In this large cohort of women, greater duration of daily physical activity (PA) was associated with less weight gain over eight years and a significant dose-response relationship was evident. Benefits were seen with an increase in PA as little as 11–20 min/d, and we found no evidence of an upper threshold. For the same activity duration, jogging/running appears to be the most protective against weight gain, indicating that the intensity of an activity can modify the duration necessary to prevent gain. Brisk walking was an independent predictor of less weight gain. Sedentary behavior was associated with weight gain independent of activity level, and the benefit of increased PA appeared to be greater in overweight/obese women than in normal weight participants.
National PA recommendations, which were developed to reduce the risk of developing chronic disease morbidities, have changed over time. These have included from 20-minute of continuous vigorous PA on at least 3 days/week (31
); 30 min/d of moderate PA on most days of the week (31
); 60 min/d of moderate to vigorous PA on most days of the week while not exceeding caloric intake to prevent unhealthy weight gain in adults (38
); at least 60 to 90 min/d moderate-intensity activity while not exceeding caloric intake requirements to sustain weight loss (38
); 30–60 minutes of moderate to vigorous PA on 5 or more days of the week (39
); and 150 minutes/week (21 min/d) (32
) to 250 minutes/week (36 min/d) of moderate-intensity (32
), or 75 minutes/week (11 min/d) of vigorous-intensity aerobic physical activity (40
). In our 8-year prospective investigation among premenopausal women, 30 or more min/d of total discretionary PA were associated with lower likelihood of weight gain, but our findings suggest a dose-response relationship with no clear threshold suggesting that persons may benefit more by exceeding the minimum recommended amounts of PA. Even a small increase in sedentary women (up to 11–20 min/d) appears beneficial. These findings are consistent with most other longitudinal studies (23
) and health related statements (37
). One cross-sectional study of 4,769 male runners (41
) and another longitudinal study of 4,599 men and 724 women (42
) showed that long-term maintenance of a given fitness level was not sufficient to prevent aging-related weight gain, and that increasing amounts of activity may be needed to maintain weight with increasing age. These findings are not entirely incongruent with ours which suggest that maintaining a high level of activity (or increasing it) reduces – but does not prevent- the risk of age-related weight gain. In fact, using linear regression models, the estimated amount of discretionary PA needed to prevent weight gain (zero average weight gain from 1989 to 1997 during 8 years) was 3.7 hours/day for women with BMI<25; 2.6 hours/day for women with 25≤BMI<30; and 1.6 hours/day for women with BMI>30. Additionally, the estimated amount of discretionary PA required to prevent a weight gain of 5% or more was 1.6 hours/day for women with BMI<25; 1.5 hours/day for women with 25≤BMI<30; and 0.75 hours/day for women with BMI>30. Because it is hard for many people to spend so much time in discretionary PA, a combination of PA and other strategies, such as restrained energy intake, may be needed to avoid age-associated weight gain. It should be kept in mind that these results cannot be generalized to all women because of the following reasons: 1) more exercise may be needed after an intentional weight loss to maintain the loss; 2) our sample did not include men or women in their 20s; 3) we excluded women who had recently been pregnant at the time that they reported weight because they had additional weight to lose; and 4) we allowed for up to 5% weight gain in our definition of weight gain prevention. In fact, the estimated amount of PA varied between 1.6 hours/day in lean women and 45 min/d in obese women when we allowed <5% weight gain. Notably, according to NIH (4
), losing 5 to 10% of baseline body weight is considered a clinically significant weight loss to reduce the risk factors of diabetes and heart disease. This explains our rationale for selecting 5% as the cut-off and anything below 5% to be within the weight maintenance range.
Schmitz et al.(23
) found that among 2,617 young adult men and women (18–30 years) followed for 5 years, body weight could be maintained whether or not PA was maintained Also, Owens and colleagues (43
) found that baseline activity was associated with weight gain prevention at 3-year follow-up in a study of 500 peri-menopausal women (42–50 years). Our results seemed to indicate otherwise as attenuation of weight gain appeared to depend on the activity during the later part of follow-up; women who remained or became highly active towards the end of the follow-up experienced less weight gain regardless of their baseline activity. Also, our results appeared to indicate that baseline activity was not associated with 8-year weight gain prevention. These discrepancies may reflect their smaller sample size and their shorter follow-up.
Among women whose only type of activity was walking, walking pace and duration were both important for long-term weight gain prevention. Non-brisk walking was not associated with weight gain prevention except for obese women. Our results are consistent with other studies (13
Comparing the different PA types, our findings strongly suggest that the form of PA is not as important as total energy expenditure. For instance, whether 600 kcalories/day were spent by jogging for 30 min/d or by brisk walking for 60 min/d, total energy expenditure mattered the most for weight gain prevention regardless of PA type.
Baseline BMI modified the association with total PA pattern, as women with excess weight benefited more than their leaner peers at any level of PA. These results are consistent with other longitudinal studies (15
). One interpretation is that heavier individuals tend to burn more calories per activity unit (say walking a mile) as compared to lean individuals (45
); Hence, once the body weight drops, more PA is needed to maintain the reduced body weight.
There are several limitations to the present study. First, dietary data were initially collected in 1991 while other baseline data were collected in 1989. However, in this analysis, excluding the dietary variables from the models did not meaningfully affect the results. Second, we could not look at the change in TV viewing as an independent predictor, because it was not included as a separate question on the 1989 questionnaire, although it has been consistently associated with obesity (46
). Therefore, we used total hours of sitting at home in 1989 and we combined the time sitting watching TV/VCR with other sitting at home for 1997 to evaluate the change from 1989 to 1997. Third, the sample was not a random sample from the United States. Moreover, we did not include women who were pregnant or recently pregnant. Thus it is not clear whether these results can be generalized to pregnancy-related weight gain. However, the relationship between pregnancy and weight gain is more complicated and deserves a separate investigation (47
). Fourth, there may be other confounders that may contribute to weight gain and to lower activity such as viral, bacterial, endocrinal, or genetic factors were not accounted for in our analysis because the information was not available to us. Fifth, there may be other types of activities such as weight training or sleep that could have an impact on the body weight; we could not account for these variables because they were not included in all of our questionnaires. Also, our measurements of PA were inevitably imperfect, which will have tended to underestimate the benefits of PA and overestimate the amount of PA needed to prevent weight gain. While objective measures of PA may have been desirable, the validity of our PA questions has been documented (29
Despite these limitations, there are several strengths to our study. First, it included a large sample of women followed with repeated assessments over an 8-year period. Second, we were able to control for sedentary behavior by using the total number of hours sitting at home. Moreover, information on a wide variety of potentially confounding behavioral and demographic variables was collected at each assessment, which allowed us to assess activity patterns and weight change associations independent of these potential confounders. Finally, we were able to exclude women with conditions affecting weight, such as pregnancy-related or post-delivery weight gain.
Our findings underscore the importance of specifying both, the duration and the intensity of PA for weight gain prevention, though other factors related to both weight gain and activity may affect the association. Increased duration of PA was associated with progressively less weight gain, and even an 11–20 min/d increase was beneficial; the benefits appeared stronger among those initially overweight; nevertheless, lean women benefited as well. Sedentary behavior was independently associated with weight gain. The results offer further justification for recommending that adults of any weight engage in activity on a daily basis.