2. Physical Activity

Twenty-four RCT articles were reviewed for the effect of physical activity on weight loss, abdominal fat (measured by waist circumference), and changes in cardiorespiratory fitness (VO2 max). Thirteen articles were deemed acceptable (346, 363, 365, 369, 375, 401, 404, 406, 432, 434, 445-447). Only one of these RCTs compared different intensities and format with a control group, although the goal was to increase physical activity and not specifically to produce weight loss (401). Results from this trial were subsequently reported after 2 years, but these no longer included the control group (447). One additional study did not have a no-treatment control group but compared three active treatment groups with each other: diet only, exercise only, and combination exercise plus diet (448).

Most RCTs described the type of physical activity as cardiovascular endurance activities in the form of aerobic exercise such as aerobic dancing, brisk walking, jogging, running, riding a stationary bicycle, swimming, and skiing, preceded and followed by a short session of warmup and cool-down exercises. Some physical activity programs also included unspecified dynamic calisthenic exercises (363, 369, 406, 446).

The intensity of the physical activity was adapted to each individual and varied from 60 to 85 percent of the individual's estimated maximum heart rate, or was adjusted to correspond to approximately 70 percent of maximum aerobic capacity (VO2 max). The measure of physical fitness included VO2 max. The frequency of physical activity varied from three to seven sessions a week and the length of the physical activity session ranged from 30 to 60 minutes. Some physical activity programs were supervised, and some were home-based. Adherence to the prescribed physical activity program was recorded and reported in some studies and not mentioned in others. Most studies did not estimate the caloric expenditure from the physical activity or report calorie intake. The duration of the intervention varied from 16 weeks to 1 year; six articles reported on trials that lasted at least 1 year (346, 363, 375, 401, 406, 432).

Rationale: Twelve RCT articles examined the effects of physical activity, consisting primarily of aerobic exercise, on weight loss compared to controls (346, 363, 365, 369, 375, 401, 404, 406, 432, 434, 445, 446). Ten of the 12 RCT articles reported a mean weight loss of 2.4 kg (5.3 lb) (or 2.4 percent of weight) (363, 369, 375, 406, 419, 432, 434) or a mean reduction in BMI of 0.7 kg/m² (2.7 percent reduction) (346, 365, 401) in the exercise group compared to the control group. In three of these ten studies, the weight loss was < 2 percent of body weight (< 2 kg) (4.4 lb) (369, 375, 434). In contrast, two RCTs showed no benefit on weight from exercise, reporting weight gain in the exercise group compared to the control group (445, 446). In one of these studies, the control group received only diet advice but nevertheless lost 9 kg (19.8 lb), whereas the exercise group lost only 7 kg (15.4 lb) (445). In the second study, there was a total of only 10 participants, all having noninsulin-dependent diabetes mellitus, and the control group lost 3 kg (6.6 lb) whereas the exercise group lost only 2 kg (4.4 lb) (446). A meta-analysis of 28 publications of the effect on weight loss of exercise compared to diet or control groups showed that aerobic exercise alone produces a modest weight loss of 3 kg (6.6 lb) in men and 1.4 kg (3.1 lb) in women compared to controls (449).

Ten articles reported on RCTs that had a diet-only group in addition to an exercise-only group (346, 363, 365, 369, 375, 406, 432, 434, 445, 448). In every case except one (365),the exercise-only group did not experience as much weight loss as the diet-only group. The diet-only group produced approximately 3 percent, or 3 kg (6.6 lb), greater weight loss than the exercise-only group. 

No single study examined the length of the intervention in relation to the weight loss outcome. Only one study compared the effect on maximum oxygen uptake of different intensities and formats of physical activity over a 1-year follow-up (401) and 2-year follow-up period (447). Better adherence over 1 year was found if the exercise was performed at home rather than in a group setting, regardless of the intensity level. Subsequently, the different exercise groups were compared with each other over the longer term (2 years), and better long-term adherence was found in the higher intensity home-based exercise group compared to the lower intensity home-based or higher intensity group-based exercise groups (447). 

The question of whether physical activity enhances long-term maintenance of weight loss has not been formally examined in RCTs. Examination of long-term weight loss maintenance produced by physical activity interventions compared with diet-only interventions cannot easily be compared between RCTs because of numerous differences between studies with respect to design, sample size, intervention content and delivery, and characteristics of the study population samples. However, a number of analyses of observational and post hoc analyses of intervention studies have examined whether physical activity has a beneficial effect on weight. Cross-sectional studies have generally shown that physical activity is inversely related to body weight (450-454) and rate of weight gain with age (455). Longitudinal studies with 2 to 10 years of follow-up results have observed that physical activity is related to less weight gain over time (456-459), less weight gain after smoking cessation in women (460), and weight loss over 2 years (461).  In addition, post hoc analyses of several weight loss intervention studies reported that physical activity was a predictor of successful weight loss (454, 462, 463). The results of these RCTs showed that physical activity produces only modest weight loss and observational analyses from other studies suggest that physical activity may play a role in long-term weight control and/or maintenance of weight loss.

Rationale: Only three RCTs testing the effect of physical activity on weight loss also had measures of abdominal fat as assessed by waist circumference (365, 369, 375). One study demonstrated that physical activity reduced waist circumference compared with the control group (365), and another study showed a small effect on waist circumference (0.9 cm) in men but not women (375). One study in men showed a small increase in waist circumference (369). Weight loss was modest in all of these studies. These studies were not designed to test the effects of physical activity on abdominal fat independent of weight loss. 

However, large studies in Europe (464), Canada (453), and the United States (465-468) reported that physical activity has a favorable effect on body fat distribution. These studies showed an inverse association between energy expenditure through physical activity and several indicators of abdominal obesity, such as waist circumference and waist-to-hip and waist-to-thigh circumference ratios.

Rationale: Eleven RCT articles testing the effect of physical activity alone on weight loss in men and women also included measures of cardiorespiratory fitness, as measured by maximal oxygen uptake (VO2 max) (346, 363, 369, 375, 401, 404, 406, 432, 434, 445, 446). All 11 showed that physical activity increased maximum oxygen uptake in men and women in the exercise groups by an average of 14 percent (ml/kg body weight) to 18 percent (L/min). Even in studies with modest weight loss (< 2 percent), physical activity increased VO2 max by an average of 12 percent (L/min) to 16 percent (ml/kg) (369, 375, 434). 

One study that compared different formats and intensities of physical activity on VO2 max reported that improvement in VO2 max was related to adherence to the physical activity regime. In that study, the lower intensity program was equally effective on VO2 max as a higher intensity program, largely as a result of different levels of adherence (401).

The results of the RCTs strongly demonstrate that physical activity increases cardiorespiratory fitness in overweight and obese individuals.