PA has many health benefits, and sedentary behaviors are associated with increased risk of many chronic diseases, including obesity and obesity-related diseases and decreased longevity (10
). Recent increases in obesity have been partly attributed to declining PA levels (20
). Although there is a consensus on the importance of PA to markers of health such as obesity, the actual amount of PA necessary for health promotion is less clear (21
PA recommendations have evolved from the amount of exercise required to improve or maintain cardiorespiratory fitness (22
) to a public health paradigm recommending that "every U.S. adult should accumulate 30 minutes or more of moderate intensity PA on most, preferably all, days of the week" (10
, p. 404). Irrespective of the uncertainty of the PA dose needed to promote health, the fact remains that as a society we practice sedentary behaviors. In an attempt to exchange sedentary behaviors for active ones, public health and clinical recommendations often start with the idea of parking farther from your destination or taking the stairs rather than the elevator. This advice implies that such active lifestyle choices, which may last <10 minutes, are of benefit to health. To our knowledge, no other nationally representative study has examined whether this strategy of accumulating PA in sessions lasting less than 10 minutes is as effective as PA accumulated in bouts lasting 10 minutes or more with relation to markers of obesity.
The principal finding from this study showed that accumulating MVPA in sessions lasting less than 10 minutes (nonbouts) was independently associated with lower BMI and smaller waist circumference even after controlling for other confounding variables, including MVPA accumulated in sessions of 10 minutes or more (bouts). Therefore, accumulating MVPA in nonbouts may be a beneficial starting point to increase PA levels and to lower BMI and decrease waist circumference. Bouts of MVPA had a significantly stronger association with both lower BMI and smaller waist circumference than MVPA in nonbouts. The stronger association reported for bout activity may be due to the greater average intensity of activity during bout versus nonbout minutes. People may essentially work harder during bout minutes than during nonbout minutes spread throughout the day. Thus, people accumulating MVPA in nonbouts throughout the day should probably work toward accumulating PA in bouts throughout the day, because this strategy was more time-efficient in decreasing BMI and waist circumference than was accumulating MVPA in nonbouts throughout the day.
After accounting for confounding variables, PA levels appeared more predictive of waist circumference than did BMI. Waist circumference and BMI are both markers of obesity. However, BMI provides an indicator of overall adiposity and waist circumference provides an indicator of abdominal fat. In addition, waist circumference is not greatly influenced by age, sex, height, or degree of overall adiposity, whereas the ability of BMI to predict overall adiposity is affected by age, ethnicity, body build, and frame size (23
). This difference could explain why PA appears more predictive of waist circumference than does BMI. Other studies have found similar results. In a randomly selected sample of adult men and women from the AusDiab study, accumulated daily steps were highly predictive of waist circumference (26
). Accumulated daily steps predicted BMI in men but not in women (26
). Other studies have found stronger relations between objectively determined PA and waist circumference than between objectively determined PA and BMI (27
Previous epidemiologic studies have investigated the effect of duration of exercise on health markers without using objective measures of PA. In the Harvard Alumni Cohort, participants were asked to recall walking, stair climbing, and participation in sports and recreational activities, while noting the frequency and duration of each sport and recreational activity (29
). After accounting for confounding variables, the duration of self-reported exercise did not have an independent effect on coronary heart disease risk.
Other intervention studies have compared the effects of accumulated bouts lasting ≥10 minutes to a single continuous bout of PA (ie, 2-3 bouts for 10 minutes compared with a continuous 30-minute bout of activity) (30
). This literature appears inconclusive. One intervention study reports similar improvements in BMI for the multiple short-bout groups and the single long-bout group (33
). Others report that only the longer (continuous 30-minute) exercise sessions were effective in improving markers of obesity (38
). Jakicic et al reported in a weight loss and weight maintenance comparison between 1 continuous bout of exercise and multiple short bouts of exercise that no significant differences between groups emerged in BMI or waist circumference after the 18-month intervention (37
). In contrast, Donnelly and colleagues compared the effects on BMI of 1 continuous 30-minute exercise session with 2 intermittent 15-minute exercise sessions (35
). After completion of the 18-month intervention, they found significant reductions in BMI in the continuous 30-minute exercise session group but observed no change in BMI for the intermittent exercise group (35
Our study has limitations. It is a cross-sectional analysis, so cause and effect cannot be determined. It is, however, representative of the national population, using strong objective measures of PA and measured markers of obesity. In the literature on methods of PA assessment, the most appropriate regression approach to demarcate PA intensity levels is much debated. We examined both time spent in MVPA, using published regression standards that accommodate both lifestyle and laboratory walking regressions to delineate PA intensity (16
), and the sum and average of raw ACC counts. From the methods we used, we could not delineate between different PA domains that would be informative to investigate (eg, the predictive capabilities of transportation activity vs household activity).
The accumulation of PA in nonbouts, a complimentary active lifestyle approach to accumulating PA in bouts, is predictive of lower levels of obesity markers. The concept of activity accumulation appears justified and represents a good stepping stone in getting inactive people to become more active. This concept should provide a strong impetus to sedentary people. However, activity accumulated in bouts lasting ≥10 minutes appears more predictive of lower levels of obesity markers, because a person's activity tends to be at a higher intensity when he or she participates in a sustained bout of PA. Accumulating activity in bouts lasting 10 minutes or more, corresponding to national public health recommendations, is potentially more effective for health promotion as indicated by its stronger predictive relation to obesity markers.