The increase over time in blood pressure levels in children and adolescents has attracted the attention of health professionals and the scientific community,¹largely due to its association with the obesity epidemic.²According to a recent study,³it is estimated that the worldwide prevalence of hypertension is 4% in children and adolescents, and the prevalence of high blood pressure according to the new guidelines of the American Academy of Pediatrics is 15%. Although these prevalence estimates may have been underestimated before the recent reclassification,⁴data have indicated that the rates in developing countries are on the rise.⁵

Over the last few years more attention has been given to the relationship between hypertension and the development of end-organ damage emanating from early dysfunctions. The main modifiable risk factors to prevent strokes and other health issues are hypertension,^{6 , 7} diabetes mellitus, tobacco smoking, and hyperlipidemia, as well as poor diet/nutrition, physical inactivity, and obesity.⁸The strong association between obesity and hypertension prompted the development of several simple and low-cost measures to assess body adiposity, e.g., body mass index (BMI), waist circumference (WC) and body fat percentage (BF).⁹It is known that the risk of hypertension increases with the increase in obesity indexes, increasing the chances of cardiometabolic problems.¹⁰It is well documented in the literature that early risk factors during childhood can lead to cardiorespiratory complications in adolescence¹¹and adulthood.¹²

Another important risk factor for hypertension is low levels of cardiorespiratory fitness (CRF). Data show that higher levels of CRF provide lower risk of hypertension.¹³Biochemical variables such as dyslipidemia,¹⁴hyperuricemia,¹⁵and altered levels of serum glucose (i.e., insulin resistance)¹⁶have shown to be important modifiable risk factors for hypertension, but CRF has an independent, predictive, and powerful negative association with blood pressure among children and adolescents.

Although the prevalence of hypertension has been well documented in several studies, there are few studies regarding incidence of hypertension in the general population.¹⁷Studies addressing the incidence of high blood pressure and its relationship with other cardiometabolic variables, including genetic profile, in children and adolescents are even scarcer. Based on these considerations, the aim of this study is to describe the incidence of hypertension and its relationship with cardiometabolic (weight status, CRF and biochemical profile) variables and genotypes of the rs9939609 polymorphism of the FTO gene in Brazilian children and adolescents.

The target population of this retrospective cohort research were volunteer children and adolescents of the city of Santa Cruz do Sul, in Rio Grande do Sul state, Brazil. All procedures were conducted at laboratories, rooms, and sports complex of the university campus, and consisted of assessment of CRF, anthropometry, laboratory data, and genetic polymorphism.

Inclusion criteria for this subsample were students who: a) attended the 2011 assessment and returned in 2014; b) took part in the blood collection; c) underwent complete anthropometric assessment; d) filled out all the forms; and e) were in the range between seven and 17 years old in both assessments. In total, results of 469 students were analyzed, as are shown in Central Figure .

Central Illustration

The participants of this study were children and adolescents between 7-17 years old, of both sexes, from 19 public and private schools, stratified by regions (north, south, east, west and center), from urban and rural areas. The sample selected is from two databases, including only the students who attended the physical assessments in 2011/2012 (baseline) and returned in 2014/2015 (follow-up) ( Figure 1 ). The databases come from a larger study named “Schoolchildren’s Health Study”, stratified by clusters. This research was approved by the Committee of Ethics in Research with Human Subjects of the University of Santa Cruz do Sul – UNISC (approval numbers 2959/2011 and 714.216 for baseline and follow-up, respectively). All parents or guardians were informed about the procedures and signed an informed consent form, authorizing the participation of the student in the study.

Figure 1

The variables included in the present study were systolic blood pressure (SBP), diastolic blood pressure (DBP), WC, BMI, %BF, lipid profile (triglyceride [TG], total cholesterol [TC], high-density lipoprotein cholesterol [HDL-C], low-density lipoprotein cholesterol [LDL-C]), glucose, and CRF.

SBP and DBP were measured twice with the student seated for at least five minutes in a quiet room. A sphygmomanometer and stethoscope were used on the right arm with an appropriate cuff for the arm circumference. SBP was determined by the first Korotkoff sound and DBP by the fifth Korotkoff sound, that is, when the sounds are no longer audible or when the timbre of the sound changes. These measurements were classified by the 90th and 95th percentiles for borderline and hypertension according to the Clinical Practice Guideline for Screening and Management of High Blood Pressure in Children and Adolescents.¹⁸

WC was measured to the nearest 1mm using a non-elastic tape (Cardiomed®). Measurement was taken at the narrowest part of the trunk between the ribs and the iliac crest. WC was classified as proposed by Fernández et al.;¹⁹abdominal obesity was defined as measures above the 75th percentile, according to sex and age. To calculate BMI, height was measured using a stadiometer coupled to the anthropometric scale (Filizola®), which was used for weight assessment, measured for each participant in light clothing without shoes. The formula BMI = weight / height²(kg/m²) was used. The result was classified according to the WHO (2007)²⁰percentile curves for age and sex. Participants with percentile <3 were classified as underweight, percentile ≥85 as overweight and percentile ≥ 97 as obese. To measure %BF, tricipital and subscapular skinfolds were measured using the Lange® caliper (MultiMed, Skinfold Caliper, USA). The %BF calculation was performed using the Heyward and Stolarczyk²¹equation and classified as very low; low; ideal; moderately high; high; and very high according to Lohman.²²The first three categories were reclassified as lower levels and the other categories as higher levels for analyses.

For the lipid profile and glucose, 10mL of blood were collected from the brachial vein after a 12-hour fast; 5mL were placed in dry Vacutainer® tubes for analysis of cardiometabolic parameters, and 5mL were placed in the EDTA Vacutainer® tubes for further analysis. Levels of TC, HDL-C, LDL-C, TG, and glucose were evaluated in serum samples. The TC, HDL-C, LDL-C, and TG values were classified according to international reference values.²³Participants were considered dyslipidemic if at least one of the parameters listed above was found to be altered. LDL-C was calculated using the Friedewald formula.²⁴Blood glucose levels were determined at the Exercise Biochemistry Laboratory of the university using the automated equipment Miura One (I.S.E., Rome, Italy) and a DiaSys (DiaSys Diagnostic Systems, Germany) commercial kit, and classified according to the American Diabetes Association protocols:²⁵normal, pre-diabetes and diabetes. For statistical purposes, the pre-diabetes and diabetes classes were grouped and considered as “high” glucose.

For genetic polymorphism evaluation, the rs9939609 FTO (fat mass and obesity-associated gene) was assessed due to its association (A allele) with the presence of obesity in previous studies in Brazil.²⁶Genotyping (alleles AA, AT, TT) was performed by real-time polymerase chain reaction (PCR), using TaqMan™ probes as previously described.²⁷The Hardy-Weinberg equilibrium was tested and confirmed (p>0.05).

CRF was assessed by indirect submaximal exercise tests. The 9-minute walk/run test was used at baseline, and the 6-minute walk/run test was used in the follow-up, according to the protocols of the Projeto Esporte Brasil (PROESP-BR).^{28 , 29} For both tests the children and adolescents were divided in groups that were adequate for the running track dimensions. They were instructed to run as long as possible, avoiding velocity peaks interspersed by long walks. During the test, the participants were verbally encouraged. At the end of the test, after a signal was given, the students stopped running and remained in place where they were standing until the distance traveled (in meters) was recorded. The result given in meters was classified as a categorical variable, dichotomized into “higher” and “lower” values according to the PROESP-BR test manual.³⁰

Participants’ characteristics of BMI, WC, %BF, glucose, dyslipidemia, and CRF are shown in Table 1 . Of the 469 children and adolescents evaluated, 202 (43.1%) were male, and 77.0% were white. Regarding the rs9939609 Polymorphism (FTO) 37.5% were classified as allele TT, 47.6% as AT and 14.9% as AA.

Table 1

Variables	n (%) Baseline	Follow-up
BMI
Underweight	6 (1.3)	4 (0.9)
Normal	283 (60.3)	289 (61.6)
Overweight	92 (19.6)	90 (19.2)
Obesity	88 (18.8)	86 (18.3)
WC
Lower risk	357 (76.1)	366 (78.0)
Higher risk	112 (23.9)	103 (22.0)
%BF
Lower	268 (57.1)	297 (63.3)
Higher	201 (42.9)	172 (36.7)
Glucose
Normal	356 (83.0)	396 (85.5)
High	73 (17.0)	67 (14.5)
Dyslipidemia
No	249 (54.8)	249 (54.8)
Yes	205 (45.2)	205 (45.2)
CRF
Lower	206 (43.9)	261 (56.5)
Higher	263 (56.1)	201 (43.5)

BMI: body mass index; WC: waist circumference; %BF: percentage of body fat; CRF: cardiorespiratory fitness.

Table 2 describes the incidence of altered blood pressure over three years. Most subjects remained classified as normotensive, while approximately one third of the normotensive sample had increases in their blood pressure levels (moving to the borderline/hypertensive categories). It should be noted that a fair number of borderline and hypertensive individuals became normotensive.

Table 2

Blood Pressure	n (%)
Normotensive (maintaining)	309 (65.9)
Normotensive to borderline	60 (12.8)
Normotensive to hypertensive	54 (11.5)
Borderline (maintaining)	2 (0.4)
Borderline to normotensive	6 (1.3)
Borderline to hypertensive	6 (1.3)
Hypertensive (maintaining)	9 (1.9)
Hypertensive to normotensive	15 (3.2)
Hypertensive to borderline	8 (1.7)

Figures 2 and 3 show the changes in ratings of PAS and PAD over time.

Figure 2

Figure 3

Table 3 shows the odds of hypertension by adiposity parameters (BMI, WC, and %BF) and biochemical data, genotype, and CRF level. Children and adolescents who were overweight and obese according to baseline BMI were more likely to become borderline or hypertensive over three years. Increased WC values were associated with a high risk for hypertension, and increased %BF was associated with changes in blood pressure category, from normotensive to borderline, and from normotensive to hypertensive over the study period.

Table 3

	Blood pressure modification across 3 years
Baseline	Normotensive  Borderline		Normotensive  Hypertensive
	OR (95% CI)	p	OR (95% CI)	p
Model 1: BMI
BMI
Underweight/Normal weight	1		1
Overweight	3.22 (1.08-9.55)	0.035	1.69 (0.58-4.90)	0.336
Obesity	4.05 (1.68-9.75)	0.002	4.84 (1.57-14.95)	0.006
Dyslipidemia
No	1		1
Yes	0.86 (0.40-1.84)	0.695	1.60 (0.714-3.61)	0.252
Glucose
Normal	1		1
High	1.54 (0.60-3.96)	0.370	1.94 (0.74-5.07)	0.177
FTO rs9939609
TT	1		1
AT	1.15 (0.482.74)	0.750	0.84 (0.36-1.97)	0.686
AA	1.68 (0.59-4.77)	0.332	0.58 (0.15-2.26)	0.433
CRF
Higher	1		1
Lower	1.60 (0.73-3.51)	0.244	1.19 (0.51-2.81)	0.683
Model 2: WC
WC
Low risk	1		1
High risk	2.46 (0.99-6.12)	0.052	3.41 (1.26-9.19)	0.016
Dyslipidemia
No	1		1
Yes	0.95 (0.45-2.01)	0.898	1.65 (0.74-3.70)	0.224
Glucose
Normal	1		1
High	1.78 (0.70-4.49)	0.225	1.92 (0.74-5.01)	0.179
FTO rs9939609
TT	1		1
AT	1.05 (0.45-2.47)	0.902
AA	1.57 (0.56-4.38)	0.387	0.53 (0.14-2.06)	0.363
CRF
Higher	1		1
Lower	1.65 (0.76-3.59)	0.204	1.34 (0.58-3.09)	0.489
Model 3: %BF
%BF
Lower	1		1
Higher	2.28 (1.04-4.97)	0.039	2.49 (1.08-5.75)	0.033
Dyslipidemia
No	1		1
Yes	0.93 (0.44-1.96)	0.845	1.58 (0.71-3.52)	0.265
Glucose
Normal	1		1
High	1.59 (0.63-4.01)	0.327	1.68 (0.65-4.34)	0.280
FTO rs9939609
TT	1		1
AT	1.12 (0.48-2.61)	0.799	0.90 (0.39-2.09)	0.807
AA	1.74 (0.62-4.85)	0.292	0.57 (0.15-2.21)	0.419
CRF
Higher	1		1
Lower	1.61 (0.74-3.49)	0.229	1.35 (0.59-3.08)	0.479

BMI: body mass index; WC: waist circumference; %BF: percentual of body fat; CRF: cardiorespiratory fitness

The aim of this study was to determine the incidence of hypertension and to relate it with cardiometabolic variables (weight status, CRF, and biochemical profile) and rs9939609 Polymorphism ( FTO ) of Brazilian children and adolescents. Our results identified 12.8% of normotensive subjects at baseline who became borderline, and 11.5% of them changed to hypertensive. Importantly, all these associations were independent of growth and development. Furthermore, regarding the adiposity variables, children and adolescents who were overweight or obese according to BMI (model 1) at baseline were more likely to become borderline, while those who were obese were more likely to become hypertensive over three years. High-risk values for WC (model 2) were associated with hypertension, and higher %BF values (model 3) were associated with the development of both borderline hypertension and hypertension.

The incidence of high blood pressure levels has been rising in children and adolescents.⁴This increase is believed to be due to the high incidence of overweight and obesity in this population, since excess weight gain, especially when associated with increased visceral adiposity, is a major cause of hypertension.³¹According to a large study including Brazilian adolescents, the highest prevalence of hypertension in Brazil are in the south region, the same population assessed in the present study. The south region of Brazil also has the highest prevalence of overweight and obesity and the most physically inactive population.³²

Our study demonstrated that children and adolescents who were overweight and obese according to BMI (model 1) at baseline were more likely to become borderline (overweight OR: 3.22, 95% CI: 1.08-9.55; obesity OR: 4.05, 95% CI: 1.68-9.75) or hypertensive (obesity OR: 4.84, 95% CI: 1.57-14.95) over three years. In addition, other studies with same-age populations have shown associations between BMI and elevated rates of blood pressure.³³A similar but cross-sectional study³⁴aimed to verify the association between overweight/obesity and high blood pressure in Brazilian students aged between six and 10 years old. Obesity increased by twice the chance of high blood pressure among children aged 6-7 years old. In children aged 8-9 years old, overweight doubled the chance of high blood pressure, while obesity quadrupled that chance.³⁴In another study,³⁵conducted with Chinese children and adolescents aged between 7 and 18 years old, a high prevalence of high blood pressure was also found among those who had overweight (19%) and obesity (23.2%).³⁵

In our results (model 2), “high-risk” WC classification was associated with the development of hypertension at follow-up (OR: 3.41, 95% CI: 1.26-9.19), which is supported by several studies.^{36 - 39} There are findings that both WC and BMI were good predictors of elevated levels of blood pressure in every phase of life,²including one study that showed that WC was a good predictor of hypertension, even when the BMI was normal.⁴⁰Another study, however, showed that high BMI, but not WC or %BF, was associated with high risk of hypertension in normal-weight Chinese children.⁴¹As it is known, abnormal distribution or excess of fat tissue impacts the renin-angiotensin-aldosterone system and the rise in production of non-esterified fatty acids and inflammatory cytokines (e.g., interleukin 6). Also, obesity is associated with a hyperinsulinemic state, which activates the sympathetic nervous system. These changes involve an increase in renal sodium reabsorption and intravascular volume, and vasoconstriction, which ultimately result in arterial hypertension.⁴²

A systematic review of abdominal adiposity and cardiometabolic risk factors showed that blood pressure measurement was the most common cardiometabolic parameter among the studies.⁴³Importantly, most of these studies confirmed the association between elevated levels of blood pressure and abdominal obesity; however, most were cross-sectional. On the other hand, some studies did not support WC as a better predictor than BMI to identify children with elevated levels of blood pressure.⁴⁴A metanalysis included 23 longitudinal observation studies and found that WC and BMI were good predictors of diabetes but not hypertension.⁴⁵

Also, children and adolescents with higher levels of %BF at baseline were more likely of becoming borderline (OR: 2.28, 95% CI: 1.04-4.97) or hypertensive (OR: 2.49, 95% CI: 1.08-5.75), corroborating some findings in the literature.^{46 , 47} A higher percentage of body fat could increase the risk of hypertension, and its reduction would be beneficial for the prevention and control of hypertension in children as shown by Tao et al.⁴⁷In their study⁴⁷and in the current study, all results related to adiposity point in the same direction: an association with high blood pressure. A study⁴⁸in an adult population showed that weight loss had beneficial effects on incident hypertension and cardiovascular events. It is recognized that lifestyle changes have an impact on blood pressure levels and reinforce that weight reduction is an important goal for the primary prevention of cardiovascular events.⁴⁸A 5% reduction of total body weight is associated with decrease of 20-30% in blood pressure levels⁴⁹which is an important index of vascular function improvement.⁵⁰Also, another study supported the association between blood pressure levels and obesity suggesting that high blood pressure might be more related to body composition rather than body weight itself, since in some situations body weight, and consequently BMI is high, but the %BF is low. The findings of the same study still suggest that losing or gaining weight, which changes the anthropometric profile, has a substantial impact on the reversal or development of hypertension, respectively.⁵¹

A tracking study indicated that blood pressure shows a moderate stability from childhood into adolescence or early adulthood, since blood pressure measurements seem to be a strong independent predictor for future measurements.⁵²Another tracking study, this time including participants from late childhood to early adolescence, showed low-to-moderate stability to SBP and DBP over three years.⁵³This trend may be explained by changes in the anthropometric profile over time⁴⁹such as increase of central adiposity, the main risk factor for metabolic syndrome,⁵⁴which can trigger changes alterations in blood pressure, blood lipids and glucose.⁵⁰In the current study, there were also cases of patients who that changed from borderline or hypertension down to the normotensive category. One possible explanation is the influence of school, in which individuals participate in physical activities that can improve their physical status and consequently their health.

None of the other risk factors showed a statistically significant association with borderline or hypertension in any of the models, perhaps because it is too early in life to show some consequence in health. Even though CRF and body composition are highly related, and CRF has shown to be an important modifiable risk factor for hypertension, and independent and powerful negative predictor of blood pressure among children and adolescents,¹⁶we found no statistically significant association between them. Therefore, the prevention of cardiovascular disease should begin during childhood through regular screening for hypertension, counseling for a healthy lifestyle, and prevention of modifiable risk factors as BMI, WC, and %BF.⁵⁵

The present study has some limitations: our analyses did not control for physical activity, maturation stage, and dietary behaviors; BMI was used as an adiposity marker, even though there are more direct methods for assessing adiposity. Also, blood pressure was measured only twice in each assessment period, despite recommendations¹⁸for three measurements at each assessment. However, our study was comparable to most epidemiological studies which, for logistical and cost reasons, take single measurements at each assessment period. As strengths, this longitudinal study used a randomly selected sample at baseline to measure blood pressure, using the auscultatory method, by the same evaluator in both periods and according to the same recommendations.¹⁸

This longitudinal study showed that there is an increase in the number of new cases, represented by a high incidence of hypertension in children and adolescents, compared to previous studies. Additionally, our findings showed that children and adolescents with higher BMI, WC, or %BF at baseline were more likely to develop hypertension throughout the three years, highlighting the importance of pediatric health care to avoid future harm.