The exercise modes included cycle ergometer, treadmill, and elliptical trainers. There was an initial ramp period (gradual increase of exercise minutes and intensity) of 2 to 3 months, followed by 6 months at the appropriate exercise prescription. All exercise sessions were verified by direct supervision or by heart rate monitors that provided recorded data.
The STRRIDE study was designed to investigate the effects of different amounts and different intensities of exercise on metabolic risk factors for cardiovascular disease and diabetes. As mentioned previously, we failed to anticipate the speed and degree to which numerous health-related variables worsened in the inactive control group. In Table 1, we identify 12 variables that were shown to significantly deteriorate for only 6 months. These health-related variables were wide ranging and included, in addition to body weight, both general and specific measures of central obesity, carbohydrate metabolism, lipid metabolism, and cardiorespiratory fitness.
One of the important findings in the STRRIDE trial was that the inactive group gained a small but statistically significant amount of weight (approximately 1% body weight gain for 6 months), whereas all the three exercise groups lost weight in a dose-response manner in the absence of reduced caloric intake. These observations support a number of conclusions. First, the weight gain in the inactive individuals was almost certainly due to a small daily imbalance of caloric intake over expenditure. Unfortunately, this small caloric imbalance can contribute to weight gain that may occur at a relatively rapid rate (2% per year, as estimated from our observations), which would be anticipated to ultimately affect metabolic health. However, a small amount of exercise eliminated the weight gain seen in the inactive group, which lends support to the hypothesis that there is a minimal amount of physical activity required for adequate weight control and that, below this critical level of activity, weight gain occurs. This hypothesis was first proposed by Mayer et al. (10). The observation that all three exercise groups in STRRIDE lost a modest amount of weight supports this concept. A summary of the STRRIDE data in relation to exercise volume (amount per week) and weight loss or maintenance is presented in Figure 2. As indicated in this figure, the relationship between exercise amount per week and weight loss has an x-intercept at approximately 8 miles of walking or jogging per week or energy expenditure equivalent. To us, this suggests that this volume is the minimal level of activity that theoretically will produce weight maintenance in overweight and obese middle-aged sedentary men and women (13).
Recent findings from the Coronary Artery Risk Development in Young Adults study add strong support for the concept that weight maintenance is a reasonable surrogate for maintenance of metabolic health (14). In this study, the authors reported that weight gain for 15 yr was associated with unfavorable changes in numerous risk factors, whereas weight maintenance resulted in stable levels for fasting glucose, total cholesterol, LDL cholesterol, and HDL cholesterol, with only minimal increases in triglycerides (approximate increase of 6-10 mg/dL) and, in African Americans, a small increase in blood pressure (approximate increase of 7 mm Hg). Furthermore, they found that whether the group was normal weight or overweight at baseline, the beneficial effects of weight maintenance on cardiovascular risk factors were essentially the same.
It is important to interpret the relationship between volume of exercise and weight maintenance with the realization that the theoretical minimal amount of exercise is different for each individual. This is illustrated by understanding that if everyone in our study did the theoretical minimal amount of exercise (e.g., 8 miles·wk−1), some proportion of individuals would still gain weight, and some proportion would lose weight. In theory, this would be true for each of the variables that displayed a dose-response relationship.
So, how much exercise is enough to prevent inactivity-related metabolic worsening, weight gain, diabetes, and premature mortality? Is 30 min·d−1 enough as the 1995 U.S. Centers for Disease Control & Prevention/American College of Sports Medicine recommendation suggests, or is 60 min·d−1 the necessary amount as some national organizations suggest? How do we reconcile these vastly different public recommendations? One prudent approach would be to recommend that all adults aim for 30 min of moderate intensity activity each day and then let body weight changes be the surrogate measure for determining if this amount of activity is adequate. For individuals who still gain weight at this activity level without significant dietary changes, then perhaps increasing to 40-45 min of daily activity would be the next step. This approach is not novel but rather is similar to titrating pharmacologic agents (e.g., statins) for desired effect on clinical risk factors (e.g., LDL cholesterol). We offer that this individualized approach might be an important component of national activity recommendations that would greatly minimize the confusion surrounding the seemingly contradictory recommendations for physical activity levels required to maintain health and wellness.
It is increasingly evident that inactivity is unhealthy and that the detrimental effects appear to be occurring more quickly than previously realized. A modest amount of daily exercise (equivalent to the 1995 CDC/American College of Sports Medicine recommendations of 30 min a day of moderate-intensity exercise (11)) seems to be effective in most overweight and mildly obese sedentary individuals for preventing further inactivity-related metabolic deterioration. Therefore, this amount of daily activity would seem to be a very appropriate starting point for a national recommendation with the caveat that, on an individual basis, the adequacy of this amount could be judged based on its effect on body weight, which appears to serve as a reasonable surrogate measure for general metabolic health. In individuals who still experience weight gain at this dose, as with other medical therapies, the dose might be titrated to a level that is effective for prevention of further weight gain. Finally, as epidemiological research has previously suggested, results from STRRIDE demonstrate that modest increases above this minimal recommendation generally lead to additional significant and widespread improvements in numerous health measures.
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