One major finding in this study was that, compared to the inactive controls, moderate intensity exercise – at an amount calorically equivalent to walking approximately 17 km (10 to 11 miles) over an average of 170 minutes per week – resulted in a significant improvement in MS (). This is an important finding, as the data directly addresses the question of how much and what intensity of exercise is needed to obtain metabolic health benefits. Our findings clearly indicate that a modest amount of moderate intensity exercise is adequate for obtaining significant health benefits. This is an exercise prescription that is likely to be perceived by the general public and clinicians alike as an obtainable goal. Further, these data lend support to the 1995 CDC/ACSM position stand, which states “all adults should accumulate 30 minutes of moderate intensity activity most, preferably all, days of the week”8
. At the same time, this study also shows that for the group that did a higher amount of more vigorous exercise – an energy expenditure calorically equivalent to approximately 28 km (17 miles) of jogging over 170 minutes per week – greater and more widespread benefits were realized. The beneficial effect obtained by the high amount exercise group was not only significantly better than observed in the inactive control group (p<0.0001), but was also better than that experienced by the low amount/vigorous intensity group (p<0.01).
In addressing the current controversy of whether 30 or 60 minutes of exercise per day should be the national recommendation, experts commonly agree that: 1) physical inactivity is detrimental to long term health, 2) some exercise is better than none, and 3) more exercise, up to a point, leads to greater and more widespread health benefits. The data from the current report together with other published data from STRRIDE, add strong support to these previously already well founded concepts8–11
A somewhat surprising finding was that although the exercise at the lower intensity was found to be effective compared to the inactive control group, the same amount of exercise at a more vigorous intensity was not significantly different from the inactive group (). It is important to point out that this same amount of exercise at a lower intensity requires more total minutes per week, which generally leads to a greater weekly exercise frequency (). As a result, we cannot rule out that greater weekly frequency and/or duration are important factors in achieving the health effects studied here. We have observed similar responses for other health outcomes in the STRRIDE study. For example, we observed that the moderate intensity group was significantly better at improving insulin sensitivity than the same amount of exercise at a higher intensity12
. This same pattern is also evident in the triglyceride response (). Also, in the lower intensity group triglycerides (), the MS Z-score and insulin sensitivity index () are improved for both men and women (the improvement in Z-score just failed significance for women, p<0.07).
The exact mechanism whereby lower intensity exercise might confer greater health benefits than does higher intensity exercise is not clear. The same amount of exercise at a lower exercise intensity increases the percentage of energy coming from fat oxidation. Conversely, the higher intensity exercise group gets more of its energy from carbohydrate oxidation13
. While caution is certainly warranted and additional confirmatory studies are needed, taken together these data suggest that for metabolic health there may be some real advantages of moderate intensity exercise and the consumption of fat oxidation as a fuel.
While it is known that men and women differ in their clinical presentation of cardiovascular disease and risk factors, an additional purpose of this analysis was to examine the relationship between MS and exercise volume and intensity in men and women separately. Although men and women were recruited with the same criteria and the women were significantly older, it is clear that women generally displayed more desirable cardiovascular risk profiles at baseline. Of note, national survey studies report that MS prevalence is almost identical for men and women, at about 23%14
. The prevalence of MS in women from the present study is 34%, whereas 46% of men in this study met MS criteria at baseline.
Given these consistent, strong gender differences at baseline, attempts to study the differences statistically between men and women in their responses to exercise are likely to be complicated. When baseline MS Z-score (or ATPIII score) is added to the model, this variable is highly significant, indicating that baseline values explain an important part of the exercise response. These findings suggest that if there is a gender difference in the effect of exercise on MS, it is likely found in the fact that in this study the women had better risk profiles at baseline.
In addition to the randomized, controlled design, other major strengths of this study include: 1) verification of time and intensity of exercise - not just attendance - for nearly all training sessions; 2) carefully defined and controlled exercise amounts and intensities, 3) a significant proportion of women and minorities in the study sample. Two limitations should be noted. While the findings should be applicable to the general population, they may not be as appropriate for special populations, such as those with hypertension, diabetes, or cardiovascular disease. Due to practical reasons, we could not compare the effects of a high amount/moderate intensity intervention. It was felt that the amount of time necessary for this group (up to 8 hours/week for low fitness subjects) would seriously hinder recruitment efforts.
In sum, in a middle-aged, overweight to obese, at-risk physically inactive population of men and women, moderate intensity exercise consistent with the current ACSM/CDC exercise recommendations for health effects had a significant impact on metabolic syndrome in both men and women.