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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES

Although public health campaigns stress leisure time physical activity (LTPA) as essential for obesity prevention, few epidemiological studies have focused on the association of specific types and intensities of LTPA and the clinical endpoints of overweight and obesity. Therefore, we prospectively assessed whether moderate- and vigorous-intensity as well as total LTPA were associated with the risk of becoming either overweight or obese using a prospective cohort design of 19,003 women enrolled in the Women's Health Study (WHS). Women reported their participation in walking and LTPA at baseline. During a median follow-up of 11.6 years, 7,865 women became overweight or obese. In multivariable-adjusted models that included demographic, lifestyle, and dietary factors, both vigorous-intensity and total LTPA showed a modest inverse relationship with the development of overweight/obesity. The hazard ratios (HR) and 95% confidence interval (CI) for the highest categories of vigorous-intensity LTPA (>2,000 kcal/week) and total LTPA (>3,000 kcal/week) compared with no LTPA were 0.79 (0.71–0.89) and 0.87 (0.78–0.96), respectively. In addition, a greater percentage of total LTPA spent performing vigorous intensity activities was associated with a lower risk of overweight/obesity (multivariable HR 0.93, 95% CI 0.87–0.98 for performing >50% compared with <50% of activity as vigorous). In conclusion, higher amounts of total LTPA should be encouraged to prevent obesity. Among those willing to participate in vigorous LTPA, and for whom such activities are not contraindicated, vigorous LPTA should be encouraged.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES

The prevalence of obesity remains unacceptably high in the United States and continues to rise worldwide (1,2). This has important public health implications, given the known association between obesity and chronic diseases (2). Although the initial response to the obesity epidemic often focused on treatment, there is growing recognition that prevention is key because biological mechanisms favor adaptation to a higher body mass once weight gain occurs (3). In the modern world, environmental factors, including a decrease in leisure time physical activity (LTPA) and the consumption of a calorie-dense diet, have been major contributors to the mismatch between energy intake and expenditure, and the resulting high prevalence of obesity.

While an extensive epidemiological literature exists on the role of LTPA as a predictor of long-term weight change during adulthood (4,5), these studies have predominantly focused on total LTPA, but not specific intensities and types of LTPA. This additional information can provide concrete, clinically relevant information to patients who target their LTPA for obesity prevention. We therefore examined the association between the intensity and type of LTPA and the risk of becoming overweight or obese in a cohort of initially normal weight, middle-aged and older US women during more than 10 years of follow-up.

Methods and Procedures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES

Study population

We analyzed data from the Women's Health Study (WHS), a completed, randomized clinical trial of low-dose aspirin and vitamin E in the primary prevention of cardiovascular disease and cancer. A detailed description of WHS has been previously published (6). Between November 1992 and July 1995, 39,876 US female health professionals aged ≥45 years without cardiovascular disease or cancer (except nonmelanoma skin cancer) were randomized into the study through 2004. Subjects for the present study were excluded if they were underweight, overweight, or obese, defined as a BMI <18.5 or ≥25 kg/m2 (n = 20,313), were diabetic (n = 945), or were missing information on LTPA (n = 213) at baseline. Women who reported cardiovascular disease or cancer postrandomization with a prerandomization diagnosis date (n = 25) and women with missing information on BMI during follow-up (n = 127) were also excluded. These exclusions resulted in a final sample size of 19,003 women.

Assessment of LTPA

On baseline questionnaires, participants were asked the average time spent per week on eight groups of recreational activities, including walking or hiking, jogging (slower than 10-min miles), running (faster than 10-min miles), bicycling (including use of stationary machines), aerobic exercise/aerobic dance/exercise machine use, lower intensity exercise (including yoga, stretching, or toning), racquet sports (including tennis, squash, or racquetball), and lap swimming. Women also reported their usual pace of walking. Each group of activities was assigned a multiple of resting metabolic rate (MET) based on absolute intensity of the activity, which was then multiplied by the woman's body weight in kilograms and by the hours per week engaged in the activity to obtain the kilocalories per week expended in that activity (7). We divided the activities into moderate-intensity LTPA (3– <6 METs) and vigorous-intensity LTPA (≥6 METs). Moderate-intensity LTPA included walking, bicycling, and low-intensity exercise; vigorous-intensity LTPA included running (≥10 min/mile), jogging (≤10 min/mile), aerobic dance or exercise machine use, racquet sports, and lap swimming. We calculated the total LTPA as well as moderate- and vigorous-intensity LTPA separately by summing the energy expenditure for corresponding activities.

The LTPA questions used in WHS were adapted from the College Alumni Health Study questionnaire, which has been validated using the double-labeled water method (8). In contrast to the College Alumni questionnaire, the WHS questionnaire listed eight specific activities commonly carried out by women. The WHS questionnaire had a 2-year test–retest correlation coefficient of 0.59 and a correlation coefficient of 0.62 compared with activities diaries recorded every 4 weeks during the year before the administration of the questionnaire (9).

Other baseline variables

On baseline mailed questionnaires, women also provided self-reports on age (in years), smoking (never, former, or current), parental history of myocardial infarction before the age of 60 years (yes/no), alcohol use (rarely/never, 1–3 drinks/month, 1–6 drinks/week, and ≥1 drink/day), history of hypertension (defined as either a physician diagnosis, a self- reported blood pressure ≥140/90 mm Hg, or use of antihypertensive medications), history of high cholesterol (defined as either a physician diagnosis, treatment or self-reported total cholesterol ≥240 mg/dl), diabetes (yes/no), menopausal status (premenopausal, postmenopausal, or uncertain), and postmenopausal hormone use (never, former, current). BMI (in kg/m2) was calculated from self-reported height and weight. Participants also completed a 131-item validated semi-quantitative food frequency questionnaire by reporting the frequency of consuming common portion sizes for each food item, on average, during the previous year. Nutrient intake was computed by multiplying the intake frequency of each unit of food by the nutrient content of the specified portion size according to food composition tables from the Harvard School of Public Health, Boston, Massachusetts (10). Each nutrient reported was adjusted for total energy intake using the residual method (11). All participants gave written informed consent, and the study protocol was approved by the institutional review board at Brigham and Women's Hospital, Boston, Massachusetts.

Outcome ascertainment

Information on body weight was updated at the 24-, 36-, 60-, 72-, and 108-month follow-up questionnaires. In addition, 17,600 of 19,003 women (92.6%) with a normal BMI at baseline agreed to continue in an observational follow-up study and had BMI updated on three annual follow-up questionnaires. BMI was categorized into three groups according to the World Health Organization criteria: normal weight (18.5– <25 kg/m2), overweight (25– <30 kg/m2), and obese (≥30 kg/m2).

Cases of women who become either overweight or obese (n = 7,865) were defined as women of normal weight at baseline who subsequently had a BMI ≥25 kg/m2 at any follow-up time-point. The “time-to-event” for each case that became overweight or obese was calculated as the estimated time when her BMI crossed the cutoff point by modeling a regression line from the last reported BMI of less than 25 kg/m2 to the first reported BMI of 25 kg/m2 or higher during follow-up. For women who remained normal weight, the time-of-censoring was calculated as the latest date when a normal BMI was reported. Incident cases of becoming obese were defined in a similar manner using a BMI of 30 kg/m2 as the cutoff point. Women who developed intermediate diabetes, the management of which typically involves weight control, were censored on the date of the diabetes diagnosis. In a similar population of female health professionals, self-reported weight was highly correlated with clinic-measured weight (Pearson r = 0.97) (12).

Statistical analysis

Moderate-intensity, vigorous-intensity, and total LTPA were categorized separately. Categories were chosen to allow assessment of dose-dependent relationships. Baseline characteristics were compared across categories of total LTPA using χ2 tests for proportions and ANOVA for means. For each category of LTPA, we computed the incidence rates of becoming overweight or obese. After checking the assumption of proportional hazards (all P >0.05), we used Cox proportional hazards models to compute hazard ratios (HRs) with corresponding 95% confidence intervals (CIs). The initial model adjusted only for age (in years). The multivariable-adjusted model additionally controlled for smoking, alcohol use, hormone replacement therapy, menopausal status, randomization arm, total caloric intake, and dietary factors, including intake of saturated fat, low-fat dairy products, high-fat dairy products, red meat, poultry, whole grains, and fiber. The effect of baseline BMI (in kg/m2) was examined separately, as BMI is part of the defined outcome and may represent overadjustment. Tests for linear trend were computed using median values within each category of LTPA as an ordinal variable. We also assessed potential confounding by adding hypertension and hypercholesterolemia into the model. We computed the ratio of vigorous/total LTPA to assess the risk of becoming overweight or obese in individuals performing ≥50% vs. <50% of their LTPA as vigorous activity.

We also conducted several secondary analyses. Stratified analyses were conducted by baseline BMI (<22.5 vs. 22.5–25 kg/m2). We restricted the analysis to nonsmokers to reduce bias due to smoking. We also analyzed the association between individual activities, focusing on the most common LTPA, and the risk of becoming overweight or obese. Finally, we repeated the main analysis using updated LTPA assessments at 36, 72, and 96 months in a time-dependent Cox model. All analyses were completed using SAS version 9.2 (SAS Institute, Cary, NC). The significance level was set at 0.05. All authors had full access to the data and take responsibility for its integrity.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES

Among the 19,003 women comprising the study population, the mean age was 54.5 ± 7.1 years. Table 1 compares the baseline characteristics by level of total LTPA. A significant association was found between increasing levels of total LTPA and age, dietary variables, smoking status, alcohol intake, hypertension, high cholesterol, and total daily caloric intake (all P <0.05). Baseline BMI did not differ substantially with higher levels of either moderate or vigorous LTPA. During a median follow-up of 11.6 years, 7,869 new cases of either overweight or obesity developed.

Table 1.  Baseline characteristics of 19,003 US women according to level of total leisure time physical activity (kcal/day)
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For moderate LTPA, no association was found between increasing levels of LTPA and the risk of becoming overweight or obese in either age-adjusted or multivariable-adjusted models. There remained no association after addition of baseline BMI to the multivariable-adjusted model (Table 2).

Table 2.  Incidence rates and hazard ratios (95% confidence intervals) of becoming overweight or obese according to level of physical activity
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For both vigorous and total LTPA, a significant graded decrease in the risk of becoming overweight or obese was observed with increasing levels of LTPA in the age-adjusted and multivariable-adjusted models. HRs (95% CI) for the highest categories of vigorous and total LTPA in the multivariable- adjusted model were 0.79, (0.71–0.89) (P trend <0.0001) and 0.87 (0.78–0.96) (P trend = 0.002), respectively. Additional adjustment for baseline BMI diminished the HRs for both vigorous and total LTPA to 0.95 and 0.92. The trend across categories remained significant for total (P trend = 0.02), but not vigorous (P trend = 0.3), LTPA (Table 2).

For all types of LTPA, additional adjustment for a history of hypertension or hypercholesterolemia did not significantly alter the HRs. Individuals who performed ≥50% of their total LTPA as vigorous-intensity activities had a modest lower risk of becoming overweight or obese after controlling for total LTPA. The HR (95% CI) for the multivariable-adjusted model was 0.93 (0.87–0.98).

In secondary analyses stratified by baseline BMI category (BMI <22.5 vs. 22.5–25 kg/m2), the risk of becoming overweight or obese was similar in women regardless of their baseline BMI category in both the age-adjusted and multivariable plus BMI adjusted models for all intensities of LTPA (all Pinteraction > 0.05). The multivariable HRs (95% CI) for the highest amount of LTPA for moderate, vigorous, and total LTPA were 0.86 (0.60–1.23), 0.82 (0.64–1.07), and 0.98 (0.78–1.23) for women with BMI <22.5 kg/m2 and 0.96 (0.81–1.14), 1.00 (0.88–1.14), and 0.92 (0.81–1.03) for women with BMI of 22.5–25 kg/m2. In sensitivity analyses that excluded LTPA due to walking at a pace <3 miles/h (which requires <3 METs and is a light-intensity activity), the results were similar (data not shown). In analyses limited to nonsmokers, the findings largely paralleled those from the main analyses for moderate- and vigorous-intensity LTPA, but differed for total LTPA. Specifically, the inverse association between total LTPA and the risk of becoming overweight or obese was no longer present in the age- and multivariable-adjusted models.

When we considered the risk of becoming obese alone as a separate outcome, the findings were largely similar to the main analysis for moderate and vigorous LTPA (Table 3). However, the significant inverse association seen with total LTPA in the main analysis was no longer present, perhaps due to a smaller number of incident cases of obesity in this subgroup analysis.

Table 3.  Hazard ratios (95% confidence intervals) for incident obesity according to level of physical activity
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In separate analyses examining the association between specific LTPA activities and the risk of becoming overweight or obese, there was no association between higher levels of walking and the risk of becoming overweight or obese. For bicycling, a significant increase in risk was seen for the individual category of 500–1,000 kcal/week in age-adjusted and multivariable-adjusted models, but this association was no longer present after additional adjustment for baseline BMI. An inverse association was found between higher levels of jogging and the risk of becoming overweight or obese in multivariable-adjusted models. This relation was no longer present after additional adjustment for BMI. For the category of aerobic dance and exercise machine use, a significant increase in the risk of overweight/obesity was found in the multivariable-adjusted model among those participants performing >1,000 kcal weekly of activity, but this association was no longer present after adjustment for baseline BMI. In contrast, a significant inverse association was found between 500–1,000 kcal of this type of activity and the risk of becoming overweight/obese even after adjustment for baseline BMI (Table 4).

Table 4.  Incidence rates and hazard ratios (95% confidence intervals) of becoming overweight or obese according to level of specific physical activities
inline image

When we repeated our main analysis with updated assessments of LTPA at 36, 72, and 96 months, the findings were largely similar. In multivariable models, no association was found between higher levels of moderate LTPA and the risk of becoming overweight or obese, whereas a significant graded decrease in the risk of becoming overweight or obese was observed with higher levels of vigorous and total LTPA. HRs (95% CI) for the highest categories of vigorous and total LTPA in the multivariable-adjusted model were 0.80 (0.71–0.90) (P trend <0.0001) and 0.88 (0.79–0.97) (P trend = 0.001), respectively. Additional adjustment for baseline BMI diminished the HRs for both vigorous and total LTPA to 0.95 and 0.92. The trend across categories remained significant for total (P, trend = 0.004) and vigorous (P trend = 0.0002) LTPA. In time-dependent analysis examining individuals who performed ≥50% of their total LTPA as vigorous-intensity activities, the HR (95% CI) for the multivariable-adjusted model was 0.82 (0.77–0.88), and was attenuated, but remained significant after adjustment for baseline BMI 0.92 (0.86–0.99).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES

This large, long-term, prospective cohort of middle-aged and older women provides insight into the relationship between both the quantity and intensity of LTPA and the risk of becoming overweight or obese. Specifically, we found a modest but significant inverse association between higher levels of both vigorous and total LTPA and the risk of becoming overweight or obese after multivariable adjustment. A similar association was not observed for moderate LTPA. In analyses of individual activities, jogging, a vigorous-intensity activity, had an inverse association with the risk of becoming overweight or obese, whereas moderate-intensity activities such as walking and bicycling did not. Overall, these findings suggest that higher total LTPA has an important but modest role in preventing the development of overweight and obesity. Expending a larger percentage of total LTPA devoted to vigorous activities may have an incremental benefit, thus allowing individuals to target more vigorous-intensity LTPA if obesity prevention is their primary goal. The lack of an inverse association for moderate LTPA and the risk of developing obesity was somewhat surprising. This may reflect less precise reporting of moderate-intensity activities, the tendency of individuals to increase their level of moderate-intensity LTPAs once they have begun to put on weight, or the fact that although moderate LTPA may still be associated with less weight gain over time, it may not prevent the development of the clinical cutpoints of overweight and obesity.

These findings add to the large body of literature on the association between LTPA and weight change, and uniquely examine the association between LTPA and the endpoint of becoming overweight or obese. The risk of becoming overweight or obese has been infrequently assessed in epidemiological studies (13). These BMI “milestones” are clinically relevant as most studies examining the association between body weight and adverse health outcomes, including cardiovascular disease, diabetes, and mortality, use overweight or obesity as the exposures of interest.

In addition, our analysis examined different intensities of LTPA. Most prior analyses of weight change have focused on total level of LTPA and have differed in terms of the range of individuals' baseline BMI (4,14,15,16,17). These differences in study design may provide insight into the lack of association of moderate-intensity LTPA and the development of overweight and obesity in our study. The largest previous study examining intensity of LTPA and weight change among women with normal BMI at baseline had findings similar to ours, specifically an association between high-intensity LTPA, but not moderate LTPA, and weight gain (16,17). As in our study, this lack of association of moderate-intensity LTPA and weight gain was counterintuitive. Differential reporting of moderate LTPA may contribute to these findings. Prior work with the physical activity index from the College Alumnus Questionnaire suggests that moderate-intensity LTPA may be less accurately reported in questionnaires compared with vigorous or total LTPA (18). Specifically, self-reported high-intensity LTPA had higher correlations than moderate LTPA with direct reporting of similar intensities of LTPA. Another explanation for the lack of association of moderate LTPA and the risk of becoming overweight or obese is that moderate activity prevents some amount of weight gain, but not sufficient weight to prevent many individuals from crossing the threshold defining overweight or obesity. Findings from a large prospective study in Washington state that included women of all BMIs at baseline provide support for this theory (17). In this analysis, higher amounts of low and moderate, in addition to high-intensity, LTPA were associated with less weight gain. The largest benefit for low- and moderate-intensity LTPA was seen among women with BMI ≥25 kg/m2. In conjunction with these prior studies, our findings support the additional insight provided by the use of the endpoints of overweight and obesity, which are used clinically and have been prospectively associated with the development of obesity-associated comorbidities.

Our findings using overweight and obesity as the endpoint compliment a recent study in the WHS that examined the amount of total LTPA and weight gain (19). Both analyses demonstrate the difficulty of maintaining a normal weight among middle-aged and older women and that relatively high levels of total LTPA are required to decrease weight gain. This contrasts with studies examining LTPA and the risk of other chronic health conditions including type 2 diabetes and coronary heart disease (20,21) in which even relatively low levels of light- and moderate-intensity activities were beneficial. Taken together, these findings suggest that the health benefits attributed to LTPA for chronic disease prevention are not solely explained by a reduction in the risk of becoming overweight or obese. Our study expands on the previous WHS study by examining the intensity of LTPA. The inverse association we found between vigorous LTPA and the development of overweight/obesity may help explain the greater benefit seen with the highest levels of LTPA in the previous study. Presumably, women participating in vigorous-intensity LTPA are more likely to achieve high amounts of total LTPA, although this was not assessed.

Major strengths of our study include the large number of cases of overweight and obesity accrued over a long period of follow-up, allowing for sufficient statistical power to detect modest effects. Detailed information on both specific type and intensity of LTPA as well as important confounders strengthened the multivariable analysis. The repeated assessments of amount and intensity of LTPA allowed us to take into account changes in activity level during follow-up. Some potential limitations deserve comment. Both LTPA and weight were self- reported and therefore are subject to misclassification. However, the high validity and reliability of both of these variables has been demonstrated in similar populations (9,12), although moderate-intensity LTPA may be less accurately reported than vigorous-intensity LTPA. The relatively low average level of LTPA in our study population may have also limited our ability to detect a threshold effect for moderate LTPA. The ability of each individual to perform absolute MET level of activity may depend on fitness. However, we used absolute, as opposed to relative, METs as it is more applicable to population-based, as opposed to individual-targeted, LTPA recommendations. Finally, despite our detailed multivariable adjustment, we cannot exclude the possibility that self-reported LTPA may at least partially serve as a proxy for an overall healthy lifestyle.

In conclusion, our findings suggest a modest association between higher levels of both vigorous-intensity and total LTPA and a lower risk of becoming overweight or obese. Performing a greater proportion of total LTPA as vigorous activity may also decrease the risk of becoming overweight or obese. While we did not observe similar findings with moderate-intensity LTPA, moderate-intensity LTPA may still attenuate weight gain, albeit perhaps not enough to prevent the development of overweight or obesity. Alternatively, the lack of association may be explained by less precise reporting of such activities. Moderate-intensity activities, such as brisk walking, should in no way be discouraged: a large body of evidence clearly supports a role of these activities in decreasing the risk of many chronic diseases (22).

Acknowledgmant

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES

This study was supported by research grants DK 081141, DK-066401, CA 47988, HL 43851, HL 65727 from the National Institutes of Health, Bethesda, Maryland. These grants provided funding for study conduct and data collection. We are indebted to the 39,876 participants in the Women's Health Study for their dedicated collaboration and to the entire staff of the Women's Health Study for their assistance in designing and conducting the study.

DISCLOSURE

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES

Dr Lee serves as a consultant to Virgin HealthMiles and sits on their Scientific Advisory Board. Dr Sesso reports receiving consulting fees from Lovate Health Sciences USA. The other authors declared no conflict of interest.

REFERENCES

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgmant
  8. DISCLOSURE
  9. REFERENCES
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