In the context of a rapidly growing prevalence of obesity in childhood, we evaluated the important predictors of hypertension across childhood. Pediatric hypertension has undergone shift from secondary hypertension (mostly caused by renal disease) to essential hypertension (as the main cause of hypertension in childhood and adolescence) [12]. The factors related to primary hypertension in children were different for systolic and diastolic hypertension. In the present study, age and BMI were the main predictors of SBP, while age and BMI with higher intakes of fruit and fast foods were in positive and significant association and higher intake of vegetables was in negative and significantly association with DBP. The univariate analysis showed a significant correlation among adiposity indicators and sedentary lifestyle (lower physical activity and spent many time for watching TV/PC) with SBP and DBP, whereas, higher intake of fruit, also, associated with DBP in this children and adolescents. In this study, we went through a hierarchical linear multivariable regression model to examine the effect of each of the predictors in a model on other variables. In accordance to our findings, Dong et al also reported that increased prevalence of hypertension is associated with higher body mass index among 943 128 participants aged 7 to 17 years [13]. Similar results were also observed in the Zeberio et al study indicating a positive association between SBP and BMI in school-aged children [14]. Higher concentrations of circulating inflammatory cytokines have also been shown to be associated with the atherosclerotic process, and CRP is one of the most susceptible indicators in obese Japanese children [15]. Increased carotid artery intima-media thickness (C-IMT) is associated with hypertension in children [16]. A study in healthy children mean aged 10.5±1.1 showed that CRP was a significant independent predictor of C-IMT and flow-mediated vasodilation [17].
Similar to our results, some studies have also demonstrated that a rise in the blood pressure is associated with increased age in children and adolescents [14, 18]. Several reports also indicated the possible role of puberty as an important determinant in the association between obesity and hypertension among adolescents specially girls. This might also explain the increase prevalence of hypertension with increased age [19]. Barba et al reported that in the period close to the completion of puberty, the association between age and BP becomes more evident among girls [18]. In the study of Oliveros and colleagues, the prevalence of prehypertension and stage 2 of hypertension was higher among younger compared with older children, while stage one of hypertension was more prevalent among older children [20]. Among the dietary factors that influenced the nutritional pattern score in the current study, a positive correlation between hypertension and increased consumption of fruit groups and fast food /junk food consumption, and inverse correlation between hypertension and vegetable groups was observed. Increased fast food consumption containing high amounts of salt, sugar, and fat, is associated with increased obesity state [21, 22]. Studies showed that high levels of fats, sugars, and salt intake of fast foods are one of the other possible reasons of increased blood pressure in children and adolescents [23, 24].
Although the possible role of increased salt and sodium consumption in the pathogenesis of hypertension has been clarified, however, recently the role of other dietary factors, food pattern and lifestyle habits in increasing blood pressure has also been focus of interest. Stamler et al. showed that in addition to sodium, several other nutrients including calcium, magnesium, potassium, and fiber are also involved in the pathogenesis of hypertension [25]. One meta-analysis of 56 studies on the effect of sodium restriction on blood pressure showed that sodium restriction could be beneficial among elderly individuals with hypertension, however, its beneficial effects are low among people with normal blood pressure [26].
The higher sodium and energy contents of fast and junk foods are possible underlying reason of the association between fast food consumption and hypertension.
Numerous studies have shown that higher intakes of fruits, vegetables and dairy products could have an effective role in prevention of childhood hypertension due to several nutrients including potassium, magnesium, calcium and fiber [27, 28]. The positive association between fruits intake and hypertension in our study in contrast with several previous studies, could be attributed to the difference in study design, target group characteristics such as age or gender distribution [27], or taken fruit and vegetables as a one group with no separation of them [28]; while we analyzed fruits and vegetables separately in two independent groups. Moreover, 100% fruit juices are also considered as fruit groups and are likely to increase blood pressure by increasing consumption of fructose-rich fruit juice.
Consumption of 100% fruit juices might be associated with hypertension through several possible biological mechanisms including increased energy intake and weight gain, and increased uric acid production [29] which is also associated with elevated blood pressure. Also, consuming whole fruits in high amounts can increase weight due to increased energy intake. Therefore, it could be suggested to study the effects of whole fruits and fruit juices separately in further researches.
No significant correlation was found between blood pressure and dairy products in the current study. Greater intakes of dairy products were associated with lower SBP in white but not black children and teens in Dellavalle et al. study, suggesting that greater dairy products intake alone might be not beneficial for all races [30].
In the current report, 43.53% of participants consumed more than 5 meals per day; however, there was no significant relationship between meals and hypertension. Some studies showed that the prevalence of obesity declined by increased number of meals [31]. Donin et al. showed that more snacks and meal consumption leads to obesity and cardiovascular problems in children [32]. Also, Toschke et al studied 4370 children aged 5-6 years, and found that the prevalence of obesity declines by increasing the number of meals [31]. In the final regression model, we did not observe any significant correlation between blood pressure and physical activity or watching TV/PC. Torrance et al demonstrated that 40 minutes of moderate to vigorous aerobic-based physical activity 3-5 days/week is required to reduce blood pressure in obese children [33]. TV commercials influence the food choices of children in different ways. Using their highfalutin and vivid messages, they encourage people to buy the advertised products [34]. A program comprising screening, early detection and health promotion through school health programs may help to prevent future complications of hypertension [35]. Although in the current study we observed a correlation between blood pressure and television viewing and physical activity in the univariate regression, however these associations had been vanished by including several confounders into the model.