This study tells us that aerobic exercise alone and combined aerobic and resistance exercise improves most parameters of metabolic syndrome. Both aerobic and combined exercise reduced waist circumference and systolic blood pressure. Aerobic exercise additionally reduced fasting plasma glucose, diastolic blood pressure, and triglycerides. Higher intensity exercise had a more favorable effects on some risk factors like blood pressure and HDL cholesterol compared to lower intensity exercise, but most changes were equivalent between intensities. Both types and intensities of exercise could reduce the risk of metabolic disease compared to sedentary control participants.
Importantly, however, some of the benefits, while statistically significant, were often small and may not be clinically meaningful. For instance, the 1.2 kg weight loss resulting from aerobic exercise, and none from combined exercise, didn’t reach the 3% to 5% weight loss considered clinically significant for health benefits. However, it should be kept in mind that the average duration of the studies was only 16 weeks; this leaves open the possibility of clinically significant weight loss if exercise is sustained over a long-term. Also, even small improvement, if sustained over the long term, could reduce the risk of developing more serious health problems such as diabetes.
While the weight loss was not very impressive, some of the other changes may indeed be more impactful. For instance, combined moderate intensity aerobic and resistance exercise improved waist circumference by almost four centimeters. This result may be clinically meaningful, since a reduction in waist circumference of over two centimeters has been associated with reduced incidence of the metabolic syndrome, as shown in Figure 3. The reduction in systolic blood pressure of 2.5 - 4 mmHg for aerobic and combined exercise may also be clinically significant; this drop is large enough to result in a roughly 10% decrease in CVD risk based on trials of blood pressure lowering medication; however this exact number should be taken with a grain of salt, since medications may impact CVD risk differently than exercise does. Similarly, increases in HDL-C in response to combined exercise and high intensity aerobic exercise were large enough to be associated with reduced heart disease.
However, the clinical implications just described remain mildly speculative, as the metabolic syndrome is essentially a related cluster of risk factors for metabolic diseases; it’s not really a “disease” in itself. So, we can't know for sure how impactful exercise will be on actual diseases like CVD from these results alone. Even so, there exists a wealth of evidence that exercise prevents CVD, so it’s a pretty good assumption that the results from this study would lead to real benefits. The size of benefit is somewhat more in question, though.
Much of this evidence suggests that activity and cardiorespiratory fitness, while interlinked, independently relate to CVD risk. Thus, it is notable that VO2peak was significantly improved in both aerobic and combined exercise programs. This improvement also confirms that the participants were taking part in the exercises that were improving their cardiorespiratory fitness. The improvements in VO2peak were greater than 1 MET (3.5 ml/kg/min), which is considered to be clinically significant.
Overall, comparisons between aerobic only and combined training were not statistically significant for any outcome. However, the magnitude of improvement was notably greater in some outcomes with combined training, such as with waist circumference (-3.8 vs -1.4 cm) and VO2peak (+4.6 vs +3.0 mL/kg/min). More research investigating combined training and comparing its effectiveness directly to aerobic training only is necessary before strong conclusions about superiority (or the lack of) can be made. The fact that combined exercise did not show as large an effect in some other measures as aerobic exercise may be due to the smaller number of studies. Since only four studies utilized combined training, it is possible that a failure to detect significant differences was a result of insufficient statistical power. More research down the road would lead to higher sample sizes that may be able to reveal a difference. Also, since studies analyzing the effects of resistance training alone were completely disregarded, it is also difficult to estimate the individual contributions of the resistance and aerobic exercise components to the previously described changes.
The amount of exercise the participants took part in is an important consideration. However, in this study, neither weekly nor total exercise time significantly affected the outcomes measured. Trials involving aerobic training alone used between three to five sessions per week, for six to 52 weeks, with individual sessions lasting 30-60 minutes each. Trials involving combined exercise used between two to three sessions per week, for 12 to 52 weeks, with sessions of between 40 and 75 minutes. This suggests that this amount of exercise takes a long time to achieve benefits. Plus, since the longest trial was a year, this leaves open the question of whether improvements would continue to be seen past this mark, or whether the moderate improvements here will top out with time. Further longer-term research is needed in order to address this question.
An important limiting factor that was not included in this meta-analysis is how well the participants in these studies adhered to the exercise regimes. There is currently little agreement on how adherence should be measured in studies such as these. Adherence involves several factors beyond just attendance. For example, participants may complete the exercise sessions but not exercise at the required intensity or duration, which can be difficult to measure. Ensuring adherence using heart rate monitors results in greater improvements in health than when adherence was defined as session attendance alone. Differences and inaccuracies in measures of adherence may have resulted in larger effects in this meta-analysis.
One final limitation to mention is publication bias. The analyses of publication bias from this meta-analysis suggested that some studies with negative results may not have been published, although an exploration of how this could impact the authors’ conclusions were not presented in this paper. This lack of published negative results can make the interventions used, such as exercise, appear more effective than they are.
This meta-analysis supports the notion that both aerobic and combined aerobic and resistance exercise is useful for individuals with metabolic syndrome. The small number of trials included prevent any definite conclusions as to whether aerobic only or combined exercise is more effective and whether there is an optimal level of exercise intensity. Most importantly, however, the overall size of the some of the beneficial changes resulting from increased exercise were moderate and their clinical significance is not completely certain.