In this survey, we observed that Chinese boys were more active than girls while physical activity and female were associated with low cardiovascular risk. After adjusting for known effects of puberty on cardiovascular risk factors, increased physical activity remained independently associated with low risk score.
The beneficial effect of regular exercise is possibly mediated through its direct effect on the circulatory system, improvement of sensitivity to insulin and adrenalin, increased non-insulin-dependent glucose uptake, improved oxidative enzymes in fat and carbohydrate metabolism . In adults, regular and accumulated physical activities have been shown to prevent premature death and other adverse health outcomes . In a large-scale, population-based cohort of Japanese adults aged 45-74 years, subjects in the top quartile of daily total physical activity level had lower risk of all-cause mortality than those in the lowest quartile .
In the youth population, association between physical activity and cardiovascular risk remains relatively unexplored. In support of our findings, in the United States, high physical activity level (≥60th percentile in the study cohort) is associated with low prevalence of metabolic syndrome in adolescents . In addition, Chinese girls had lower cardiovascular risk than boys after adjusting for pubertal staging and physical activity. In this regard, there are major gender differences in lifestyle, levels of physical activity, energy expenditure, body fat composition and hormonal levels. These differences may contribute to gender-related predisposition to health problems such as higher risk of hypertension, dyslipidemia and central obesity in men and higher risk of psychological problems such as depression and insomnia in women . In a review consisting of 38 prospective cohort studies from Medline search (1966-2000) conducted primarily in the United States and Europe, women adhering to guidelines for physical activity had lower all-cause mortality rate than men with similar degrees of physical activity , suggesting that the protective effects of physical activity may be confounded by gender.
In this survey, only 21.5% of Chinese youth reported high level of exercise, a finding similar to the European survey in school children , with boys being more active than girls. These gender differences had also been reported in a Swedish study of 993 high school students aged 16-19 years surveyed by questionnaires . In another study involving 2185 European children aged 9 and 15 years in whom physical activity levels were measured by accelerometers, boys were more active than girls . In the European Youth Heart Study which included 1732 school children from Denmark, Estonia and Portugal (aged 9 year-old and 15 year-old), the authors reported graded and negative association between clustering of cardiovascular risk factors and physical activity. Based on these data, the authors recommended at least 90 minutes of daily physical activity in youth to prevent cardiovascular risk . However, in this study, only pre- and post-pubertal school children were included and did not address the potential effects of puberty on cardiovascular risk. In our survey, onset of puberty was associated with increased cardiovascular risk score probably due to hormonal changes and their effects on insulin resistance.
There are ongoing debates on the optimal frequency, duration and intensity of physical activity [1, 2]. Based on a systematic review of over 850 publications, school-aged youth were recommended to engage in at least 60 minutes of daily physical activity of moderate to vigorous intensity . Here, most guidelines recommend high level or vigorous physical activity in young and active individuals while moderate activity appears to confer most benefits in the majority of people especially amongst the older and less active individuals [1, 2]. In support of this notion, some studies have shown that while there was no association between mortality and light or low level of physical activity (<4 METs), vigorous or high level of physical activity (≥6 METs) was associated with increased mortality rate .
The first guideline for physical activity in the youth was published in 1988 by the American College of Sports Medicine which recommended that children and adolescents should have 20-30 minutes of vigorous exercise daily . In 2007, the Regional Office for Europe of the World Health Organization (WHO) recommended children and young people to have at least 60 minutes of moderate-intensity physical activity daily. The guideline also highlighted that moderate-intensity physical activity at least twice weekly would enhance and maintain muscular strength, flexibility and bone health .
Several limitations of this study need to be addressed. First, due to budget restrictions, we used questionnaire rather than objective assessment tools like accelerometer to measure physical activity level. Nonetheless, questionnaires are widely accepted in large-scale surveys with reasonable reliability for analysis. For the same reason, we only analyzed fasting plasma glucose and did not measure insulin level to estimate insulin resistance as reported by other authors . Second, we used cardiovascular risk factors to derive the risk score and long term prospective studies will be needed to confirm the validity of these risk scores (and associated physical activity level) in predicting cardiovascular diseases. Third, it might be difficult for children to recall physical activity levels over a long period of 12 months. However, such recall bias is likely to weaken rather than strengthen the risk associations.