Is exercise enough to improve the metabolic syndrome?

Metabolic syndrome is defined^[1] as having at least three of five clinical risk factors that are shown in Figure 1: elevated fasting glucose, abdominal obesity, high blood pressure, elevated triglycerides, and reduced high-density lipoprotein (HDL). The prevalence of metabolic syndrome is increasing worldwide^[2], and an estimated 33% of adults in the U.S.^[3] now meet the criteria for metabolic syndrome. This represents an enormous public health challenge because metabolic syndrome is associated with a 50% increased risk of developing cardiovascular disease^[4] and at least a twofold risk of developing type 2 diabetes^[2].

Exercise has the potential to improve metabolic syndrome and reduce the risk of developing more severe diseases like heart disease and type 2 diabetes. Previous meta-analyses have shown exercise to benefit people with metabolic syndrome^[5] and patients with type 2 diabetes^[6]. However, as these analyses used studies which involved a combination of diet and exercise, the contribution of exercise per se to producing these benefits is uncertain. In addition, most studies included in these meta-analyses utilized endurance exercise interventions. At least one meta-analysis^[7] of resistance training has suggested it may benefit blood pressure but not other criteria of metabolic syndrome.

Different types of exercise and levels of exercise intensity may also have different effects on the metabolic syndrome. The above-mentioned meta-analysis in participants with type 2 diabetes suggested that there was no difference between high-intensity and low- to moderate-intensity exercise for changes in glycemic control. Another meta-analysis comparing^[8] high intensity interval training to steady state medium intensity was unable to find enough evidence to draw any conclusions around the most effective exercise intensity. Thus, how exercise type and intensity impacts factors of the metabolic syndrome remains unclear.

In the study under review, the authors set out to evaluate the health effects of exercise intensity and type in people with metabolic syndrome by conducting a meta-analysis of randomized, controlled trials.

Metabolic syndrome is an increasingly prevalent collection of risk factors for cardiovascular disease and diabetes. Exercise has been shown to improve the health of people with metabolic syndrome, but the role of exercise type and intensity remains uncertain. However, previous reviews of the evidence, which included both exercise only and exercise plus diet interventions, have not isolated the role of exercise. This study pooled the existing experimental evidence from previous trials to investigate the role of exercise alone in improving metabolic syndrome.