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Introduction

    The modern society has undergone major change in lifestyle resulting from multiple changes in the economic, social, cultural and scientific coming of the industrial revolution, coupled with the advent of capitalism, in recent years. Despite these changes contribute to a better quality of life and longevity, they are strongly reflected in public health conditions, contributing to further development of chronic degenerative diseases, in particular the increasing of cardiovascular diseases (CVD)(1,4,5)

    CVD are the leading cause for morbidity and mortality in developed countries, underlining the rising rates in countries development process. In 2004, according to World Health Organization (1) CVD accounted for about 17.1 million deaths worldwide, and it is estimated that by 2030 the number of deaths from these chronic diseases are expected to increase by 23.6 million.

    Although it is obvious and urgent to develop strategies to combat CVD, such measures have collided with the difficulty in reducing their risk factors. The clustering of abnormal metabolic risk factors responsible for cardiovascular events, which represent about 30% of causes of death worldwide(3), has been generally referred to as metabolic syndrome (MS).

    The clustering of multiple risk factors was first reported in adults, but more recent investigation has shown that single risk factors as well as multiple-clustered risk factors have now been observed in children and adolescents (4, 6). The prevalence of MS is high among European obese children independently of the criteria used(3).

    MS is seen and recognized in public health as a major cause for health problems worldwide, which are associated with an increase risk of developing CVD, myocardial infarction, and type II diabetes. The most recognized indicators of CVD risk factors are hypertension, dyslipidemia, abdominal obesity, glucose intolerance and insulin resistance, as standardized by international standards (7, 8).

    Several studies have demonstrated that there is evidence of a direct relationship between physical inactivity and high prevalence of MS components (9,17)., despite the burden of genetic factors, the change in lifestyle, including nutritional habits and physical activity (PA) (5). In addition, several studies have reported that moderate​​-to-high cardiorespiratory fitness (CRF) level is a protective factor of MS.

    There is accumulating evidence that PA can have beneficial effects on the risk factors of CVD in children (21). Public health policy to promote PA in children, especially the most sedentary children, may be a key element to prevent the onset of CVD later in the children's lives.

    Therefore, the aim of this review was to summarize the latest developments with regard to PA and MS (or clustering of CVD risk factors) in children and adolescents in the last 5 years.

Methods

    This review consisted of a search of published literature in the English language. Studies were located by searching online databases (PubMed, Scopus and Web of Science) using the following keywords: "metabolic risk", "metabolic syndrome", "physical activity", "children" and "adolescents" and possible combinations. Studies were selected between May and June 2012 and the inclusion criteria were: (1) studies of national or international that report the relationship between MS and PA in children and adolescents, and (2) studies published between January 2008 and June 2012.

Results

    We identified 10 studies (table 1) that met the inclusion criteria for this study (10-20) which investigated the relationship between PA and MS. We observed that these articles were come from three continents (America, Europe, Asia) and seven different countries (UK(10,11), Spain(12), United States(13,14), Brazil(15), Denmark(16), Portugal(18) and Korea(19)). Some studies also adopted ethnic criteria for sample selection (11, 13,17), and two of studies come from USA, with populations of African Americans and other Latin, and one from UK, South Asian and black African-Caribbean. The sample of the different studies consisted of children and adolescents of both sexes. The number of participant range from 100 to 4450 aged between 6 and 18 years old

    Regarding to the characteristics of the studies, we found that the majority were cross-sectional studies, with a total of nine cross-sectional studies and only a 3 years longitudinal study (16), which is a study from the University of Southern Denmark, including 484 children aged six years of both sexes. The authors concluded that clustering of CVD risk factors developed between the age of six and nine years, and at nine years of age, clustered CVD risk was highly associated with low fitness level.

    Concerning the criteria used to determine MS, some studies used adaptations of cutoff values ​​suggested by international organizations or adjustments were made in previous studies while other studies have built a Z score of MS adjusted for sample characteristics.

    Regarding the assessment of PA levels different instrument were used such as questionnaires, accelerometers and pedometers. Three studies used Questionnaires (13,15,20), one study used pedometers (18) and seven studies used accelerometers (10,11,12,14,16,17,18,19).

    According to the studies selected for this review, we observed that it appears to be a strong relationship between PA and the metabolic risk factors. The Owen's study (11) provides strong evidence that low levels of PA are associated with increased cardio metabolic risk. In Hong’s study (19), it was found that PA was inversely associated with metabolic risk factors (β = -0185), and also concludes that PA is an independent predictor for the clustering of metabolic risk factors in children.

Table 1. Methodological characteristics and results of studies

Discussion

    The studies that examined the relationship between PA and MS showed, in general, inverse associations between the two variables, where children and adolescents more active have lower risks of developing MS(20,19,18,17,15,14,13,12,11) and high prevalence of MS and children physically inactive.

    The articles selected for the study, we observed that the results was expressed by odds ratio and other standardized regression coefficient for (β). Subjects with lower levels of PA were more likely to develop syndrome compared to subjects with higher levels of PA, and studies that showed the results by means of standardized regression coefficients (β) values ​​for the association between PA and MS varied greatly. Note that Casazza et al. found a positive (expected) between PA and HDL-C in the total sample (β = 0.18) and between Caucasians (β = 0.46) and between sedentary lifestyle and level of blood glucose the total sample (β = 0.22) and Hispanics (β = 0.38). For the portion of the sample african-american authors also found a negative association between PA and triglycerides (β = -0291).

    Bayle (23) has found in his research differences in cardio metabolic risk between physically active and physically inactive participants, according to previously proposed health-related threshold values, and between tertiles for physical fitness subcomponents. In this study the clustered risk was significantly lower (p < 0.001) in the fit (mean 1.21 ± 3.42) compared to the unfit adolescents (mean -0.74 ± 2.22), However, no differences existed between tertiles for any subcomponent of PA. Summarizing the authors say these findings suggest that CRF may play an important cardioprotective role in children and adolescents and highlights the importance of promoting CRF in adolescents.

    The Owen’s(11) Study showed that low levels of objectively measured overall PA are associated with increased adiposity and less favorable card metabolic risk profiles in children.. In this study it was found that similar increments in PA levels were associated with lower diastolic blood pressure (1.0 mmHg, 95% CI 0.6-1.5 mmHg) and LDL-cholesterol (0.04 mmol/l, 95% CI 0.01-0.07 mmol/l), and higher HDL-cholesterol (0.02 mmol/l, 95% CI 0.01-0.04 mmol/l). In addition, associations were broadly similar for strength in all ethnic groups. All associations between PA and cardio metabolic risk factors were reduced (albeit variably) after adjustment for adiposity.

    In the study by Martinez(12) showed us that VPA and MVPA were significantly related with CRF. And only CRF, and not patterns of PA, was inversely and independently associated with the three MS scores although with different magnitudes (ranged: β = 0.22 to 0.36, p < 0.05). In another study from Brazil(15) they found similar results. However, in some studies was not found a weak relationship between PA and MS. The different results may be related to the different methods used for collecting the information, with no unanimous criteria for determination of MS in children and adolescents, and the size and different sample characteristics.

    With regard to sample characteristics it was observed a wide variation both in size (between 100(10) and 4450(20), studies in PA / MS, and in the subjects' age (between 06 and 19 years). These differences can be identified as influential factors of the different results and the difficulty in making comparisons, since there is no consensus on the minimum age for diagnosis of MS in children.

    It is clear that both high levels of CRF and PA have a protective effect against the risk of MS, emphasizing the need for a healthy lifestyle in the prevention of CVD (12,13,14,15,19,20). This fact is supported by the significant impact of exercise on insulin resistance, since the trained muscles tend to be more sensitive to insulin (15, 16).

    The review of studies demonstrated the inverse association between high levels of PA and risk factors for MS. Despite the importance of the results, some limitations of this study should be mentioned: (1) the restriction to studies published since 2008, (2) the absence of unanimous criteria for determination of MS in children and adolescents, (3) the diversity of instruments used for data collection, and (4) the different statistical methods adopted for presentation of results.

Conclusion

    Starting from what was analyzed and exposed in this review, despite the ethnic, cultural and dimensional samples, the methodological differences between studies, as well as the inconsistency of the cut-off values ​​to determine the presence of MS in children and adolescents, the results relate to existence of a negative association between PA and MS.

    So with regard to the recommendations of the authors, future interventions designed to improve metabolic health in children and young people should aim to promote both PA and physical fitness. Objective and subjective measures should be used together to better capture the activity behavior.

References

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