the 13-year study spent the equivalent of 60 min/day in moderate-intensity activity—the level recommended by the IOM for the prevention of unhealthy weight gain.13
These results highlight two important points for weight gain prevention. First, once overweight, it may be “too late” since physical activity—at least, at levels carried out by study participants—was not associated with less weight gain. Second, sustaining high levels of physical activity (~60 min/day) is needed to successfully maintain normal BMI and prevent weight gain: women with this level of physical activity at baseline only (who may not have sustained the level over the study duration) gained weight at a similar trajectory as less active women ().
The rate of weight gain in this study—2.6 kg over 13 years—was very similar to that observed between 1992 and 2000 among nationally representative women aged 51–61 years.8
This seemingly small amount of weight gain is sufficient to adversely affect health.2, 3
Preventing weight gain is preferable to treating overweight/obesity because of the limited sustainability of weight loss.5, 6
For example, in a recent randomized trial of weight loss in overweight/obese women, even with extensive counseling on diet and exercise and the provision of treadmills in subjects’ homes, the 8–10% weight loss at 6 months could not be sustained at 24 months.25
In another study where overweight/obese men and women had lost 8.5 kg, weight regain of 4.0–5.5 kg over 30 months occurred in all groups, randomized to different behavioral interventions to maintain weight loss.26
Because weight gain results from an imbalance between energy intake and expenditure, one important question for weight gain prevention, among individuals consuming a usual US diet, is the amount of physical activity needed. The available data are unclear, particularly for long-term weight gain prevention. Current national and international recommendations for preventing unhealthy weight gain have variously targeted 45–60 min/day of moderate-to-vigorous activity, but acknowledge the limited basis for this amount.10, 12, 13, 20, 27
Cross-sectional studies consistently indicate an inverse, dose-response relation; however, the direction of the relation is unclear with this study design (does physical activity lead to lower body weight, or is higher body weight a deterrent to activity?)10
Prospective cohort studies with follow-up of 6.5 years or longer show consistent associations between increased physical activity levels, generally assessed at two time points only, and less weight gain.10
It is difficult to infer the amount of physical activity carried out in these studies because studies typically analyzed physical activity as a continuous variable, or grouped subjects into categories of increase, decrease, or no change in physical activity.10, 28
Several randomized controlled trials testing different amounts of physical activity exist, where the primary outcome was not weight, but where weight was recorded.10
These trials are short-term, lasting 8–16 months, and suggest that physical activity on the order of 13–26 MET-hr/week over this short interval results in <3% weight change.10, 29–31
The data from our long-term observational study are congruent with this activity range.
Most of the studies discussed above have used self-reported physical activity, as has our study, which may be limited. However, studies with objective measures of physical activity and weight change are sparse. The IOM recommendation on the amount of physical activity to maintain weight was based on data from doubly-labeled water studies world-wide.13
The expert panel estimated the activity level among 407 normal weight men and women to be 60 min/day of moderate-intensity activity. However, not examined by the panel were the 360 overweight or obese subjects in the doubly-labeled water studies, where physical activity levels were similar to, or even higher than, those among normal weight subjects.12
Thus, these data, in addition to being cross-sectional, do not fully support the IOM conclusion that 60 minutes/day of activity is needed for maintaining normal weight. Another study of 74 Pima Indians reported a negative, non-significant correlation between physical activity, measured using doubly-labeled water, at baseline and weight change over 4 years; the lack of significance may be due to the small sample.32
Strengths of the present study include the large number of women followed prospectively for a long period, with multiple assessments of physical activity and body weight. Women were classified according to clinically relevant groups corresponding to physical activity recommendations. We also adjusted for many variables that could potentially confound the physical activity-weight relation.
Limitations include self-reported recreational physical activity and weight. However, the physical activity questionnaire used has shown good reliability and validity,18, 19
and physical activity in the present study has shown expected associations with chronic disease outcomes.33–35
Self-reported body weight also is highly correlated (r=0.96) with directly measured weight in health professionals.21
We did not have detailed information on other measures of body composition, nor on medications potentially affecting weight. Further, participants are not representative of the US population; however, their rate of weight gain was similar to that of comparably aged women in the general population.8
While we adjusted for baseline diet, we did not have repeated measures of diet over time. We also did not examine activity levels needed for weight management among women restricting caloric intake, since our intent was to investigate women with a usual diet.
In conclusion, in this large prospective study of women consuming a usual diet, sustained moderate-intensity physical activity for ~60 min/day was needed to maintain normal weight and prevent weight gain. These data suggest that the 2008 federal recommendation for 150 min/week, while clearly sufficient to lower the risks of chronic diseases, is insufficient for weight gain prevention absent caloric restriction. Physical activity was inversely related to weight gain only among normal weight women; among heavier women, there was no relation, emphasizing the importance of controlling caloric intake for weight maintenance in this group.