Hypertension in Blacks of West African Origin

BACKGROUND:

The increased risk of hypertension in United States Blacks remains a major unsolved public health problem. Comparisons between macro-population groups in the United States suffer from confounding by social factors and genetic admixture. The study's international comparative design provides a unique opportunity to ascertain the evolution of hypertension among populations with a common genetic background currently living in widely varied social conditions. A well-organized network of investigators currently exists in the societies of the Black diaspora and provides for the first time an opportunity to conduct cross-national research on this question.

DESIGN NARRATIVE:

During the first phase of the project over 12,000 adults between the ages of 25 and 74 were sampled from seven communities: West Africa (Nigeria, urban & rural; Cameroon, urban & rural), the West Indies (St. Lucia, Barbados) and Maywood (lllinois). The screening examination provided information on blood pressure (BP), height, weight, waist/hip ratio, alcohol use, education/occupation, sodium/potassium excretion and social stress. These data provided estimates of hypertension prevalence in each geographic region and the contribution of the known risk factor to higher rates among United States Blacks. In the first phase considerable effort was directed toward establishing a strong research infrastructure in each of the sites and demonstrating the feasibility of cross-national blood pressure comparisons.

In the second phase the research was continued and extended in three areas: 1) Population studies on risk factors. At the start of the study in 1991, it was not clear that adequate procedures were available to allow standardization of mean blood pressure across sites, especially since differences between some sites might be as little as 2 mmHg. Experience since that time suggested that it would be feasible to collect definitive data on this question. In-depth studies were carried out at one of the African sites to investigate the earliest stages of hypertension risk for a population. Investigators in Jamaica and the United Kingdom joined the collaborative group to examine the effect of migration from the Caribbean to the United Kingdom. 2) Prospective study of blood pressure and mortality. It was well known that hypertension accounted for much of the excess mortality among United States Blacks. It was less well recognized, however, that death rates among Blacks in the United States were higher than reported in the Caribbean or Africa. Hypertension was likely to account for that differential. A follow-up study in each region estimated the relative and attributable risk from hypertension. 3) Heredity/Genetics. The populations in this study shared a common ancestral origin in West Africa. Family studies were undertaken to determine the heritability of blood pressure/hypertension within and between sites. In addition, pilot studies on genetic distance and candidate genes for hypertension were conducted.

The study was renewed in FY 1999 to examine in further detail the role of the two most potent hypertensive risk factors, namely dietary sodium and obesity. Randomized studies will be carried out in Nigeria, Jamaica, and Chicago to determine the relative sodium sensitivity of these populations and the factors which condition the blood pressure response. The renin-angiotensin system and renal sodium handling will be compared as causal mechanisms. In addition, body composition will be studied in a large sample of each of the three populations to determine the role of body fat versus lean body mass. Insulin and leptin will be examined and physical activity will be measured by stable isotopes to assess the role of a sedentary lifestyle as a contributor to the hypertension risk experienced by the obese. Existing cohorts will be followed to determine hypertension sequelae and to examine changes in blood pressure with age.