The main findings of this meta-analysis suggest that in patients with the MetS, dynamic endurance training is associated with favourable effects on most cardiovascular risk factors related to the MetS, that is WC, HDL-C, SBP and DBP. Our results further demonstrate that in these patients dynamic endurance training also favourably affects other important cardiovascular risk factors including LDL-C, total cholesterol, BMI and \( {\dot{{V}}\text{O}}_{{ 2 {\text{peak}}}} \).
It is generally accepted that we need better and more affordable prevention and treatment strategies to improve wide-scale cardiovascular health outcome and to prevent the epidemic of MetS from reaching global proportions and straining public health and the economy [18]. Exercise is a key component in the treatment of patients with the MetS and in the prevention of CVD morbidity and mortality. The results of this study are in line with previous meta-analyses and extensive reviews that focused on the effect of exercise on single risk factors in populations with different cardiovascular risk factors [4–7].
First of all, we observed a significant improvement in measures of body composition including reduction in abdominal obesity, assessed by means of WC, body weight and BMI and a non-significant reduction in fat mass. Improvements in body composition characteristics are suggested to be associated with beneficial changes in lipids and lipoproteins through mechanisms related to insulin resistance [23, 24]. Indeed, the results of our meta-regression show a trend (p = 0.06) between changes in WC and HDL-C. In line with the meta-analysis of Kelley et al. [25], we observed significant increases in HDL-C and decreases in LDL-C and total cholesterol, without an effect on triglycerides. Wilson et al. [26] found that an HDL-C level between 41 and 46 mg/dL (1.06–1.19 mmol/L) for men resulted in a relative risk (RR) of all-cause death of 1.27 and HDL-C levels between 23 and 44 mg/dL (0.59–1.14 mmol/L) in an RR of 1.47 for women, compared with groups with values of more than 55 and 70 mg/dL, respectively. In our meta-analysis, HDL-C levels rose from 1.05 mmol/L to 1.13 mmol/L in a sex-mixed population, still showing non-optimal HDL-C plasma levels. However, it is stated in a meta-analysis of Kelley et al. [25] that a decrease of 1% in HDL-C would be associated with an increase in the risk for coronary heart disease of 2–3%. Assuming that the reverse is true, the approximate increase of 8% observed in our meta-analysis should decrease the coronary heart disease risk by 16–24%.
In addition, patients with the MetS do have type 2 diabetes or at least have an increased risk for developing it. Whereas plasma insulin levels remained statistically unaltered, we observed a trend towards lower values of mean plasma glucose following dynamic endurance exercise (p = 0.06). Earlier, Snowling and Hopkins [27] reported in patients with type 2 diabetes significant reductions in fasting glucose, HbA1c and insulin sensitivity after dynamic endurance training, with some evidence of small additional benefits resulting from combining endurance and resistance training. Moreover, it was suggested that the effects were somewhat larger for those with a more severe disease status, in particular for HbA1c. The lack of a significant effect on glucose-insulin dynamics in the current meta-analysis might be partly explained by the fact that the majority of participants were pre-diabetic but did not have diabetes. Indeed, the study of Balducci et al. [20] in patients with type 2 diabetes and the MetS did result in significant decreases in HbA1c after 12 months of exercise training, which confirms our suggestion. In addition, measuring plasma glucose and plasma insulin after an oral glucose tolerance test is often a better marker of changes in glucose tolerance after an exercise or dietary intervention. In the study of Watkins et al. [15], significant decreases in 2 h of plasma insulin were found in the dynamic endurance exercise group but not in the control group after 26 weeks, while there were no changes in fasting glucose and fasting insulin.
Following dynamic endurance training, a mean reduction in BP of 7/5 mmHg was achieved. These mean differences are statistically significant but more importantly, the magnitude of these differences is likely to be clinically relevant. Results from large, prospective intervention studies suggested that small reductions in resting SBP and DBP of 3 mmHg can reduce coronary heart disease risk by 5%, stroke by 8% and all-cause mortality by 4% [28].
Although the observed effect on each of the individual risk factors might seem small, when we combine all the effect sizes, that is a BP reduction to a mean of 131/82 mmHg, a decrease in WC to an average of 101 cm and for plasma glucose and triglycerides to a mean of 5.67 and 2.0 mmol/L, respectively and an increase of HDL-C to 1.1 mmol/L, a large number of patients would probably no longer be classified as having the MetS. Earlier, Katzmarzyk et al. [29] showed that 30.5% of patients (mean age 44.7 years) with the MetS at baseline, were no longer classified as having the MetS after a 20-week supervised dynamic endurance training programme. This reduction was mostly due to an improvement in triglycerides (43%), BP (38%) and WC (28%) whereas beneficial effects on HDL-C and plasma glucose were seen in fewer patients (16 and 9%, respectively). Similarly, Anderssen et al. [30] reported that 23.5% of 34 patients no longer had the MetS after an exercise intervention of 1 year. The only study included in this meta-analysis reporting on the number of patients with the MetS showed similar decreases with a 37.5% (from 8/8 to 5/8) and a 45% decrease (from 11/11 to 6/11) following 16 weeks of continuous and interval training, respectively [18]. Although it is difficult to quantify exactly the overall risk reduction associated with all observed changes, these results are compatible with an overall improvement of cardiovascular risk. Moreover, the results of this meta-analysis study are supported by studies that have examined the cross-sectional relationship between physical activity and the MetS. For example, in 1298 healthy police employees of the Utrecht Police Lifestyle Intervention Fitness and Training (UPLIFT) study, average physical activity intensity, average time spent at physical activity, physical activity volume and physical fitness, were each associated with reduced odds of the MetS [31]. Taken together, the results of these studies, and those from the present meta-analysis, reinforce the notion that physical activity is an important treatment option for the MetS, as stated in Katzmarzyk et al. [29].
Finally, low aerobic capacity has been shown to be a stronger predictor of CVD and mortality compared with other established risk factors [32]. The five studies that reported on the effect of dynamic endurance training on \( {\dot{{V}}\text{O}}_{{ 2 {\text{peak}}}} \) showed a significant mean increase of 6 mL/kg/min or 19.3%. An increase of one metabolic equivalent task (=3.5 mL O2/kg/min) in exercise capacity referred to an RR for all-cause death of 0.84 [33]. Earlier, physical activity and physical fitness have been shown to be inversely associated with the clustering of metabolic abnormalities [31]. With regard to physical activity, it seems that intensity and more specifically higher intensity is the main characteristic of physical activity determining its effect on the combination of CVD risk factors [31]. However, as physical fitness exerted greater effects [31], this suggests that in daily practice physical activity should preferably increase physical fitness to maximize its effect on the prevention of the MetS. This might most likely be achieved by exercise at higher intensity [31]. Given the lack of adequate reporting of training characteristics, we were unable to investigate the effect of intensity in this meta-analysis. A considerable number of additional studies will be required to adequately discuss this aspect.
With regard to the effect of other training characteristics on cardiovascular risk factors in patients with the MetS, more research is warranted to allow for appropriate exercise prescriptions. That is, only one intervention group investigated the effect of dynamic resistance training on the clustering of cardiovascular risk factors in individuals with the MetS and found that only WC was decreased significantly [21]. In a study of Banz et al. [34], with obese subjects, the effect of dynamic resistance training also resulted in significant changes in WC, but in no other significant beneficial cardiovascular effects. Smutok et al. [35] found significant changes in glucose and insulin responses after an oral glucose tolerance test was administered after a dynamic resistance training programme in individuals with two cardiovascular risk factors, but blood lipids and lipoproteins or BP outcomes didn’t improve significantly. It seems that dynamic resistance training has potential beneficial effects on the cardiovascular risk profile, but that dynamic endurance training is more beneficial [34, 35]. In a large meta-analysis of Cornelissen et al. [36], the effect of resistance training on BP and other cardiovascular risk factors was reviewed. They found positive effects on SBP, DBP and \( {\dot{{V}}\text{O}}_{{ 2 {\text{peak}}}} \), but no changes were seen in HDL-C, triglycerides and plasma glucose.
The effect of combined exercise on the clustering of cardiovascular risk factors in patients with MetS was discussed in two studies [20, 21]. Balducci et al. [20] concluded that combined endurance and resistance training had larger beneficial effects on upper body strength and body weight than dynamic endurance training only. No significant differences were found for the other outcome measures [20]. Stensvold et al. [21] found slightly larger benefits for the dynamic endurance training on SBP and DBP than that of combined exercise. The training responses were similar for WC, HDL-C, triglycerides and plasma [21]. However, more research is needed to investigate the potential benefits of combined training programmes.
The results of this meta-analysis should be interpreted within the context of its limitations: First, only one study fulfilling the inclusion criteria included a dynamic resistance intervention [21], and only two trials used a combined intervention of dynamic resistance training and endurance training [20, 21]. Further research about the effect of dynamic resistance training only or in combination with dynamic endurance exercise training is warranted in patients with the MetS. Second, a sensitivity analysis demonstrated that omitting the only trial in which all patients had type 2 diabetes [20], the effect of exercise on DBP, HDL-C and WC were no longer significant (p = 0.07; p = 0.44; p = 0.40, respectively). The severity of the MetS risk factors or a pathological state (type 2 diabetes) can probably influence the effects of exercise training, with more severe CV risk factors resulting in potentially larger improvements. Third, due to the small number of study groups, a subgroup analysis for sex or medication intake was not possible. More research is needed on the effect of variables that could potentially affect the outcomes of an exercise programme, especially the effect of sex and medication. Further, the mean age of participants was 52 years with a range between 46 and 64 years; therefore it may not be possible to generalize these results for younger or older individuals with the MetS. Finally, it has been suggested that one exercise dose does not fit for all components of the MetS and that recommendations should vary regarding the different risk factors [37]. Therefore, it is necessary to strike a balance for the most optimal exercise programme to improve the cluster of cardiovascular risk factors in patients with the MetS.