Excess adiposity, particularly if centrally deposited, is associated with greater risk of cardiovascular disease, stroke, diabetes, cancer, and other chronic diseases in women (5,14,20,29). The menopause transition was shown to be associated with an average 10% increase in fat mass accompanied by an increase in waist circumference and an increase in total body weight despite a slight (1%) but significant decrease in muscle mass (34). These adverse body composition changes at midlife not only have been explained by both increasing age and ovarian aging associated with menopause but also are likely due to changes in dietary intake and/or a decline in physical activity (PA) and energy expenditure (EE). The changes in PA and EE may be accompanied by further increases in fat mass and decreases in muscle mass as women age (8,33,36), thus influencing total body weight in opposite directions. An age-stratified analysis of a subgroup of the Women’s Health Initiative’s (WHI’s) large cohort of postmenopausal women who were weighed in a clinic each year during a 7-yr period found that younger women (50–59 yr) gained weight on average, whereas older women (70–79 yr) lost weight and those age 60–69 yr tended to maintain their weight (13).
Weight gain in middle-age women is associated with increased health risks (21), whereas weight loss in older women is associated with increased frailty (2). Both relationships are at least in part due to fat mass gain and lean mass loss as women age (15). From a public health perspective, identifying factors that can both prevent increases in body fat, particularly central fat deposition, and maintain muscle mass, as well as stamina and muscular strength, and thus prevent the frailty associated with aging is of great relevance. Most promising in this regard may be PA, which is believed to minimize fat mass gain and lean mass loss (9,22,23,31), as well as a redistribution of fat to central depots (16). In the Healthy Women’s Study (28), both baseline PA levels and postbaseline changes in activity were inversely associated with 3-yr changes in body weight, independent of baseline weight, menopausal status, or hormone use. Moreover, prospective data from the WHI indicate that there is a strong graded inverse association between increased PA and incidence of obesity and cardiovascular events (23). However, the effect of habitual PA or change in PA on body weight and fat distribution during the decades after menopause has not, in general, received much attention. The present analysis focuses on the possible role of PA on changes in body weight and fat distribution, as reflected by the waist-to-hip ratio (WHR), taking into account initial body weight, body mass index (BMI), and WHR in a large multiethnic cohort of postmenopausal women to understand potential differences by age.
The study population was drawn from the multiethnic cohort of 68,132 postmenopausal women enrolled from October 1993 to December 1998 into the WHI Clinical Trials (CT) of Diet Modification and/or Hormone Therapy at one of 40 US clinical centers. Women were eligible for the WHI CT if they were age 50–79 yr, postmenopausal, planned to live in the clinical center area for at least 3 yr, had no history of breast or other cancer except nonmelanomatous skin cancer within the past 10 yr, and were free of any serious health conditions that might reduce survival within 3 yr. Details of recruitment, baseline data collection, and baseline characteristics of the CT cohort have been published previously (10). All procedures and protocols were approved by the institutional review boards at each participating institution, and all participants provided written informed consent. The present analyses further excluded participants who had missing baseline information for variables used in statistical models, including PA, physical measures, dietary intake, smoking, alcohol, hormone use, education, and sleep information. These exclusions reduced the sample to a final analytic cohort of 57,735 participants.
Clinic visits, body weight and anthropometric measurements, and questionnaires
WHI CT participants came into a WHI clinic at baseline, 6 months from enrollment, and annually thereafter, during which trained clinical staff recorded body weight and height as measured with a calibrated balance beam or digital scale and a wall-mounted stadiometer, respectively, with participants wearing no shoes and having removed heavy clothing and pocket contents. BMI was calculated as body weight (kg) divided by height in meters squared (m2). Using a conventional measuring tape, waist circumference was measured at the midpoint between the last floating rib and the upper part of the iliac crest at end expiration, and hip girth was measured at the site of maximum extension of the buttocks to determine WHR.
Participants completed questionnaires at baseline regarding (self-declared) ethnicity; highest level of education attained; smoking status; menopausal status; hormone use (postmenopausal and oral contraceptive use); hours of sleep per night; physical functioning; physician diagnosis of diabetes, hypertension, and heart conditions; alcohol intake; use of lipid-lowering drugs and antidepressants; past and current PA, described below; and dietary intake measured by a calibrated food frequency questionnaire (12,29). Physical function performance measures (e.g., timed walk, grip strength) were obtained using a well-validated measure of self-reported physical function, the RAND-36 physical function scale. The scale is scored from 0 to 100, with higher scores indicating better physical function (23). Updated information was obtained annually. Follow-up time for each woman was determined from enrollment to administrative censor date or loss to follow-up. For this article, an average of 8 yr of follow-up is reported for all relevant measures.
Assessment of PA
Information on walking and recreational PA was used to generate a summary variable in MET-minutes per week. Participants were asked how often they currently walked outside the home for more than 10 min without stopping and the usual duration and speed of their walks. Categories of frequency were rarely/never, one to three times per month, one time per week, two to three times per week, four to six times per week, and seven or more times per week. Duration categories were less than 20 min, 20–39 min, 40–59 min, and 1 h or more. Four speed categories were created: less than 2 mph (casual strolling or walking, 2.0 METs), 2–3 mph (average or normal walking, 3.0 METs), 3–4 mph (fairly fast walking, 4.0 METs), or more than 4 mph (very fast walking, 4.5 METs).
To capture recreational PA, women were asked how often they currently exercised at strenuous levels (that increased HR and produced sweating, 7–8 METs) by checking categories on a Likert-type scale ranging from never to 1, 2, 3, 4, or 5 d·wk−1 or more and for how long they exercised at each session (<20 min, 20–39 min, 40–59 min, or ≥1 h). Examples provided of strenuous activities included aerobics, aerobic dancing, jogging, tennis, and swimming laps. Women were asked similar questions about moderate-intensity and mild physical activities. Examples provided of moderate-intensity activities (4.5–5.0 METs) included biking outdoors, using an exercise machine, calisthenics, easy swimming, fairly fast walking, and popular or folk dancing; for mild activity (3.0 METs), examples included slow dancing, bowling, and golf. Using a standardized classification of PA intensity, EE was summarized in MET-minutes per week (MET·min·wk−1) computed as the summed product of frequency, duration, and intensity for reported activities.
On the basis of the questions asked, women were classified into four groups of PA levels: sedentary, i.e., those who engaged in ≤100 MET·min·wk−1; low activity, >100–500 MET·min·wk−1; moderate activity, >500–1200 MET·min·wk−1; and high activity, >1200 MET·min·wk−1. Achieving >500–1200 MET·min·wk−1 broadly is equivalent to engaging in at least 150 min·wk−1 of moderate-intensity PA, which is the minimum health-related dose of activity recommended by the federal government.
Baseline descriptive characteristics were compared among the sedentary, low, moderate, and high PA categories using linear models. For each baseline covariate, a linear trend variable of exercise category (0 = sedentary, 1 = low, etc.) was modeled as a function of the covariate of interest, with the P value from the resulting type 3 test used to determine statistical significance for the covariate presented. Absolute changes in body weight, BMI, and WHR were calculated by taking the difference from baseline at each time point. Multivariable analyses were conducted to examine the effect of PA on change in body weight, BMI, and WHR over time, using a repeated-measures linear regression to account for the correlation of within-woman repeated observations. Each of the three models modeled change over time (dependent variable) as a function of baseline PA and was adjusted for baseline variables of age, ethnicity, education, weight, BMI, WHR, total energy intake, percentage energy from saturated fat, smoking, alcohol use, fiber intake, fruit/vegetable intake, grain intake, average hours of sleep per night, hormone use, and physical function level. For each of these outcomes, unadjusted means are presented by year of follow-up (Figs. 1–3). Results are presented with β coefficients from the model, 95% confidence intervals (CI), and the corresponding P value for the PA variable.
Subgroup analyses were performed to determine whether the relationship of PA to changes in body weight and fat distribution varied by age (50–59, 60–69, and 70–79 yr); obesity status (by BMI (kg·m−2) category), e.g., normal weight (18.5 to <25), overweight (25 to <29.9), obese (≥30); WHR by quartiles (<0.767, 0.768–0.813, 0.813–0.865, >0.865); or physical function (construct score ≤70, 71–90, >90). In addition to the two-way subgroup models, a subset analysis was conducted to examine the three-way interaction of baseline PA level, age, and BMI in relation to postbaseline changes in body weight to determine variation by age/BMI combinations. All models were adjusted for age, ethnicity, education, weight, BMI, WHR, total energy intake, percentage energy from saturated fat, smoking, alcohol use, fiber intake, fruit/vegetable intake, grain intake, average hours of sleep per night, hormone use, and physical function level. These covariates were chosen on the basis of past analyses conducted within the WHI cohort and knowledge of potential associations with age-related body weight changes. Analyses were performed using SAS 9.0 (SAS Institute, Inc., Cary, NC).
Table 1 presents study sample characteristics for selected baseline sociodemographic and lifestyle factors by PA group. Twenty-six percent of study participants were in the sedentary group, 31% (nearly one-third) of the participants were in the low PA group, 26% were in the moderate PA group, and 17% were in the high PA group. Compared with the moderate and high PA groups, a greater proportion of women in the sedentary and low categories were obese, were current smokers, reported being treated for diabetes, and were distributed in the highest quartiles of WHR, total energy intake, and energy intake from saturated fat. Small but statistically significant differences in the distribution of sleep duration, hormone therapy use, and oral contraceptive use were seen between women in the lowest versus the highest activity groups. The mean number of postbaseline clinic visits was 6.1 ± 2.3 with no differences in the number of follow-up examinations among activity groups (5.9, 6.1, 6.3, and 6.3 for sedentary, low, medium, and high, respectively).
Change in body weight and fat distribution
As previously reported (13), an overall trend for weight gain was observed in women age 50–59 yr (mean gain = 1.3 kg) as well as those age 60–69 yr (mean gain = 0.5 kg), whereas women age 70–79 yr lost weight (mean loss = 0.75 kg) across the 8-yr study period. As shown in Figure 1, these age-related differences in weight change did not differ by PA group except in the 70- to 79-yr group, in which the women reporting >1200 MET·min·wk−1 of activity had less weight loss as compared with the other three MET-minutes-per-week groups. Changes in postbaseline annual measures of WHR and BMI are shown according to baseline age and PA level in Figure 2 and 3. Among women age 50–59 and 60–69 yr, BMI increases (Fig. 2) did not differ significantly among PA groups during follow-up. Among women age 70–79 yr, weight loss and decreases in BMI were less in participants reporting >1200 MET·min·wk−1 of activity compared with the less active groups (Fig. 2). During the 8 yr of follow-up, waist-to-hip ratio tended to increase, regardless of age or PA subgroup, and this increase in the WHR (mean = 0.015) in women age 70–79 yr occurred despite their tendency toward weight loss, suggestive of increased central adiposity (Fig. 3).
Effects of PA on changes in body weight and fat distribution
In the fully adjusted multivariable repeated-measures model of anthropometric change by baseline PA (Table 2), there were trends to indicate differences in PA in relation to weight changes. Less weight gain (P = 0.08) was observed both in those women expending >500–1200 MET·min·wk−1 (coefficient = −0.06, 95% CI = −0.19 to −0.07) and in those expending >1200 MET·min·wk−1 (coefficient = −0.10, 95% CI = −0.25 to −0.06) compared with the group expending ≤100 MET·min·wk−1.
In the fully adjusted multivariable repeated-measures model of weight change by PA by baseline age, BMI, and WHR (Table 3), there was a significant interaction (P < 0.001) of age and PA in relation to weight changes from baseline. Among women age 50–59 yr, there was significantly less weight gain in those expending >500–1200 MET·min·wk−1 (coefficient = −0.30, 95% CI = −0.53 to −0.07) and >1200 MET·min·wk−1 (coefficient = −0.46, 95% CI = −0.73 to −0.21) compared with those expending ≤100 MET·min·wk−1. Among women age 70–79 yr, higher PA (>1200 MET·min·wk−1) was associated with significantly greater weight gain (coefficient = 0.34, 95% CI = 0.04–0.63). Associations between PA levels and body weight changes were not modified by baseline BMI (interaction P = 0.31), WHR (interaction P = 0.07), or physical function (interaction P = 0.18) (Table 3).
To further examine the interrelationship of baseline PA, age, and BMI with weight changes during follow-up, a test of interaction was conducted among these exposure variables, revealing no evidence of interaction (P = 0.423).
We report here that habitual PA levels are significantly associated with previously reported differences in body weight changes by age in postmenopausal women (age 50–79 yr). We are thus extending findings from a subset of this large ethnically diverse cohort of postmenopausal women who showed differential body weight changes by baseline age, with women age 50–59 yr gaining weight, those age 60–69 yr maintaining weight (through an early weight gain and a later weight loss), and those age 70–79 yr having a net weight loss during 7 yr of follow-up (13), to report the influence of PA on weight changes over time, with an additional year of follow-up in a larger cohort. Specifically, greater levels of habitual PA attenuated the weight gain in women age 50–59 yr and, to a lesser extent, in women age 60–69 yr; on the other hand, among older women age 70–79 yr, higher activity levels attenuated weight loss. In our analysis, neither baseline BMI nor WHR modified the association between PA and body weight changes.
Recently, Lee et al. (19) investigated prospective weight and PA data of the Women’s Health Study (11) and determined that PA was associated with less weight gain only in women with a BMI lower than 25. Moreover, to attenuate weight gain during the 13-yr follow-up, women on average participated in ∼60 min·d−1 of moderate-intensity activity (equivalent to the >1200 MET·min·wk−1 of the current study). The study by Lee et al. also demonstrated that higher levels of PA were associated with less weight gain only among women age <65 yr, consistent with the results presented here.
Novel findings of the current study are that weight changes with habitual PA differ depending on age and that regardless if the change is weight gain or loss, greater MET-minutes per week of PA attenuated weight changes across age groups. With greater MET-minutes per week of PA, maintenance of weight (or an attenuation of weight gain) across the 50- to 69-yr age groups and an attenuation of weight loss in the ≥70-yr age group were observed. Achieving >500 to 1200 MET·min·wk−1 broadly is equivalent to engaging in at least 150 min·wk−1 of moderate-intensity PA, which is the minimum health-related dose of activity recommended by the federal government (26). Achieving >1200 MET·min·wk−1 is approximately equivalent to engaging in at least 400 min·wk−1 of moderate-intensity activity, which is recommended for weight management by the Institute of Medicine (25).
Our findings suggest that promoting PA among postmenopausal women may be important for managing body weight changes associated with aging. As reported in other studies (1,3,19,37), we found that higher activity levels protect against weight gain in middle-age women even after accounting for differences in other factors that influence weight changes. This is important given the propensity for women during the menopause transition to experience weight gain, particularly through abdominal fat accretion, and an associated increased risk of several chronic diseases (4,17). In a shift of popular thought, higher baseline PA was associated with less weight loss in this study’s cohort of older postmenopausal women age 70–79 yr. With this, the issue of lean mass becomes an important one. Lean muscle mass is known to be metabolically active, which may contribute to less overall weight gain in the younger cohorts of this study. Habitual PA in the older cohort may preserve lean muscle mass, (presumably) attenuating weight loss, and may prevent a loss of physical function in this older cohort, a concept that has been reported by other investigators (6,13,15,32). Loss of lean body mass accounts for a substantial amount of weight loss at older ages (27) and gives rise to the development of sarcopenia and associated clinical manifestations. Sarcopenia is a proinflammatory catabolic condition (18) predisposing older women to loss of skeletal muscle function (8,36) that likely contributes to accelerated declines in functional capacity (7) and increased mortality. Thus, prevention of weight loss and preservation of lean body mass through active lifestyles may be an important component in lowering the burden of disability and mortality in older postmenopausal women.
Other observational studies on PA and weight gain in women have a reported effect modification by baseline BMI levels (1,19). Findings by Blanck et al. (1) in postmenopausal women and by Lee et al. (19) in a cohort of mostly postmenopausal women indicate that PA protects against weight gain only in women who were not overweight or obese at baseline. We did not observe a significant interaction between baseline BMI, WHR, and activity levels in relation to weight gain. Women who were obese at baseline and who reported the highest EE experienced significantly lower weight gain than their less active obese peers, but this effect was constrained only to women age 50–59 yr. Although the absence of a significant interaction between activity, baseline BMI, and WHR in relation to body weight changes in our study is consistent with findings reported by Sternfeld et al. (37) in a large cohort of middle-age women, this issue requires further investigation in women across a broad age range.
Strengths of this present study include the prospective design, the large size, the diversity of ethnicity and socioeconomic status of the cohort, the detailed assessment of PA as well as sedentary behavior, long-term follow-up, and high retention rates. Several limitations of the present analyses deserve mention. Despite the fact that we controlled for a large number of potentially confounding variables in our multivariate analyses, residual confounding by lifestyle-related factors cannot be excluded. Self-reported PA is an additional limitation; although the PA questionnaire used has shown good reliability and validity (24), misclassification through self-report may affect the accuracy of estimates in activity. The magnitude and consistency of the relations between PA and weight across the variety of analytical approaches used here suggest that confounding by diet is not a likely explanation for the findings (29). An important limitation of these analyses is that no body composition measurements were conducted to differentiate fat mass from fat-free mass; therefore, we cannot assess whether PA is preventing fat mass gain and/or lean mass loss differentially by age group. In addition, adjusting for the intensity and mode of exercise to demarcate resistance training from yoga-type and lower-intensity aerobic exercise was not feasible because of the nature of the data collection. Although research indicates that moderate-intensity activity of approximately 45–60 min·d−1 attenuates fat gain and promotes lean mass development in older postmenopausal women (1,15,27,37), research supports resistance training and moderate- to high-intensity exercise as a reasonable component of any strategy to lose weight (35).
In conclusion, the results of the present study highlight the paradox of weight and PA: in younger age groups, higher levels of habitual PA ameliorate the weight gain associated with aging; in the older cohort, higher levels of habitual PA reduce the unintentional weight loss due to muscle and bone loss associated with increasing age; both findings may improve health outcomes in the later years of life.
The Women’s Health Initiative (WHI) program is funded by the National Heart, Lung, and Blood Institute, National Institutes of Health, US Department of Health and Human Services, through contracts N01WH22110, 24152, 32100-2, 32105-6, 32108-9, 32111-13, 32115, 32118-32119, 32122, 42107-26, 42129-32, and 44221. A listing of WHI investigators can be found at http://www.whiscience.org/publications/WHI_investigators_shortlist.pdf.
The authors declare no conflicts of interest.
The authors thank the WHI investigators and staff for their dedication and the study participants for making the program possible.
The results of the current study do not constitute endorsement by the American College of Sports Medicine.
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