This meta-analysis provides evidence of the efficacy of an LMI in resolving the proportions of patients with MetS in comparison with conventional education. The proportion of patients with resolved MetS in the intervention group was approximately two times higher in the intervention group than in the control group. Values for five of the six components of MetS (excluding HDL) were significantly reduced in the LMI groups compared with their control groups. Several sensitivity analyses supported these results.
The major goals for subjects in the intensive LMI groups included achieving and maintaining a weight reduction through diet and moderate-intensity physical activity. Most of the control group received only standard lifestyle suggestions. The results of the previous meta-analyses for exercise performed by Carroll and Dudfield  were 1.8 mg/dl (95% CI 1.1 to 2.6 mg/dl) for HDL (20 RCTs) and -19 mg/dl (95% CI -26 to -13 mg/dl) for triglycerides (19 RCTs). Because the effect size changes between the study groups were homogeneous, the authors used a fixed-effects model (weighted mean difference method) for the estimation of an overall effect size. The pooled estimate of the effect size was small but significant for both HDL and triglyceride. Our meta-analysis suggested an effect size of 1.3 mg/dl (95% CI -0.6 to 3.1 mg/dl) for HDL and -12 mg/dl (95% CI -22 to -2 mg/dl) for triglyceride by the random-effects model, and 2.9 mg/dl (95% CI 2.5 to 3.3 mg/dl) and -19 mg/dl (95% CI -20 to -17 mg/dl), respectively, by the fixed-effects model. The differences between the two meta-analyses were the type of intervention and study duration. Namely, in our analyses, the LMIs comprised 'exercise and diet' or 'diet only', whereas the in the studies assessed by Carroll and Dudfield, the LMI was 'exercise'. Our meta-analysis required a duration of '6 months or over, median 1 yr', which was longer than the '12 to 52 weeks' required in the meta-analysis by Carroll and Dudfield. Although we cannot deny possible bias, dietary lifestyle intervention is possibly a more effective method than exercise intervention.
A previous meta-analysis of the Mediterranean diet using epidemiological studies  found a beneficial effect of the Mediterranean diet on individual components of MetS, including waist circumference, HDL, triglycerides, systolic and DBP, and glucose metabolism. However, it should be noted that in that meta-analysis of the Mediterranean diet, the subjects included high-risk participants. In our analysis, we used only RCTs for patients with MetS, and used several models for analyses, such as sensitivity analyses, because of the heterogeneity of the studies. Nevertheless, our results suggested that the Mediterranean diet and general lifestyle modification, including dietary education, were beneficial.
Strengths and limitations
To our knowledge, this is the first study employing a meta-analysis of RCT s that has examined the effects of lifestyle modification for individuals with MetS. However, the educational training used was not uniform across the studies. The strengths of our study were that it analyzed only RCTs and assessed the magnitude of the effects primarily according to the relative proportion of patients with resolved MetS, and by the difference in mean changes in individual MetS components. The results suggest that lifestyle modification was more likely than conventional education to result in resolution of MetS. Although we found a weak tendency toward reduction in fasting blood glucose, we consider that fasting blood glucose is not a sensitive index for glucose intolerance. In our experience from a previous study on the effects of lifestyle modification in preventing T2D, a significant effect was shown in the level of plasma glucose 2 hours after an oral glucose load of 75 g, but such an effect was not shown for fasting blood glucose . To confirm whether fasting blood glucose level is a good index as the component for glucose intolerance, further evidence is required.
Certain limitations of this study should be considered. The first is publication bias. From the visual observation of sample size on effect size (figure not shown), the result did not seem to be greatly affected by sample size. We performed electronic searches and a hand search. However, this study used a small number of studies with high heterogeneity, thus a funnel-plot assessment may be not appropriate to examine the publication bias. This analysis was confined to English-language articles, which could also introduce publication bias. However, Moher et al. found that language-restricted meta-analyses overestimated the treatment effect by only 2% on average compared with language-inclusive meta-analyses, although the language-inclusive meta-analyses were more precise. Furthermore, we included only RCTs in the present meta-analysis, which could also introduce bias. However, considering that the quality of studies of lifestyle and dietary education may be affected by many confounding biases, these limitations may be acceptable. Publication bias is always a concern in meta-analyses, and although it may be small, we cannot deny the possibility of such bias. We assessed the studies based on the information reported in the papers mainly for the elements of risk of bias table in the relevant Cochrane review . Although the power was low, the significance levels in the subgroup analyses were similar to those of the primary analyses.
A second limitation relates to study duration. We collected data only from studies with a follow-up period extending for more than 6 months. This may be acceptable, however, because earlier assessments could be biased as a result of changes made only because subjects were conscious of being studied. The duration of follow-up in the included studies was 6 months [20, 22, 23], 1 year [16, 18], 2 years , and 3 years [17, 19]. In the prevention and resolution of MetS, maintaining long-term control is likely to be warranted .
A third limitation is the variability in the LMIs. Lifestyle modification was not uniform, and varied depending social circumstances, therefore heterogeneity might be accepted. In most trials, the control diet was the subject's usual diet, whereas the lifestyle modification included some special diets such as the Mediterranean diet [17, 21] and DASH diet , among others, and most of the studies included recommendations for exercise. Because of this heterogeneity, we used the random-effects model as the primary analysis. Although the quality and content of lifestyle modification varied, the results indicated that it was effective. Because the number of studies was too small to perform difference-by-subgroup analyses, we could not conduct these analyses by intervention styles. Furthermore, even though our search method used systematic review and added hand searching, we could have inadvertently missed eligible studies. The results should be interpreted carefully, considering the risk of bias across studies.
Implications for practice and research
Many of the changes noted in this review were significant and improvements were observed, but the absolute values were small in terms of magnitude. It should be emphasized that these outcomes measured are surrogate outcomes. Important patient outcomes such as incidence of CHD and T2D should be investigated in future studies.
As the results of the present study show, there is a need for adequate assessment of lifestyle modification for resolving MetS. Future research evaluating lifestyle modification with and without weight loss, lifestyle modification of diet versus aerobic exercise versus combination therapies, lifestyle modification versus medicine, and other comparisons is needed to clarify which aspects and what degree of lifestyle modification are best to resolve MetS.