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Blood pressure (BP) is a physiological variable. Therefore since it is a physiological variable then it can be affected by environmental and social changes. How do racial differences come into play here, for instance? Since blacks face more (perceived) discrimination, then they should, on average, have higher BP levels than whites. This is what we find—but the effect is mostly seen in low-income blacks. How do psychosocial factors come into play here in the black-white BP gap?

BP is regulated by cardiac output, vascular resistance of blood flow, blood volume, arterial stiffness, and, of course, the individual’s emotional state which can decrease or increase BP. Neural mechanisms also exist which regulate BP (Chopra, Baby, and Jacob, 2011). Knowing how and why BP increases or decreases will have us better understand the social contexts of increased BP in low SES blacks.

BP is a complex physiological trait. It can go up and down due to what occurs in the immediate environment. Values of 120/80 mmHg are cited as ‘average’ values, but we have no idea what an ‘average’ BP is. Nevertheless—like most/all physiological variables—there is a wide range of what is considered ‘normal’. Due to the variance in human physiological systems, what is ‘normal’ for one individual is not ‘normal’ for another. Variation in BP (like, say, 120 SBP (systolic blood pressure) to 140 SBP) is ‘normal’. I believe even around 110 for SBP is within that range. For DPB (diastolic blood pressure) between 75 and 90 is within normal diurnal fluctuations due to activity/eating/etc (Taylor, Wilt, and Welch, 2011). BP, like testosterone, is one of those tricky variables to measure and so must be measured upon waking to see if there are any problems. So even for a trait like BP, there seems to be a ‘normal range’.

About 33 percent of blacks have hypertension (HTN) (Peters, Arojan, and Flack, 2006). Urban blacks are more likely to have higher BP levels than whites, but “At present, there is no complete explanation for these differences and further research is required” (Lindhorst et al, 2007). Low SES is correlated with higher levels of BP in black Americans—especially those with darker skin—but not Africans in Africa (Fuchs, 2011), suggesting that this is an American phenomenon that needs to be addressed. One good explanation, in my view, is the social environment. Physiological traits are extremely malleable due to the need to be able to ‘change gears’ in an instant, for instance to either fight or flight. Though, in our modernized world, these responses—mostly—have no need and so (due to our supposed civilized behavior), one’s BP rises due to social stress and other environmental factors and it is due to the urban environment.

What is the cause of high BP in blacks?

One explanation that has been given to explain higher rates of BP in blacks when compared to whites is discrimination. However, studies show mixed evidence on whether or not so-called discrimination raises BP (Couto, Goto, and Bastos, 2012). The same American effect (American blacks having higher BP than American whites) is seen even in the UK London area (Agyemang and Bhopal, 2003). This, yet again, is more evidence that the social environment drives these differences—again, regardless of whether or not any of the discrimination is real or imagined. Say most of it were imagined: it’d be irrelevant because the imagined discrimination leads to very real physiological outcomes in BP.

The country of birth also has an effect on BP. In one study, it was noted that Africans had significantly higher BP when compared to Asians (which is identical/lower) and native French living in France (Bahous et al, 2015). Ethnic differences in BP increase due to similar sodium intake is lower than what is usually cited (Graudal and Jurgens, 2015). However, other authors have pointed out that basing conclusions off of observational studies have problems, like the estimation of sodium intake being inaccurate since it’s a one-time measure; (Gunn et al, 2013; Cobb et al, 2014)

There is also evidence—along with pathways—that show how certain social activities work to lower stress and BP, including participation at church (Livingstone, Devine, and Moore, 1991). Black Americans can make other lifestyle changes in order to decrease BP, such as exercise and other lifestyle interventions. Redman, Baer, and Hicks state that “gene-environment interactions, job-related stress, racism, and other psychosocial factors to racial/ethnic disparities” need to be explored as causes for higher rates of HTN in blacks compared to whites. And with the knowledge of how all physiological systems work in terms of stress and other factors, should be explored as causes for this disparity.

Grim et al (1990) state that factors that influence high BP in blacks compared to whites are inherited and that is the major source of variation between these populations. However, the other mounting social/physiological evidence deserves an explanation; that is not inherited, and what we know about how our physiology responds to stress and discrimination—whether real or imagined—are extremely important and lead to extremely real, and important, outcomes in these populations. It is also argued that since blacks en route to America during the slave trade died from salt-depletive diseases, that blacks with a higher genetic propensity to absorb salt survived and this is why blacks have a higher propensity to absorb salt and are more ‘salt-sensitive’, which also could explain higher rates of HTN in American blacks compared to their cousins in Africa (Wilson and Grim, 1991). However, Curtin (1992) disputes this because “There is no evidence that diet or the resulting patterns of disease and demography among slaves in the American South were significantly different from those of other poor southerners”.

However, in regards to the social environment, Williams (1992) drives one of the best arguments I have encountered in this literature so far, stating that while genetic factors play a small part in regards to the BP gap between blacks and whites, social factors are arguably more important than genetic ones (and with our homeodynamic physiology, this does make sense). Dressler (1990) for instance, argues that skin color is a proxy for both social class and discrimination and these factors explain a large amount of the variation. Psychosocial variables can also explain heightened BP (Marmot, 1985; Cuffee et al, 2014). Yan et al (2003) also note how “time urgency/impatience” and “hostility” “were associated with a dose-response increase in the long-term risk of hypertension.” Henry (1988) also argues that calcium, obesity and genetic factors cannot be the aetiology of HTN in blacks, while also proposing that high sodium intakes are due to psychosocial stress (Williams, 1992: 136).

Obesity also leads to hypertension (Re, 2009) while blacks are more likely to be obese than whites, however, black American men with more African ancestry are less likely to be obese (Klimentidis et al, 2016). This would imply that the greater amount of African ancestry in American blacks both protects against obesity and along with it HTN. Williams (1992) makes a convincing argument that environmental and social factors are the cause for the black-white BP gap. And while genetic factors are important, no doubt, environmental and social factors are arguably more important to this debate.

Kulkarni et al (1998) show that increased stress leads to subsequent BP elevations which, over time, will lead to HTN. In a 2009 meta-analysis, Gasparin et al show how “individuals who had stronger responses to stressor tasks were 21% more likely to develop blood pressure increase when compared to those with less strong responses.”

Further, in support for the ‘perceived stress’ hypothesis in regards to blacks ‘perceiving’ stress and discrimination, “stress denial in combination with abdominal obesity, alcohol consumption, and smoking may be proxy for a high stress level” (Suter et al, 1997). Carroll et al (2001) also show how there are is “modest support for the hypothesis that heightened blood pressure reactions to mental stress contribute to the development of high blood pressure.” Sparrenberger et al (2009) also did a systematic review of observational studies, finding that “Acute stress is probably not a risk factor for hypertension. Chronic stress and particularly the non-adaptive response to stress are more likely causes of sustained elevation of blood pressure.”

Lastly, Langford (1981) shows that when SES is controlled for, the black-white BP disparity vanishes, implying that social and environmental—not genetic—factors are the cause for elevated HTN levels in black Americans. Sweet et al (2007) showed that for lighter-skinned blacks, as SES rose BP decreased while for darker-skinned blacks BP increased as SES did while implicating factors like ‘racism’ as the ultimate causes. This is solid evidence that both skin color and SES are predictors of higher prevalence of BP in black populations, and since other studies show that this is not noticed in higher class blacks, nor is this noticed in blacks in Africa, then the main causes of this disparity are social and environmental in nature.

(Non, Gravlee, and Mulligan, 2012). Their study suggests that educating black Americans on the dangers and preventative measures of high BP will reduce BP disparities between the races. This is in-line with Williams (1992) in that the social environment is the cause for the higher rates of BP. One hypothesis explored to explain why this effect with education was greater in blacks than whites was that BP-related factors, such as stress, poverty and racial discrimination (remember, even if no racial discrimination occurs, any so-called discrimination is in the eye of the beholder so that will contribute to a rise in physiologic variables) and maybe social isolation may be causes for this phenomenon. Future studies also must show how higher education causes lower BP, or if it only serves as other markers for the social environment. Nevertheless, this is an important study in our understanding of how and why the races differ in BP and it will go far to increase our understanding of this malady. This is a very convincing argument that education and not genetic ancestry cause disparities in BP between blacks and whites.

WebMD states that, of course, both environmental and genetic factors are at play in regards to black’s increased propensity for acquiring HTN. Fuchs (2011) also states that “They [environmental and behavioral factors] could act directly or by triggering mechanisms of blood pressure increase that are dormant in blacks living in Africa” and explain why black Americans have higher rates of BP than Africans in Africa. Further, race and ethnicity are independent predictors of HTN (Holmes et al, 2013).

Conclusion

Blacks and whites do differ in BP, and its aetiology is both complex and hard to untangle Genetic factors probably don’t account for a lot of this variance since Africans in Africa have low levels of BP compared to their black American cousins. Numerous lines of evidence shows that social and environmental factors are the cause, and so to change this, all people—especially blacks—should be educated on how to change these problems in our society. Whether discrimination is real or imagined, the effects of it lead to real physiological outcomes that then lead to increased health disparities between these populations.