The main results of this national study showed that elevated BP was prevalent among Tunisian adolescents, more so in boys, that prevalence increased strongly with BMI and WC, while associations with behavioral and socio-economic factors were much less straightforward.
Apart from a possible overestimation of BP due to methodology limits, the observed prevalence of pre-HT and HT in Tunisian adolescents are among the highest rates reported in the literature for this age class, where prevalence ranged between < 1% and 5.1% for HT [15
] and from < 10% to < 40% for pre-HT [32
]. Comparing the prevalence of elevated BP with data in the literature is difficult because it is easily subject to bias related to the class of age chosen as well as regional differences in the definition of elevated BP, the distribution of reference BP data and the method used to measure BP [31
]. However in many countries, it remains largely underestimated [41
]. Recently, Hansen et al. [42
] showed that among a cohort of 14187 children and adolescents aged 3-18 followed regularly in a large academic urban medical system in northeast Ohio (USA), only 26% of HT and 11% of pre-HT cases had an appropriate diagnosis recorded in their electronic medical record. The authors suggested that this low diagnosis rate could be accounted for by the lack of knowledge concerning normal BP ranges and the lack of awareness of a patient's previous BP readings. Thus, it would be useful to provide a practical tool for physicians to enable them to easily identify the threshold of normal BP values for adolescents according to age, gender and height.
The present study identified gender differences mainly in the prevalence of elevated BP as Tunisian males were more affected than females. Gender differences in the risk of developing elevated BP have been reported by several authors in different populations [37
]. According to Dasgupta et al., the gender differences in the risk of elevated BP can be explained by the impact of sex steroids on BP [44
]. This factor is also strongly suggested by experimental models [46
The prevalence of elevated BP and HT were higher among overweight and obese Tunisian adolescents. The association between BP level and overweight status has been documented in many young populations thanks to prospective and cross-sectional studies [11
]. According to a recent review, there are three main mechanisms of obesity-induced- hypertension: activation of the sympathetic nervous system, renal, and hormonal dysfunction [50
On the other hand, in the models including both BMI and WC as covariates, a significant association was found between WC and BP status for males and females adolescents, especially with elevated BP. These results show that BMI and WC, despite being strongly correlated, once adjusted for one another, remain significantly associated with elevated BP, indicating that they are independently associated with elevated BP. These findings are in agreement with data in the literature [49
]. Indeed, according to many authors, the distribution of body fat in particular abdominal fat, is even considered to be more predictive of health risk than whole body composition measures such as BMI [54
]. The association between WC and elevated BP has been attributed to hyperinsulinemia induced by excess abdominal fat [55
Comparison (detailed data not shown) of our BP data with data gathered among Tunisian adolescents (aged 15-19) during the Tunisian 1997 national survey using comparable data (1996 BP data featuring only one measurement compared to the first measurement of our 2005 survey), showed that SBP and DBP had decreased in girls but remained stable in boys. Adjustment for BMI, age, height, and area (urban/rural) did not change these trends. BMI is a major determinant of BP, but the absence of an increase in BP over the time in Tunisia and reported in several other studies, suggests that, despite the significant increases in the prevalence of overweight [57
], other factors may also have an influence on the evolution of BP [58
]. Such factors may include diet characteristics, i.e. the intake of fruits, vegetables, or dairy products, which leads to an increase in calcium consumption, which traditionally was rather low; as a matter of fact, increased consumption of dairy products has been repeatedly associated with reduced risk of elevated BP [59
]. In accordance, the assessment of dietary intake in a subsample of the same subjects [62
], underlined a main dietary pattern of modernization associated with urbanization and regional socio-economic development, this pattern being associated with a favorable effect on BP in girls.
According to some authors, studies using uniformly standardized methodology showed a positive trend in the prevalence of HT among adolescents especially with increasing prevalence of obesity, in contrast to other studies which showed a decrease [32
]. On the other hand, Chiolero et al. showed that at the population level, a marked increase in the prevalence of obesity in children and adolescents in a rapidly developing country was not associated with a commensurate secular rise in mean BP [58
] despite the strong relationship between obesity and elevated BP at individual level in the same study. On the other hand, some authors showed that obesity during childhood is associated with elevated BP in early adulthood [64
]. All these results lead us to formulate the following hypothesis: persistent exposure to overweight is a risk factor for elevated BP later; the association observed at individual level may be explained by premature exposure to overweight in early childhood and the recent exposure to overweight could expose the subjects to the risk of elevated BP in early adulthood. Hence, the effect of the increase in the prevalence of overweight in Tunisian adolescents will probably lead to an increase in the prevalence of BP later in adulthood.
In our study, elevated BP and/or HT among males and females was not strongly associated with the other factors we analyzed (socio-economic factors, physical activity, stress level) as much as with intermediate outcomes such as BMI and WC. Prevalence of elevated BP increased somewhat with an increase in sedentarity level among Tunisian adolescents; this result is consistent with most other studies [37
]. Hence, the promotion of weight control using appropriate strategies (measures that target environmental factors as well as behavioral ones) in Tunisian children and adolescents aimed at reducing overweight, could also help reduce elevated BP and many other risk factors of chronic diseases during adolescence and adulthood. Our data did not reveal any link between perceived stress and BP. Indeed, there is a controversy in the literature about this relationship as some studies based on declared perceived stress report decreasing associations [68
The absence of monitoring of BP among adolescents reflects the facts that the majority of physicians and parents in Tunisia think that elevated BP is rare among adolescents and are probably not aware of current epidemiologic trends in adolescents' health. Indeed, the prevalence of overweight and obesity (risk factors for elevated BP) among adolescents has increased in Tunisia [20
], due to the epidemiological transition and its impact on the environment and changes in lifestyle: indeed the analysis of changes in the food consumption behavior among 15-19-year old Tunisian adolescents between 1997 and 2005 showed for instance that the intake of total fat, saturated fat and total sugars increased while the intake of PUFAs decreased [57
]. Pre-HT left untreated during adolescence predisposes to persistent HT in adulthood [71
]. Thus, it is reasonable to take action to prevent elevated BP during childhood and adolescence and to contribute to reducing morbidity and mortality related mainly to cardiovascular diseases. However, whereas screening should be focused on overweight and obese young people; primary prevention should concern the whole young population.
As for its strengths and limitations, the study was based on a large random sample of the Tunisian adolescents, the first at such a large scale in Tunisia. However, the cross-sectional design has known limitations regarding causal interpretations of observed associations between the measured covariates and elevated BP and HT. For financial and practical reasons, assessment of the BP status, was based on two measurements of BP made during the same visit. Indeed, we measured BP in non-stress conditions (at home, no white coat), and a recent study showed that BP screening based on three or more measurements per visit was no better than two [72
], but not using sets of measurements made during repeated visits as advised in the literature [30
], may have led to overestimation of elevated BP [15
]. Not taking into account dietary habits was another obvious limitation of this study but this aspect had already partly been dealt with by the same authors though only on a sub-sample of the subjects [62