Why Is Hypertension More Common in African Americans?

Thomas G. Pickering, MD, DPhil

Introduction

In the last article of this series, some recent findings were discussed that suggest that depression may be a factor contributing to the onset of hypertension, particularly in African Americans.[1] It is well known that hypertension is more common in African Americans than in Caucasians, but the reasons for this remain obscure. The issue revives the old nature-nurture debate: do African Americans have a genetic predisposition that leads to hypertension, or does it have to do with their environment? As one might guess, the answer almost certainly lies somewhere in between, in accordance with Oscar Wilde's adage that truth is rarely pure and never simple. Some recent studies have thrown light on this question, and to my mind, favor the predominance of environmental factors.

One of the problems in this debate is that we are trying to relate two poorly defined entities. The concept that racial differences in chronic disease are due to genetic differences among races has been repeatedly challenged. There is no way to consistently classify people by race: there are continuous gradations between ethnic groups, and genetic analyses have found that there is more genetic variation within one ethnic group than between one group and another.[2] The American Anthropological Association recently stated, "...human biological variations should not be reduced to race. It is too complex and does not fit this outdated idea. Race is real. Rather than being based on biology, it is a social and political process that provides insights into how we read deeper meaning into phenotypes. Racialization and racism come about because, in a racialized culture, we read meaning into skin color and other phenotypic variants. Rather than biology affecting behavior, ideology and behavior affect individuals 'under the skin.'"[3]

A huge effort has already been made to try to understand the reasons for the higher prevalence of hypertension in African Americans, almost all of which has involved the underlying assumption that there is some genetically determined physiologic difference. So far, the results have been disappointing at best.[4] For one thing, it is becoming clear that, with rare exceptions, human hypertension is determined by several genes, and it is unlikely that genetic factors account for more than 50% of hypertension. In order for a genetic factor to be accepted as a cause of hypertension in blacks (that is, people of African descent, who do not necessarily live in America), it is not sufficient merely to show that there are ethnic differences in the allelic polymorphisms of candidate genes; it is necessary that the observed differences can account for the differences in blood pressure. So far, although there have been descriptions of both genetic and physiologic differences between African Americans and Caucasians, it is not clear whether these actually contribute to the blood pressure differences. The latest example is a hypothesis that creatine kinase, which has been observed to be present in greater amounts in the skeletal muscle of African Americans than Caucasians, could be a genetic factor that predisposes African Americans to hypertension, if it is also increased in vascular smooth muscle and cardiac muscle.[5] At the present time there is not a scintilla of evidence that this is the case, or that it affects blood pressure.

One practical aspect of these discussions is whether hypertension should be treated differently in African Americans and Caucasians. Much has been written about angiotensin-converting enzyme inhibitors being less effective in African Americans, but one of the largest studies to examine this, conducted by the Veterans Administration, found that the response to captopril and other antihypertensive drugs was influenced to a greater extent by whether the patients lived inside or outside the "stroke belt" of the southeastern United States than by their ethnicity.[6] Furthermore, the differences in response rates that have been observed have usually been relatively small, and while they may achieve statistical significance, they do not achieve clinical significance. Finding the right antihypertensive drug is still, unfortunately, a matter of trial and error.

One of the most powerful pieces of evidence that environmental, and particularly psychosocial, factors are important in the development of hypertension is a series of epidemiologic studies conducted over many years, which have shown that when people move from a traditional tribal society to an urban, westernized lifestyle, their blood pressure rises. Many of these studies have been conducted in Africa, and hypertension is (or was) relatively rare in rural Africans. A good example is the Luo study from Kenya.[7] People who had moved to Nairobi had higher pressures than people living in the villages, even if they had been in the city for only 1 month. They also had higher heart rates, consistent with activation of the sympathetic nervous system. While these studies indicate that there is something pressor about our western lifestyle, they do not tell us whether it is stress or diet that is more important (it is probably both).

Another study from Africa sheds light on the question. Egypt, being at the intersection of the Arab countries and Africa, has a multiethnic society, with dark-skinned Nubians in the south and paler-skinned people in the north. The authors measured skin color and blood pressure, and found no relation between the two in men, and a rather weak association in women that was nonlinear (the women with the highest pressures tended to have brown rather than black skin). In contrast, studies conducted in the U.S. have generally shown that darker skin is associated with higher blood pressure.[8] The genetic school would explain this on the grounds that skin color is a marker of the admixture of African and European genes, and that African genes lead to high blood pressure, while the environmental school would say that the color of one's skin determines the way one is perceived in society, and that this influences the blood pressure. The Egyptian study favors the latter, and is corroborated by an epidemiologic study from Cuba, where Communism has presumably broken down the cultural barriers that separate African Americans and Caucasians in the U.S., and where the ethnic blood pressure differences are quite small.[9] Studies from other countries also show that the higher blood pressure seen in African Americans is not invariable. In England, the Health Surveys for England (equivalent to the National Health and Examination Survey, or NHANES, in the U.S.) found that the prevalence of hypertension was much higher in African Americans than Caucasians, even after controlling for potential mediators, such as age, body mass index, and alcohol intake.[10] However, another study conducted in English factory workers found no ethnic differences in blood pressure.[11]

While these variable findings are difficult to explain on a genetic basis, they also cannot be accounted for by differences in the major environmental risk factors for hypertension, such as obesity and salt intake, since these have been controlled for in several of the studies that have disclosed an ethnic difference in pressure.

The risk factor that has been almost totally neglected in studies of racial differences in blood pressure is the social environment. While there is a pronounced gradient in the prevalence of cardiovascular disease according to socioeconomic status in most affluent countries, a socioeconomic gradient of blood pressure is not seen very consistently.[12] A series of classic studies conducted by Ernest Harburg in Detroit in the 1970s was the first to really probe this question.[13,14] He showed that blood pressure in African Americans living in the inner city was highest in people living in the worst-off neighborhoods. A more recent analysis of the Atherosclerosis Risk in Communities (ARIC) study by Diez-Roux et al.[15] found that "neighborhood" effects were independently related to blood pressure after controlling for other risk factors. In four U.S. communities, people living in the neighborhoods with the lowest median house prices tended to have the highest blood pressure. This was most pronounced in the one community (Jackson, MI) that was largely African American.

This brings us to the environmental factor that stares us in the face as a potential cause of the higher prevalence of hypertension in African Americans, which has largely been ignored by researchers (perhaps because it is so politically sensitive): racial discrimination. The only epidemiologic study I am aware of in which this was investigated with regard to blood pressure is the CARDIA study of young African American and Caucasian men and women (one of the studies showing that depression is a risk factor for hypertension in African Americans).[16] Subjects were asked if they had experienced racial discrimination, and how they had reacted to it. Although 80% of the African American subjects reported having experienced some discrimination, the relationship with blood pressure was not what might be expected. Systolic pressure was 7.4 mm Hg higher in men who reported no discrimination, and who accepted unfair treatment and talked to others about it. It was also higher in women who reported no discrimination. This is a strange result: While it shows that there is a substantial effect, it is the opposite of what one would expect. The authors of the study had to perform some contortions to explain the findings, and suggested that the people who reported no discrimination were actually in a state of denial, or "internalized oppression," and found the topic too painful to acknowledge.

In evaluating the effects of psychosocial influences on blood pressure, such as racial discrimination and living in disadvantaged neighborhoods, we are faced with the problem of having to assign numbers to phenomena that are inherently subjective and hard to define. This is, of course, a common problem in behavioral science, and in many cases we must rely on self-reporting, usually using a standard questionnaire. One of the limitations of this procedure is that people are not always accurate in how they answer such questions because they may give the answers that they think are expected of them rather than what they really believe. However, whether something is hard to measure should not deter us from making the effort to do so, particularly when the answer to the question being posed is so important. This is an area that needs more research, and it is time to move on from the simplistic concept of "race as genes."

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