The current study presented the prevalence of obesity and hypertension, and a positive associated between Hb, hematocrit level and blood pressure, in children and adolescent. To our knowledge, this is the first study that investigated the association between hemoglobin and hematocrit level, and systolic and diastolic blood pressure in a large Korean children and adolescents, aged 10–18 years. Our study population was representative of the Korean children and adolescents because the KNHANES is an official, nationwide database that reflects geographic and demographic differences in Korea.
Recently, the prevalence of overweight and obesity has increased rapidly in children and adolescents. Obesity is a multifactorial disease, caused by genetic, biological, environmental, behavioral, psychological factor (23). Childhood obesity leads to adult obesity (24, 25), and increased the risk of cardiovascular disease and mortality in adulthood (26). Severe obesity in children associated with an increased prevalence of cardiometabolic risk factor, including low HDL cholesterol, high SBP and DBP, high TG level, and high glycated hemoglobin level (27).
The prevalence of overweight and obesity was reported more than 10 % in developing countries and even higher (20%) in developed countries (28). Reports on the prevalence of childhood obesity in Korea vary from data to data, but it has increased over time, too. In 2013, the prevalence of overweight and obesity in children age < 20 years was 21.2 in boys and 13.2 in girls, and the prevalence of obesity was 4.8 in boys and 3.1 in girls (28). In our study, the overall prevalence of obesity and overweight was 21% in boys, and 17.8 % in girls, and the prevalence of obesity was 10.5% in boys and 9.2% in girls, aged 10–18 yeas, from 2007 to 2017. Our study did not analyzed the annual prevalence of obesity and overweight. And the difference in prevalence was caused by the age difference between subjects and the method of analysis.
The prevalence of elevated blood pressure and hypertension in children is reported around 6% and 3%, respectively (9, 29). In obese adolescents, the combinded prevalence increases to around 30%(29). Cho H et al reported the prevalence of hypertension increased over time, it was from 6.9% (KNHANES 2007–2009) to 9.0%,(KNHANES 2013–2015) in Korean children and adolescent aged 10–18 year (30). The prevalence of HTN were higher in boys than girls, and increased with severity of obesity. Especially, in obesity group, the prevalence of HTN were higher than average participant, it increased from 14.9% (KNHANES 2007–2009) to 27.7% (KNHANES 2013–2015). In our study, the prevalence of HTN were higher in boys than girls, and increased with severity of obesity, too. However, our study design and analysis of method are different with previous study, the prevalence of HTN was about 19.19 % in overall children and adolescent aged 10–18 year (KNHANES 2007–2017), especially 30.4% in obesity group.
The results of our study showed that the prevalence of obesity and HTN was higher in boys than girls, and the prevalence of HTN was higher in the obesity and overweight group than normal weight group, in both sex. Hb and Hct level tended to increase with the degree of obesity and blood pressure in both sex, but there were sexual divergence. In boys and girls, Hb and Hct increased in hypertension than normal BP. In boys, Hb and Hct level were significantly increased in OB group, than NW and OW group. On the other hand, in girls, Hb and Hct were no difference according to obesity. This indirectly suggest that Hb and Hct are closely related to blood pressure than obesity.
Several studies have investigated the association between Hb concentration, Hct and BP, however, most of study performed in adult (19–22). Previous studies presented that Hb concentration increased in humans with HTN, and it is positively associated with SBP and DBP. Result of our study is in line with the finding of previous studies(19–22, 31–33). Atsma et al reported that Hb level was positively associated with both SBP and DBP in healthy adults whom Dutch voluntary blood donors (18 ~ 70 years of age)(20). They presented that 1.3 mm Hg and 1.8 mm Hg increase in SBP with every 1mmol/L increase in the Hb level for men and women, respectively. And DBP rose 1.4 mm Hg and 1.5 mm Hg increase with every 1mmol/L increase in the Hb level in men and in women, respectively. Lee et al reported that SBP in men with Hb concentration ≥ 13.0 g/dL, and in women with Hb concentration ≥ 11.0 g/dL, increased by 2.6 mm Hg with every 1mmol/l increase in the Hb concentration. DBP increased by 3.2 mm Hg with every 1mmol/L increase in the Hb concentrationin both men and women in Korean adult population (aged ≥ 20 yeas)(22). In our study, the regression coefficients was relatively lower than the previous study. SBP increased by 1.3mmHg in boys, and 0.5mmHg in girls with every 1g/dL increase in the Hb level, DBP increased by 1.1 mmHg in boys, and 0.8 mmHg in girls with every 1g/dL in the Hb level. In our study, subjects were 10–18 years old, younger than in previous studies, and the unit of Hb was used as g/dL, instead of mmol/L, so the regression coefficient seem so be smaller. It is estimated that the difference in study design, ethnicity and characteristics of the study subject caused the difference in the results.
Göbel et al found significant correlations between mean arterial blood pressure and red blood cell count, Hct, and Hb concentration in healthy subjects. They suggested that Hct plays a role in determining the viscosity of the blood, and that it may be involved as a rheological factor controlling blood pressure (21). Our study did not collect data of red blood cell count, but indicates that Hct and Hb level are involved with blood pressure determination, although the mechanisms for an elevated BP with increasing Hb and Hct level are not entirely known. Previous stud presented higher Hct levels were associated with a higher incidence of hypertension(34). Increased Hct level associated with increased blood viscosity, and that increased viscosity is a determinant of vascular resistance, and it is expected to contribute to blood pressure(35, 36). The fact that polycythemia vera is often associated with cardiovascular disease supported this theory as plausible (37). Moreover, Kawamoto reported that Hb is strongly related to arterial stiffness, which, in turn, increases SBP and DBP(38). Therefore, they suggested a slightly low Hb level was beneficially associated with arterial stiffness in women. Because Hb is a scavenger of nitric oxide (NO), produced in the endothelial cells(39). NO act as vasodilator and to prevent atherosclerosis development and its complications(40). Increased levels of free Hb induce vasoconstriction due to NO scavenging and consequently, an elevation of BP.
The current study has some limitations. First, since our study is a cross sectional study, it is inherently difficult to draw conclusion about any causal relationship between level of Hb or Hct and BP. Second, it is possible to approximate the prevalence of hypertension, but it is difficult to understand the actual prevalence of HTN, because white coat HTN was not excluded. Finally, the cause of the sexual difference in the relationship between Hb or Hct and blood pressure was not fully elucidated. Periodic menstrual bleeding in girls might have influenced the difference of results. However we have no data about pubertal status, diagnosis of anemia and menstrual loss in the present study. Subsequent prospective studies are needed to clarify the mechanisms by which blood pressure increase with increasing Hb and Hct level, and to determine the cause of sexual difference.
In conclusion, Hb and Hct levels were positively associated with SBP and DBP in Korean children and adolescent, aged 10–18 year. Both Hb and Hct levels are involved with blood pressure, and their increase could be a potential risk factor for increasing blood pressure. In particular, obesity could have a synergistic effect that cause high blood pressure. It is important to identify and manage risk factors for cardiovascular disease from childhood and adolescence. Establishing their risk factors and management would be the cornerstone of long – tern health care that influences lifelong qualities. Further research is required to identify and verify the factors that increased blood pressure in children of all ages, including newborn, and different ethnic group.