The high prevalence of overweight and obesity observed in this study is alarming especially when compared with the prevalence in a small number of previous reports from comparable locations, which ranged from 0.3% to 5% [5
]. An exception to this however was the study by Owa [23
], also in the same environment which reported a prevalence of 18%. The wide variation in prevalence is largely attributable to differences in population type, geographical location and age range studied [5
]. In the study by Owa, where the prevalence of obesity was similar to that of the current study, the age range studied was wide, consisting of 5–15
year olds, and there was no distinction between overweight and obese subjects. The urban location of our study may also partly explain the relatively high prevalence, as an urban preponderance of childhood obesity has been well described in the developing world [3
In a study among adolescents performed in the same environment as this study about a decade ago [7
], a low prevalence of obesity of 0.4% was reported. The authors noted, as have other African authors [3
], that with increasing affluence in our society associated with a gradual adoption of Western lifestyles [3
] and reduction in physical activity, childhood obesity may become a potential public health problem in Nigeria and other developing countries. This is especially true of highly industrialized cities like Lagos, which may account for the much higher prevalence in the current study that is similar to those obtained from studies in the Western world [11
]. The low level of physical activity in our cohort, despite the availability of the playground, is worthy of note. Urbanization and acquisition of a Western lifestyle may also apply to the subjects in the study by Owa [23
], who were from the elite class and lived in a highly developed part of the area of study. This is in contrast to the developed world where obesity seems to be more predominant among rural dwellers [11
In our study, BP increased with age, as has been documented by other studies [3
]. In addition, SBP and DBP were positively correlated with BMI in both sexes. In the total study population, there were 7- and 2-fold increases, respectively, in the risk of hypertensive range SBP in obese females and males compared with their normal BMI counterparts. The risk of hypertensive range DBP was about 4-fold and almost 2-fold for males and females, respectively, compared with normal BMI subjects. In addition, the overall risk for pre-hypertensive range BP in obese subjects was 2–2.5 times higher than for normal BMI subjects. The association between BMI and BP has been attributed to various interacting complex systems that have been observed in obese children. They include over-activity of the renin-angiotensin and sympathetic nervous systems, insulin resistance and abnormalities in vascular structure and function [28
A higher risk of hypertensive range SBP and DBP was documented amongst the cohort in group B (the older age group) implying that if weight control interventions are implemented from early childhood, obesity and its attendant comorbidities may be prevented. The USPSTF recommendation for obesity screening from the age of 6
] is therefore appropriate, as obesity-related hypertension has been demonstrated even in pre-adolescent children [29
The increase in prevalence of hypertensive range SBP in obese children compared with their counterparts with normal BMI was significant in females and not in males. An increase in hypertensive range SBP with age in children with normal BMI was observed only in males in our cohort. This phenomenon has been attributed to hormone-related sexual dimorphism [27
]. Even though we did not apply the Tanner staging of puberty, the non-observance of this phenomenon amongst females implies that obesity may exert a greater influence over SBP than hormonal-related pubertal changes. Further studies will be needed to verify this.
Publications on the relationship between BMI and BP are limited in our environment. From the few accessible, the general trend was an increase in BP with BMI, though with some variations [27
]. In a study that specifically correlated obesity with hypertension in adolescents [27
], the authors demonstrated 2- and 3-fold increases in point hypertension in overweight/obese males and females, respectively, compared with their normal BMI contemporaries in an urban area. In the semi-urban area, the increase was about 5-fold for girls but was absent for boys. However, the study did not distinguish between overweight and obese subjects and SBP and DBP. It is possible that if overweight and obesity were separately assessed, a more significant increase in BP with BMI would be observed in obese subjects.
The findings in this current study are strikingly similar to that of a Canadian study [11
] that reported a 19.5% prevalence of hypertension in obese children compared with 4% in non-obese children. Similarly, the prevalence of pre-hypertension was 18.2% and 5.7% in obese and non-obese children, respectively. In the same study, there was a higher prevalence of obesity-related hypertension in children aged 13–17
years compared with the lower age group of 4–12
years. These findings were independent of family history of hypertension or kidney disease, similar to our observations and further substantiated by a similar recent study from Sudan [32
A recently concluded longitudinal follow-up of 13-year-old adolescents into young adulthood in the USA [16
] revealed that high BP and BMI act independently and have an additive effect in predicting the highest levels of cardiovascular risk in young adults. It can therefore be inferred that an increase in the prevalence of adolescent hypertension may herald an increase in the prevalence of adult hypertension, cardiovascular morbidity, and mortality in Nigeria. The threat this poses on the already inundated health-care system is enormous and calls for action by all stakeholders to combat this epidemic at every level of care.
A limitation of this study was that BP readings were obtained on a single day and hence the persistence of elevated BP could not be ascertained. This is significant as some studies have demonstrated a reduction in the prevalence of elevated BP following repeated measurements in the same cohort [33
]. In a large Swiss study [33
], there was a 5-fold decrease in the proportion of children with elevated BP following triplicate BP measurements. However, overweight or obesity still accounted for 37% of cases of hypertension in their cohort. In view of this limitation, a follow-up to this study with recall of all the students with pre-hypertensive and hypertensive range BP is planned. Information about sexual maturation and dietary intake such as level of salt consumption which may affect BP was also not available.