Our study shows that exposure to the stroke belt is associated with higher prevalence of hypertension among a national sample of adults 45 and older. After adjustment for age, race, sex, physical activity level, body mass index, smoking, alcohol use, education, and income, the prevalence of hypertension was significantly most strongly related (p < 0.0001) to lifetime as well as adolescence or early adulthood exposure to the stroke belt than exposures at other times. Birthplace and current residence in the stroke belt were each independently associated with hypertension, however, lifetime, adolescence or early adulthood exposures were more predictive than the joint model with both birthplace and current residence. There was consistency across the race-sex groups in the adolescence/early adulthood/youngest ages with the exception of black women, where the later age periods of exposure to the stroke belt were most predictive of hypertension.
Previous studies suggest a relationship between birthplace in the Southeast and stroke mortality. Our findings extend those previous reports by showing that early life exposure to the stroke belt is associated with hypertension, which is the major population attributable risk factor for stroke. (22
) Individuals are more likely to spend a greater period of their early life in the region of their birthplace. Moreover, health attitudes and behaviors established in adolescence and early adulthood are likely to persist in later years.
The findings of our study further suggest that regional differences in lifestyle during adolescence and early adulthood have the most substantial contributions to regional differences in hypertension. One could speculate that these are periods where life style choices, such as dietary patterns and exercise habits, are established. The southern region of the US consisting of the eight traditional stroke belt states plus eight other states in the mid-Atlantic area plus the District of Columba has the highest sodium and lowest potassium intakes, dietary factors that interfere with achieving an optimal blood pressure. (23
) In addition, data from multiple sources show that there are generally lower levels of healthy behaviors in the southern region of the country. (24
) The 2005 US Behavioral Risk Factor Surveillance Study (BRFSS) shows that 7 of the 16 states with the highest percentage of persons who reported no leisure time physical activity (> 26%) were stroke belt states. (25
) In 1991 BRFSS data, the geographical areas with the highest prevalence of 2 or more cardiovascular risk factors were the Midwestern and southern states, and from 1991 to 1999, the prevalence increased by 10% or more in 36 states, significantly so for 21 states, including 7 of the 8 stroke belt states. (24
) The 2001 BRFSS showed that four of the top 10 states in obesity prevalence were within the stroke belt. (24
) Our study does not have data on lifetime salt consumption but additional analyses could be performed using the single measurement obtained through baseline dietary assessment, a method Strazzullo et al used in their meta-analysis of salt intake and CVD outcomes. (27
Southern states have been reported to have among the poorest childhood health circumstances including high rates of low birth weight, high infant mortality, child mortality, teen mortality, and teen birth rates. (28
) This suggests that we consider the fetal origins theory that prenatal and early life conditions are associated with predisposition to diseases and risk factors later in life, sometimes referred to as the Barker hypothesis. (29
) A recent meta-analysis of almost 2000 adults from 20 Nordic cohorts supports the inverse relationship between birth weight and systolic blood pressure, with or without adjustment for BMI, showing heterogeneity in shape and strength of association by sex and age. (30
) One of the few studies to be conducted in the US, actually of a cohort within a stroke belt state (Louisiana), the Bogalusa Heart Study, found that birth weight was significantly inversely associated with progression of systolic and diastolic blood pressure as well as pulse pressure through early adulthood, and this was after adjustment for later health indicators such as BMI. (31
) Lower birth weights in the Southeast could explain a portion of the higher hypertension prevalence in this portion of the United States. (32
) This is an area that merits further study. Unfortunately REGARDS did not obtain data on birth weight.
The findings in our study are subject to some important limitations. The cross-sectional design provides only a one-time assessment of blood pressure so the estimate of prevalent hypertension for any individual may be incorrect. Error in either direction is possible, however, misclassification was minimized because our definition included individuals who reported taking antihypertensive medications but who were normotensive at time of exam. We do not have data on lifetime socioeconomic conditions or environmental exposures that are known to contribute to the development of hypertension. (34
) Inherently, self-reported measures of smoking status, physical activity, alcohol, and income are prone to bias, however we have actual measurement of BMI. Additionally, while the validity of the assessment of physical activity is well established, it is easier to work up a sweat in a hot climate than a cooler northern climate.
While strengths of this study include a national, general population sample, a large African-American population and the availability of lifetime residential history, it is possible that those agreeing to participate are non-representative of the general population. We do not have information on cooperation rates by strata. There is also a potential bias in that approximately 25% of the participants who initially consented by telephone could not or did not continue to the in-person exam. While this would affect the estimate of prevalence of hypertension, it has only a minor role in the association of hypertension with the stroke belt exposure measures. In comparison with other cardiovascular cohort studies however, our cooperation rate compares favorably, in particular given that participation involved allowing a stranger into the home and disrobing for an electrocardiogram.
The addition of adolescent (ages 13 to 18) or early adulthood (ages 19 to 30) stroke belt exposure had a similar impact on the ability to predict hypertension as education, income, alcohol use, and smoking, and had only marginally smaller impact than exercise. However, while the difference between exposures at these ages compared to other ages was statistically significant, there was only a marginal increase in the predictive ability. While this difference is small, that exposures to the stroke belt during adolescent or early adulthood are more strongly associated with later prevalent hypertension may offer clues to guide interventions to reduce the burden of the disease.
Our data also indicate risk for hypertension associated with multiple stages of life including the latter stages, especially for black women. Thus, strategies for hypertension prevention should be designed to reach across age, sex and race. It is not clear if the association between early life exposure to the stroke belt causes risk of future cardiovascular disease from vascular damage incurred in early adulthood or whether it is the exposure to poor lifestyle that influences learned habits that extend into later life. More research is needed on what components related to early life in the stroke belt could be contributing to the excess stroke risk later in life.
Our data suggest that after control for risk factors for hypertension, the adolescence period and the young adulthood period of exposure to the stroke belt are most predictive of hypertension later in life. This suggests a window of opportunity to be targeted for intervention to prevent the development of hypertension. It is during these age periods that individuals begin to have more choices and more control over behavioral and lifestyle factors in such areas as diet, physical activity, and smoking that impact the development of hypertension. There are many proven nonpharmacologic approaches to preventing hypertension. (35
) Because hypertension is the major contributor to diseases such as stroke, coronary artery disease, and end-stage renal disease that are among the leading causes of mortality, community and environmental strategies to prevent hypertension need to start earlier in life, specifically in adolescence and young adulthood.