Hypertension is a serious public health challenge worldwide, being quantitatively the major risk factor for premature cardiovascular disease [1–3]. According to recent estimates, it is more common than cigarette smoking, dyslipidemia or diabetes, the other major cardiovascular risk factors [1,4–6]. The National Health and Nutrition Examination Survey (NHANES), in its wave from years 2005 through 2008, estimated that approximately 29–31% of adults in the United States have hypertension . This translates into 58–65 million hypertensive patients in the United States adult population . In addition, a pooled analysis of global national and regional data  showed that 26.4% of the adult population in 2000 had hypertension (26.6% of men and 26.1% of women), and 29.2% were projected to have this condition by 2025 (29.0% of men and 29.5% of women). The estimated total number of adults with hypertension in 2000 was 972 million of whom 333 million were living in economically developed countries and 639 million were living in economically developing countries. The number of adults with hypertension in 2025 was also predicted to increase by about 60% to a total of 1.56 billion .
However, there is considerable variation (Fig. 1) among countries and geographic regions for the reported prevalence of hypertension and hypertension control rates [9–13].
Exploring racial/ethnic and socioeconomic disparities in hypertension prevalence across the world [14,15], some surveys recognized acculturation to western lifestyle, modernization and urbanization as key factors to explain regional differences in hypertension prevalence [14,15].
Individuals from western societies typically have higher prevalence of hypertension and higher blood pressure (BP) values than those from other world regions, with this difference decreasing as nonwesterners adapt to western culture and lifestyle . In particular, the more acculturated individuals have an average 4 mmHg higher SBP than less acculturated individuals.
Moreover, urbanization affects food consumption patterns, with increased consumption of fats, oils and animal-based foodstuff. These diet changes increase bodyweight, which is a well established risk factor for the development of hypertension.
Conversely, traditionally living populations in rural areas around the world still actively practice horticulture, fishing and hunting. Their diet remains rich in potassium, fibres and omega-3 fatty acids, and low in saturated fat. They usually have lower BP than individuals from western societies [11,14].
In the present issue of the Journal of Hypertension, Malekzadeh et al. present the results of a cross-sectional analysis of the Golestan Cohort Study (GCS) aimed at defining the rates and correlates of hypertension in Iran. The study enrolled Iranian individuals during years 2004–2008 and included 50 045 adults aged 40–75 years. Of the total cohort, about 80% of the participants were urban residents and 21 350 individuals (42.7%) were hypertensive.
The authors provide two pieces of relevant information. First, they documented an association between hypertension and place of residence (rural vs. urban). Second, their findings support the notion that obesity, in addition to age, is a major determinant of hypertension. Notably, in multivariable models (fully adjusted for sex, age, literacy, ethnicity, physical activity, smoking habit and wealth score), the associations between hypertension, BMI and place of residence were more pronounced than the univariate models .
Discussing the report by Malekzadeh et al., some sets of considerations deserve to be made in interpreting the study's results.
Although hypertension is more prevalent in economically developed countries (Figure 1), the larger population of developing countries results in a larger absolute number of individuals affected. Furthermore, by 2025, almost three-quarters of people with hypertension will be living in developing countries [9,10]. Just recently, reasons for the increasing prevalence of hypertension in developing countries have been analysed. Generally, the social and environmental changes that accompany modernization and urbanization are important determinants of health [16,17]. New health risk activities related to urbanization and acculturation to western lifestyle include tobacco use, saturated fat consumption, excessive alcohol intake, sedentary lifestyle and physical inactivity.
In all developing countries surveyed, hypertension was more prevalent in urban and semi-urban regions than in rural areas (Fig. 2), with the difference in prevalence depending on the country's extent of urbanization [12,18–22]. Furthermore, rural–urban migration seems to significantly affect prevalence of hypertension, with rural migrants to urban settings experiencing a rise in SBP and DBP after only few months of migration [23,24].
Nevertheless, Malekzadeh et al. documented a lower prevalence of hypertension among people living in towns than people living in rural areas [odds ratio (OR) 0.90; 95% confidence interval (95% CI) 0.86 – 0.94]. Interestingly, these results seem to refine the hypothesis of a new possible positive effect of urbanization and modernization on the risk of hypertension and cardiovascular disorders .
As recently suggested , urbanization and development of modern cities might bring population health gains through physically safer jobs, dietary diversity, access to mass media and modern technologies, preventive health programmes and environmental improvements.
Of note, few other studies have documented a positive effect of urbanization and modernization on BP. A report from the Egyptian National Hypertension Project showed that hypertension prevalence as well as awareness, treatment and control rates varied by region, with urban areas having the highest rates of awareness, treatment and BP control . Similarly, epidemiological data from adults in Yemen [19,27] showed that the lowest prevalence of hypertension was registered among individuals living in an urban area with a higher degree of modernization and acculturation than inland and coastal areas. In addition, hypertension awareness and BP control were also favourably affected by living in the urban area. Accordingly, the authors concluded that the spreading information and advice on cardiovascular prevention, as well as a facilitated contact with health services, may exert a positive effect on hypertension burden [19,27].
The relationship between BMI and BP has long been the object of epidemiological and clinical investigations. Mendelian randomization studies estimated that each 10% increase in BMI is associated with an elevation of SBP by 3.9 mmHg .
Although the potential mechanisms through which increased BMI directly causes hypertension are still an area of research, human and animal studies have elucidated that obesity-related hypertension is the result of a combination or overlap of a number of different processes. Overweight and obesity are associated with adipose tissue dysfunction characterized by enlarged hypertrophied adipocytes, increased infiltration by macrophages, marked changes in adipokines secretion and free fatty acids release [29,30]. This contributes to insulin resistance [29,31], augmented chronic vascular inflammation and oxidative stress [32–34], enhanced activation of the renin-angiotensin-aldosterone system [35,36] and sympathetic overdrive , eventually leading to hypertension.
From an epidemiological standpoint, the causal association between obesity and elevated BP has longe since been demonstrated by large population-based studies. Notably, a stable linear relation between adiposity and BP has been reported across traditionally living societies , economically developed  and developing countries [39–42].
BMI was a powerful predictor in the NHANES III Study . With normal weight individuals as the reference, the OR for the presence of hypertension was 1.7 for overweight individuals, 2.6 for class I obesity, 3.7 for severe or class II obesity and 4.8 for morbid or class 3 obesity .
A recent study by Gurven et al. compared prevalence and correlates of hypertension between Tsimane adults enrolled in the Tsimane Health and Life History Project and western individuals included in the NHANES study. Among Tsimane adults, a population of forager-horticulturalists in the Bolivian Amazon, prevalence of hypertension increased with age, but the rate of hypertension was smaller than in western countries. Interestingly, age-related increases in BP were modest when compared with western individuals, but BMI had the most substantial effect on BP level between both Tsimane and western individuals.
In Chinese women from rural areas followed up for 28 months, the risk of progression to hypertension was associated with advancing age, BMI, salt intake and low physical activity .
Similarly, Malekzadeh et al. observed that when compared with individuals with normal BMI, the risk of hypertension was higher among overweight (OR 1.67, 95% CI 1.60 – 1.74) and obese (OR 2.57, 95% CI 2.46 – 2.70) individuals.
Combining the results of the analysis by Malekzadeh et al. with previous reports from other countries, it is likely that BMI exerts a significant and similar impact on the risk of hypertension in all the areas of the globe. This suggests that BMI differences drive differences in hypertension prevalence among areas with substantial racial/ethnic and socioeconomic disparities and that weight control throughout the entire life may play an important role to prevent the rise in BP in both developed and developing countries.
Nonetheless, healthcare systems have not yet adapted to cope with these new challenges and very limited resources are globally allocated to improving healthy lifestyle choices.
This study was funded in part by the Fondazione Umbra Cuore e Ipertensione – ONLUS, Perugia, Italy.
Conflicts of interest
None of the authors of this study has financial or other reasons that could lead to a conflict of interest.
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