Cardiovascular disease risk factors have previously been reported in adults [1
] and in children and adolescents [2
]. Clustered risk factors have been found predominantly in adults, but the coexistence of several cardiovascular risk factors is now increasingly documented in schoolchildren from the age of nine [4
]. It has also been shown that clustering of cardiovascular risk factors in children is predictive of future cardiovascular disease [5
]. Clustering was observed in 10–15% of children [6
] and 7.1% of adolescents [7
]. This is of public health concern as clustered cardiovascular risk is reported to be a stable trait from childhood to adulthood [8
]. A review of the literature indicated that studies have consistently identified that clustering of cardiovascular risk factors were stable from childhood or adolescence to adulthood. Further studies concluded that clustering risk factors in an individual were a better and more sensitive method of assessing cardiovascular health in children [10
]. This refined method has resulted in researchers advocating for prevention programs targeting children from a young age, as they have the potential to reduce clustered cardiovascular risk factors [8
In order to develop effective intervention programs to reduce the progression of cardiovascular diseases, studies have identified factors that predict cardiovascular risk in children [12
]. Physical activity (PA) and cardiorespiratory fitness (CRF), for instance, are associated with a reduction in cardiovascular risk factors [13
]. Researchers have found that PA and CRF affect cardiovascular risk in different ways [14
]. For instance, weight status mediates the relationship between CRF and clustered risk, while the association between PA and clustered risk is not confounded by weight status [14
]. Further findings reveal that for children with low fitness levels, there are more chances of presenting with a clustering of cardiovascular risk compared to children with high fitness levels [2
]. Ultimately, physical fitness was found to strongly predict cardiovascular disease risk compared to body weight [7
]. This has major health implications, as only 50% of South African children meet the international recommendation of 60 min of PA per day [16
]. The evidence suggests that there are differences regarding PA [18
] and CRF levels [20
] between boys and girls. Specifically, girls seem to be less active, perform relatively poorly on fitness tests and participate in more sedentary activity compared to boys [18
]. The reasons for these gender differences might be related to the high motivation boys have to participate in PA [18
], and changes during puberty such as increased testosterone production and muscle mass, while girls tend to have increased fat mass [21
The goals of this cross-sectional study were threefold. First, to describe the cardiovascular health risk, PA behavior and CRF levels of school-aged children from disadvantaged neighborhoods in Port Elizabeth, South Africa. Second, to find out whether boys and girls, as well as younger and older children differ regarding cardiovascular health risk, PA behavior and CRF levels. Third, to examine whether independent associations exist between PA and CRF with a composite score of cardiovascular risk.
Informed consent was obtained from 1369 children, of whom 832 children had complete data sets on the cardiovascular risk markers, PA and CRF. The descriptive statistics of all independent and dependent variables for the final sample are presented in Table 1
. Kolmogorov–Smirnov and Shapiro–Wilk tests indicated that none of the dependent variables was normally distributed (p
> 0.05). Nevertheless, severe non-normality was only observed for diastolic blood pressure and triglycerides (accordingly, these variables were log-transformed). In the mixed linear regression analyses, MLR was used to handle non-normal distribution of dependent variables. Table 1
shows that nine participants had no information with regard to their sex. However, Little’s MCAR tests showed that data were missing completely at random, χ2
(df = 16) = 22.7, p
= 0.121, so FIML could be applied to impute missing data. Overall, only 24.2% of the study participants did not meet recommended PA levels, defined as <60 min MVPA per day.
indicates that children aged 9–13 years had higher HDL cholesterol and systolic/diastolic blood pressure levels compared to younger children aged 5–8 years. Younger children reported a higher ratio of total cholesterol to HDL. Older children were more sedentary and completed more laps in the shuttle run test and had lower LPA levels and lower VO2
max scores. Similar results were found among younger/older children not achieving recommended PA standards (χ2
(1832) = 1.3, p
= 0.259, 23.0% vs. 26.6%).
Girls displayed higher values for diastolic blood pressure, body fat, total cholesterol to HDL ratio, and LPA. Additionally, girls displayed a higher clustered cardiovascular risk score compared to boys. On the other hand, boys achieved higher scores for MPA, VPA, MVPA and completed more laps in the shuttle run test. No differences were found with regard to VO2max. The most substantial sex-related differences were found for MVPA, with 21.9% of explained variance. Girls were less likely to meet international PA recommendations than boys (χ2 (1823) = 95.7, p = 0.000, 38.3% vs. 9.2%).
displays information about whether CRF and PA indices reflected independent correlation with cardiovascular risk factors. After we controlled for MVPA, class-in-school, age and sex, higher CRF levels were negatively correlated with lower percentage body fat and lower clustered cardiovascular risk. A negative association was also found between estimated VO2
max and percentage body fat, triglycerides, and clustered cardiovascular risk. Higher sedentariness levels were correlated with higher percentage body fat and higher triglycerides levels, whereas higher LPA was associated with higher systolic blood pressure. MPA was negatively correlated with body fat percentage and total cholesterol. Finally, high VPA and MVPA were both associated with lower percentage body fat and lower clustered cardiovascular risk.
illustrates that when categorizing children into quartiles based on their estimated VO2
max or MVPA, a gradient of lower clustered cardiovascular risk between participants with higher estimated VO2
max (F(3817) = 2.89, p
< 0.05, η2
= 0.11) or MVPA (F(3817) = 2.89, p
< 0.05, η2
= 0.11) appeared.
This is one of few studies to investigate the clustering of cardiovascular risk and its association with PA and CRF in South African schoolchildren. Overall, our findings indicated that 24.2% of the study participants failed to meet the international recommendation of PA levels. We also observed that older children (9–13 years) presented with higher systolic/diastolic blood pressure, HDL levels, engaged in more sedentary activity and had lower LPA and VO2
max scores than their younger counterparts (5–8 years). Additionally, we found similar results for children not meeting 60 min of MVPA per day in both younger (23%) and older (26.6%) children. Consistent with the existing literature [34
], girls showed higher scores for most of the variables (diastolic blood pressure, percentage body fat, total cholesterol to HDL ratio and LPA) and they had a higher clustered cardiovascular risk score compared to boys. As expected, girls had a higher likelihood for not meeting the PA recommendations than boys, 38.3% vs. 9.2%, respectively. Moreover, higher CRF, VPA and MVPA were negatively associated with lower percentage body fat and lower clustered cardiovascular disease risk.
Our study found a positive result regarding PA participation, with 24.2% of the children not meeting recommended PA levels. Similar findings have been reported in children and adolescents (5–18 years) [35
]. PA has been identified to have multiple beneficial health outcomes, namely cardiometabolic health, muscular fitness, bone health and CRF [36
]. Among younger and older children, we found similar results regarding children not meeting the PA recommendations. Meanwhile, older children had a higher score for systolic/diastolic blood pressure and HDL cholesterol levels than younger children, who reported a higher ratio of total cholesterol to HDL. Our results corroborate those reported by Bugge and colleagues [9
], who found that cardiovascular risk factors increased with age. Additionally, we found that older children were more sedentary and had lower LPA and lower VO2
max scores. These results are significant as Andersen and colleagues [2
] have demonstrated that clustered cardiovascular risk factors were not apparent in younger children (6–7 years) but started developing by age 9 in 13.8% of children, with low fitness levels being strongly related to clustering. In the longitudinal study, they were not able to explain why clustered cardiovascular disease risk was only evident after children started school, but concluded that having low fitness levels at 6–7 years predicted the development of cardiovascular risk later in life [2
A review on children living in Sub-Saharan Africa found that girls engaged in less PA than boys, and showed higher sedentary behavior, and performed more poorly in aerobic fitness [18
]. In our study, girls presented higher values for individual and clustered cardiovascular risk than boys and were less likely to meet PA recommendations. A possible reason for these findings is related to girls having higher percentage fat, which contributes to a higher prevalence of overweight/obesity, that in turn may result in individual or clustering of cardiovascular risk factors such as hypertension, dyslipidemia or impaired glucose regulation [37
]. This contradicts the results of Musa et al. [7
] who found, in Nigerian adolescents, a higher prevalence of clustered cardiovascular risk in boys than girls. Interestingly, boys in that study displayed significantly lower BMI, percentage body fat and abdominal fat, and they performed better in the fitness test than girls. According to previous cross-sectional and longitudinal surveys, overweight and unfit children have a greater risk of increasing cardiovascular disease risk than their healthy weight and fit counterparts [39
]. Our observation regarding higher MVPA scores with regard to sex corresponds with a previous cross-sectional survey in South African school-aged children (10–15 years) that found boys to have significantly higher MVPA scores than girls [34
]. However, the Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) study found contrary results showing that adolescent girls had higher MVPA and less sedentary behavior than boys [41
]. Despite the contradicting results found in the HELENA study, several studies consistently report that boys participate in more PA than girls. A recent analysis of several population-based surveys reported that there was a global trend showing girls to be less active compared to boys [42
]. The study highlighted the urgency for global and national action to reduce insufficient PA, with particular emphasis on adolescent girls [42
The results presented in this paper corroborate previous research [6
], in which negative associations of higher CRF with lower percentage body fat and lower cardiovascular risk were reported. Our data showed that being sedentary was associated with higher percentage body fat and triglyceride levels, while higher LPA was associated with high systolic blood pressure. Low PA levels have been associated with increased cardiovascular risk factors. For instance, unfit and overweight children showed the highest prevalence of increased total cholesterol and systolic/diastolic blood pressure compared to their fit and normal weight counterparts [39
]. In their review, Ruiz and colleagues [43
] reported that low fitness levels increased the likelihood of cardiovascular disease risk 5.7 times in boys and 3.6 times in girls. We found a significant negative correlation between VPA and MVPA and percentage body fat and clustered cardiovascular risk. Previous cross-sectional studies reported that clustered cardiovascular risk could be lowered by at least 0.06-SD with every 1-SD increase in VPA [44
], whereas a 0.5-SD increase in MVPA was associated with approximately 30% reduction in clustered risk [45
]. Furthermore, lower clustered cardiovascular risk among children with higher CRF and MVPA levels was observed. Similar to our findings, a longitudinal study found that, compared to children in the most fit quartile, those in the lowest fitness quartile had a 34.9 times greater risk of having clustered cardiovascular risk [2
The strengths of our study were that we used measures of PA and cardiovascular risk markers from a relatively large sample of schoolchildren from grades 1–4. We used validated and objective methods to measure PA and CRF (accelerometry and 20 m shuttle run test). Additionally, we made use of a summed score of risk markers related to cardiovascular disease. This clustered risk score is relatively stable and can compensate for the fluctuations in individual risk factors [6
]. For data analyses, we used statistical software that can compensate for missing data in a meaningful way. Likewise, we used a robust estimator to deal with non-normally distributed data.
Despite the strengths of our study, the findings should be interpreted with caution as we observed several limitations. We were unable to report inference about causality and its direction due to the cross-sectional nature of the study. We did not find any differences based on children’s ethnic background (Black African and Coloured), and therefore decided not to include this variable as a potential confounder. We also did not assess whether children engaged in intense PA the day before data assessment. This is something that was not done in previous studies investigating the relationship between PA and CRF [6
]. Furthermore, our findings cannot be generalized to the entire South African population as children from one province were included in the investigation. Finally, we cannot generalize our findings to children from different social strata attending more wealthy schools or rural schools. Although several confounding factors were controlled for, it is conceivable that other factors such as genetic variations, dietary patterns and energy intake might have influenced our findings.