The growing childhood obesity epidemic1 is concerning, especially because obese children may experience metabolic complications and are at high risk for the early development of conditions that are more commonly observed in adults,2 particularly high blood pressure (HBP), changes in serum triglycerides and elevated fasting glucose.3

The prevalence of childhood hypertension had been expected to be approximately 1-2%,4 however, rates ranging from 3.0 to 15.9% have been observed in different scenarios,5–19 and this rise is associated with increases in excess weight,2,3,18,19 increased abdominal fat,7,9–14,17 and unhealthy lifestyles.9,10,15,20

Additionally, hypertension in childhood has important implications for children's health, since it is commonly related to the development of other cardiovascular risk factors3,21 and can persist into adulthood.22,23 However, although early diagnosis of hypertension in children is of the utmost importance24 and blood pressure (BP) measurement is a low-cost, noninvasive and relatively accurate procedure for identifying this condition,25 little is known about the risk factors associated with HBP in childhood.

The aim of this study was to estimate the prevalence of HBP and its association with anthropometric indicators of adiposity in Portuguese schoolchildren.

The present study was conducted with 1555 6- to 9-year-old children (50.5% female) residing in the 18 districts of mainland Portugal (mean age 7.58 years; standard deviation 1.10). The prevalence of high-normal BP and hypertension was 4.5% and 3.7%, respectively. Both high-normal BP and hypertension were significantly more prevalent among females than among males (5.6% vs. 3.4% and 4.3% vs. 3.1%, respectively, p=0.04) (Table 1). However, the prevalence of high-normal BP and hypertension showed no significant association with age (p=0.15). An increase in weight status was positively associated with the prevalence of high-normal BP and hypertension (p<0.01), which was also observed for high WC (p<0.01) and high BFP (p<0.01) (Table 1).

There was an association between maternal excess weight and increased prevalence of high-normal BP and hypertension in children (p=0.01); however, this association was not observed for paternal excess weight (p=0.85). The prevalence of high-normal BP and hypertension was higher among children who did not perform physical activity during their leisure time (p<0.01), and conversely, these prevalences decreased as the time spent in physical activity per day increased (p=0.01 for overall trend) (Table 1). SBP and DBP were positively correlated with all anthropometric indicators (BMI, WC, BFP, and adiposity index; p<0.01 for all) (Table 2).

Due to the significant difference in the prevalence of HBP between males and females, multinomial logistic regression analysis was stratified by gender. The risk of high-normal BP was higher for males with obesity (odds ratio [OR] 6.13, p<0.01), high WC (OR 3.14, p=0.01), high BFP (OR 3.06, p=0.01), and high adiposity index (OR 1.69, p=0.01) and the risk of hypertension was greater among males who had high adiposity index (OR 1.82, p=0.01). The risk of high-normal BP was higher for females with obesity (OR 4.25, p=0.01), high WC (OR 2.36, p=0.01), high BFP (OR 2.83, p<0.01), and high adiposity index (OR 1.67, p<0.01). Additionally, the risk of hypertension was greater for females with overweight (OR 2.43, p=0.03), obesity (OR 5.26, p<0.01), high WC (OR 3.47, p<0.01), high BFP (OR 2.29, p=0.03), high adiposity index (OR 2.03, p<0.01), for daughters of overweight mothers (OR 3.23, p<0.01), and for those who did not perform physical activity during leisure time (OR 2.15, p=0.04) (Table 3).

High-normal BP and hypertension were prevalent among 6-9-year-old Portuguese schoolchildren. All the anthropometric indicators assessed were strongly associated with the prevalence of high-normal BP and hypertension, especially among females, as well as some lifestyle-related behaviors and maternal excess weight.

The prevalence of hypertension found among these Portuguese schoolchildren was comparable to that observed in Italian children aged 6-18 years (3.5%),5 Spanish children aged 6-16 years (3.2% for males and 3.1% for females),17 and Chinese children aged 7-18 years (prehypertension 3.9%, hypertension 3.3%).16 However, the prevalence of high-normal BP and hypertension observed in the present study was lower than that observed among Canadian children aged 4-17 years (prehypertension 7.6%, hypertension 7.4%),8 Brazilian children aged 7-17 years (high-normal BP 13.9%, hypertension 12.1%),11 and Italian children aged 6-11.9 years (hypertension 5.2-7.8%).18 These differences in the prevalence of HBP can be explained, in part, by the variability in procedures applied in BP measurements and categorization for children and adolescents. Furthermore, differences in the ages of the studied groups may have impaired the comparability of results, since increased age is associated with changes in BP.5,17,21

Adiposity was more strongly associated with HBP among children with excess weight than in those with normal weight. Similarly, Tu et al.,37 in a cohort study of American children with a mean age of 10.2 years, found that the effect of adiposity on BP was modest in children with BMI below the 85th percentile, but above this point, the effect of adiposity on BP increased up to four-fold. This relationship highlights the effect of adiposity on BP levels.37

Increased BP in childhood is concerning for several reasons, one of which is that HBP predisposes children to developing cardiovascular disorders, including intermediate markers of target organ damage such as increased carotid-intima media thickness.3,21 There is also evidence that changes in BP in childhood tend to persist into adulthood.22,23

The identification and treatment of children at higher risk for developing hypertension is therefore an important step in reducing the excessive burden of cardiovascular disease. Furthermore, not only does the etiology of cardiovascular disease have its roots in childhood, but the lifestyles that influence the onset of chronic disease are also acquired in early life and tend to persist into adulthood.38

In this context, considering that routine BP measurement is recommended after the age of three,25 Rinaldi et al.39 state that screening for alterations in BP among children, who are mostly asymptomatic, is fundamental to prevention and reduction of cardiovascular disease, and that the school environment is an appropriate place for measuring and monitoring BP.

A significant association has been observed in some studies between sedentary behavior and/or low levels of physical activity and HBP.10,15,20 However, in the present study, no significant association between high-normal BP and hypertension and indicators of physical activity and sedentary behavior was observed after adjustment in the multinomial model. Similar results were observed among Italian5 and Mexican9 children.

Due to its cross-sectional design, the present study was unable to determine causality between adiposity and HBP among the studied children. However, similar associations have been found in longitudinal studies.7,23,37 This study has some limitations, including the lack of information on parental BP, which could reflect the genetic origin of cardiovascular disorders.5,21 However, Tringler et al.15 found no significant association between hypertension in children and parental hypertension (p=0.35). In the present study, in the absence of such information, parental BMI was used as a proxy to examine the effect of parental characteristics on children's BP levels. Similar results were observed among five-year-old Australian children.40

The analyses in this study represent a breakthrough in the understanding of the impact of adiposity on BP levels in schoolchildren after controlling for possible influencing factors. An unexpectedly high prevalence of high-normal BP and hypertension was observed among Portuguese schoolchildren. Higher values for body fat indicators were strongly associated with HBP, reinforcing the usefulness of these indicators in screening for health problems among children, and specifically the risk of elevated BP levels. Prevention of childhood obesity is therefore of paramount importance to reduce the risk of HBP and cardiovascular disease.

This study was supported by a grant from the Portuguese Fundação para a Ciência e a Tecnologia FCOMP-01-0124-FEDER-007483. Additionally, the first author was supported by CAPES, Ministry of Education of Brazil (process number: 8349/12-6). The Fundação para a Ciência e a Tecnologia and CAPES had no role in the design, analysis or writing of this article.

The PPSOC was approved by the Directorate-General for Curriculum Innovation and Development of the Portuguese Ministry of Education. Additionally, authorization for data collection was obtained from the schools, and the participants’ parents or guardians signed a consent form indicating their agreement to participate.

PRMR contributed to analysis and interpretation of data, the drafting, writing, and revision of the manuscript, and final approval of the manuscript. RAP contributed to analysis and interpretation of data, critical revision of the manuscript, and final approval of the manuscript. AG, IMC, HN, and VRM contributed to the design of the data collection instruments, review and revision the manuscript, and final approval of the manuscript. CP contributed to the conception and design of the study, coordination and supervision of data collection, review and revision of the manuscript, and final approval of the manuscript.

The authors have no conflicts of interest to declare.