Metabolic syndrome and quality of life: a systematic review 1

ABSTRACT Objectives: to present currently available evidence to verify the association between metabolic syndrome and quality of life. Method: Cochrane Library, EMBASE, Medline and LILACS databases were studied for all studies investigating the association with metabolic syndrome and quality of life. Two blinded reviewers extracted data and one more was chosen in case of doubt. Results: a total of 30 studies were included, considering inclusion and exclusion criteria, which involved 62.063 patients. Almost all studies suggested that metabolic syndrome is significantly associated with impaired quality of life. Some, however, found association only in women, or only if associated with depression or Body Mass Index. Merely one study did not find association after adjusted for confounding factors. Conclusion: although there are a few studies available about the relationship between metabolic syndrome and quality of life, a growing body of evidence has shown significant association between metabolic syndrome and the worsening of quality of life. However, it is necessary to carry out further longitudinal studies to confirm this association and verify whether this relationship is linear, or only an association factor.


Introduction
Metabolic Syndrome (MS), understood as a complex set of cardiovascular risk factors, related to abdominal fat accumulation and resistance to insulin, is strongly associated with high cardiovascular morbimortality (1)(2)(3)(4) , even when type 2 diabetes is not present (4) . The analysis of the Heart Outcomes Protection Evaluation -HOPE study corroborated this idea and adds that the increase in the risk is directly and progressively associated with the increase in waist circumference (WC) (5) . requiring the presence of AO as well as 2 or more criteria for the MS diagnosis, also proposing reduction in the WC reference values ≥94cm for men and ≥80cm for women, and glucose levels ≥100 mg/dl (3) .
The incidence level of MS has been increasing progressively in the last decades, estimating a prevalence of up to 23.7%, according to ATP III criteria when adjusted for age, according to a study carried out in the USA with a sample of 8.814 adults (6) .
Despite all progress made in understanding and treating MS, it is still an important public health issue.
Moreover, the study of the impact of MS on the quality of life (QOL) has been receiving little attention in medical literature and because of that, is still controversial and is not well understood. Our aim was to present currently available evidence for all studies investigating the effects of the MS on the QOL to verify the association between MS and the QOL.

Method
The databases searched were the Cochrane

Results
Although there are few studies in this area, most of them show association between MS and worsening in QOL (7)(8)(9)(10)(11)(12)(13)(14)(15)(16) , even more significant when regarding subjects who also have depression (17) . However, a cross-sectional study assessing 390 obese patients, out of which 269 filled MS criteria, showed that MS in itself was not associated with a reduction in QOL, but only showed significant correlation when associated with other factors, such as depression (18) .
A recent study involving 4.480 subjects revealed that the number of components diagnosed with MS was inversely associated with General Health, in both genders, although it was positively associated with Mental Health (7) . Another two studies (19)(20)  Similarly, cross-sectional studies reveal that this association between MS and QOL differs according to the gender (19)(20)(21)(22)(23)(24) .
According to the results of a cross-sectional study with 4.463 subjects of both genders, the decrease in QOL scores is directly proportional to the increase in the number of MS components in men as well as women, although this association is significant only in women (21) .
Likewise, two other cross-sectional studies with 950 and 2.264 subjects of both genders, respectively, also showed that this association between MS and decrease in the QOL scores was only significant in women (20,22) . smoking, physical activity, etc., this difference was also significant in women. This study also revealed that there were no differences for mental health or perceived stress between subjects with and without MS (23) .

Results of a Swedish
Similarly, even though another instrument for measuring QOL was used, a cross-sectional study with 9.570 men and women from Iran also showed association between QOL domains (social relation and physical health) and MS only in women, after adjusted for confounding factors (24) .
In addition, cross-sectional studies carried out with women (25) , with significant samples of 6.913 (26) and 6.805 (27) subjects respectively showed significant association between MS and worsening of QOL. Although some studies show this association, it only takes place in the Physical Health domain of QOL (26)(27) .
A randomized controlled trial with 201 obese women with 1 or more MS components, followed by 12-month, demonstrated that, after the intervention, the prevalence of MS decreased and the QOL scores increased in most domains in the group of intensive intervention, compared to the group of moderate intervention (29) .
According to data from another randomized controlled trial with 390 obese patients of both genders that showed at least two MS criteria, after the 6th month of intervention there was significant improvement in several QOL domains of the SF-36, and this association was more significant in women in the 24-month follow-up (28) .

Similarly, an intervention study conducted in
Brazil with a 9-month follow-up also showed significant improvement in QOL scores in most SF-36 domains, especially in the group of intensive intervention (30) .
Another two randomized controlled trials followed by 1 year, also demonstrated significant improvement in QOL, specially in Mental Health domains (31-32) .

MS showed not only beneficial changes in Mental Health
(social functioning), but also in Physical Health (general health and Physical component score) (33) .
A cohort of 1.785 subjects, showed that low QOL scores, in the physical health domain of the SF-36, were associated with MS and significantly predicted 5-year mortality (34) .
Similarly, another cohort study with 657 subjects, during 7 years, also showed that MS, anxiety and depressive symptoms are independent predictors of poorer subjective health and QOL. MS was associated with weaker self-rated health in men, but weaker perceived life satisfaction in women (35) . However, a cross-sectional analysis of a study with 361 subjects in two weight-loss programs revealed that, although an association between MS and low scores of QOL were found only in the physical health domain of the SF-36, this association was not kept after being adjusted for BMI, which means that this QOL worsening will be explained by the BMI increase and not by the MS itself (36) .
Nevertheless, only one cross-sectional study did not find significant association between MS and QOL after adjustments such as age, gender, smoking and so forth (37) .     All things considered, we note that the study of the relationship between MS and QOL, due to its relevance been receiving little attention in medical literature.
The present review has some limitations: the design of the studies, i.e., a high proportion of cross-sectional studies and the different HRQOL instruments used. The contribution of the present study was to draw attention to the effects that the MS can have on QOL, in an attempt to improve prevention and treatment strategies for MS, considering the fact that MS is still an important public health issue.