The genetics of racial differences in hypertension susceptibility
Part three of the “genetics and race” debate in the pages of The New Republic is up. One point the debaters seem to have oddly fixated on is the racial disparity in the prevalence of hypertension– for those of you unaware, African-Americans have higher rates and more extreme forms of the disease.
The debaters at TNR seem to be aware of two hypotheses for why this disparity could exist: first, the “slavery hypothesis“, according to which the slaves that survived the long trip from West Africa in sub-human conditions were those that were able to retain sodium and resist dehydration (sweating causes the loss of salt, which must then be replaced), leading to a massive selective event for improved salt retention that today manifests itself as hypertension. The alternative, they seem to think, is that discrimination and an abundance of unhealthy fast food restaurants in African-American neighborhoods in conjunction lead to the differential susceptibility.
There’s no doubt that environmental factors play a role in hypertension, and it’s likely that differential exposures to those factors play a role in this phenomenon. But in terms of genetics, it’s certainly not the slavery hypothesis or nothing. I actually see no reason to invoke it– the key observation (perhaps not a groundbreaking one) is that African slaves came from Africa, which is quite different from North America. One obvious difference between the two continents is climate. Is it possible that West African populations were adapted to an West African climate; perhaps in such a way that increases hypertension susceptibility in the US today? Absolutely.
I present as evidence this (open access) paper, which states (in the title) that “Differential Susceptibility to Hypertension is Due to Selection during the Out-of-Africa Expansion”. I strongly recommend at least the introduction for anyone interested in this topic.
The authors look at a number of alleles involved in salt retention and blood pressure, and perform analyses simlar to those done in the paper on tonal languages and two brain-expressed genes– they examine the correlations of the allele frequencies with distance from the equator. When they compare these correlations with those from randomly chosen loci, they find the salt-retention alleles are outliers, suggesting they’ve undergone selection in reponse to latitude (which they interpret as a proxy for climate). While in the tonal languages case I urged skepticism about causation, these alleles are already known to be involved in hypertension. This just presents evidence that the differences in allele frequencies between populations is due to natural selection, as opposed to some neutral process.
So African populations (and thus, African-American populations as well) have different allele frequencies of at genes known to be involved in hypertension. There is no doubt that this divergence plays a role in the different susceptibilities of African-American and European-American populations to this disease.
Hypertension rate is quite high among Africans in Africa as well:
“Nationwide prevalence study of hypertension and related non-communicable diseases in The Gambia” – Tropical Medicine and International Health, 1997
“Geographical variation in prevalence of hypertension within The Gambia” – Journal of Human Hypertension, 2001
“Determinants of Hypertension in West Africa: Contribution of Anthropometric and Dietary Factors to Urban-Rural and Socioeconomic Gradients” – American Journal of Epidemiology, 1996
The hypertension rates mentioned in these articles seem quite high.
1) Is salt loss really likely to have been a major problem when sailing the briny deep?
2) I understand that the crew of the slave ships died at a rate comparable to the captives’ rate. That (if true) implies that the problem was likely to be infectious disease, I suspect.
It’s interesting that for the three graphs, the strongest correlation is for latitude and the weakest for average temperature. But looking at it closely, it seems that you need both heat and humidity to drive the allele frequency to really high levels. I think they could actually make an even stronger case if they had access to some sort of ‘misery index’ (heat + humidity) data for the locations in question. Not that p < .001 is shabby, and they clearly were thinking along those lines already since they included the rainfall figures…
Anyone know what asian population it is that lives in a place where the average temperature is -10 degrees C? Some sort of Samoyedic people like the Nenets?
I think they could actually make an even stronger case if they had access to some sort of ‘misery index’
sjournals.org/perlserv/?request=slideshow&type=figure&doi=10.1371/journal.pgen.0010082&id=42181
check figure 4 :)
http://genetics.plo
Interesting theory, however European Americans and Hispanics (of more Iberian ancestry) are well below the latitude of Europe. Even New York City is far south of Rome and more similar in Latitude to North Sicily or so.
Most U.S. cities, like Washington D.C. or below are in the same latitude as Tokyo, Northern India, and North Africa.
It was interesting though that a few weeks back it was discovered that there was a common gene in white Americans that lead to hypertension but it was not common in African Americans, so as to not be the source of the African American hypertension rate.
The article below is interesting because one can argue that Carribean blacks live in a tropical climate such as West Africa (or closeer to than the U.S.) however black American hypertension is lower than many white populations in Europe.
“Higher rates of hypertension among African Americans compared with whites has produced speculation that black populations are more susceptible to hypertension and led to a genetic hypothesis to explain these differences, the researchers write. However, this research has been limited mainly to the US. International studies comparing populations could shed light on the issue, but comparisons have been hampered by a lack of standardized methods between surveys. Within the past decade this has changed, with more comparable methods being developed, they write.
This report used 11 such surveys to glean information on about 85 000 participants in three black populations in the US, Jamaica, and Nigeria and eight white populations in the US, Canada, and Europe. Cooper et al report that US black populations did have a higher prevalence of hypertension than both African and Caribbean blacks and higher than white North Americans, but the prevalence was actually lower than most of the predominantly white European populations.
There was significant variation in hypertension prevalence within both black and white populations, ranging from 27% to 55% for whites and 14% to 44% for blacks. “
http://www.medscape.com/viewarticle/538627
This in no way means genetics is not at play due to a bottleneck or environmental pressure making genes express differently, however baring in mind American whites have lower hypertension rates than many Western Europeans and Canadians, and the majority of black Americans live in the sub-tropical South of the United States…I can’t imagine that environmental pressure due to climate is so much more in Georgia than it is in Jamaica to nearly double the outcome of hypertension cases.
I would like to see what the hypertension rate of Africans living in Europe is or specifically Carribeans living in the UK.
I would also like to see a a comparison of food average yearly intake of sodium for African Americans as compared to white Americans. Then look at the same thing for Afro-Carribeans in the UK and whtie Brits. This might prove to be quite instructive about the significants of dietary habits.
Interesting theory, however European Americans and Hispanics (of more Iberian ancestry) are well below the latitude of Europe. Even New York City is far south of Rome and more similar in Latitude to North Sicily or so.
latitude is not the causal agent here, obviously. it’s a proxy for climate. the climate in NYC is not similar to that in sicily.
p-ter:
“latitude is not the causal agent here, obviously. it’s a proxy for climate. the climate in NYC is not similar to that in Sicily.”
Quite true. I guess I was thinking that this may have something to do with sunlight exposure and calcium production…well that was where I was headed.
If you are talking about temperature, then I would say that you should be able to see a clear cline in hypertension if you took samples in Toronto of Afro-Caribbean, NYC of African Americans, Washington DC, Cleveland, Atlanta, Miami, and then some Caribbean nations down to Trinidad.
At ever point the temperature will increase and “summer” will be longer, so you should see a corresponding increase in hypertension because all of these populations are predominately descendant of slaves, however you still might want to norm for European(or other) ancestry, especially in the United States.
I would still want to see the difference in diet and how Afro-Caribbean fair in Toronto and London as compared to African Americans at various climates in the United States, from Atlanta to Stamford.
Unfortunately I don?t know of any studies such as these.
It is interesting to me also that American whites have lower heart disease rates than many Western Europeans. According to your theory it would imply (assuming no significant dietary differences) that white Europeans are not as adaptive to Europe as blacks are to West Africa, but then again one can argue that people have no been in Europe as long as in tropical Africa.
If you are talking about temperature, then I would say that you should be able to see a clear cline in hypertension if you took samples in Toronto of Afro-Caribbean, NYC of African Americans, Washington DC, Cleveland, Atlanta, Miami, and then some Caribbean nations down to Trinidad.
At ever point the temperature will increase and “summer” will be longer, so you should see a corresponding increase in hypertension because all of these populations are predominately descendant of slaves,
the hypothesis is this– African populations were adapted to retain salt, that constraint has been somewhat relaxed in European populations. In modern times and western societies, salt intake has increased. All other things being equal, then, people of African descent should retain more salt and have higher rates of hypertension.
I’m not sure current climate is that relevant, though it could play a role.
It is interesting to me also that American whites have lower heart disease rates than many Western Europeans. According to your theory it would imply (assuming no significant dietary differences) that white Europeans are not as adaptive to Europe as blacks are to West Africa,
this theory has nothing to say about heart disease, per se, except in that hypertension is a risk factor for heart disease. there are other things at play.
p-ter:
That’s my fault I miswrote. The article I got that information from above says nothing about heart disease, but about “hypertension”.
American whites and blacks have lower hypertension rates than many Europeans, such as Germans, Fins, and Spaniars. WHite Americans specifically have lower rates than Canadians, Italians, and Swedes, British.
For some reason when I click on the link above the article does not show up:
It does appear on google. Title is:
“Higher rates of hypertension in African Americans unlikely to be related to genes”
On Medscape Medical News.
Speaking of differences between white Americans and western Europeans, there has been a bit of a hubub among economists on differences in height:
http://econlog.econlib.org/archives/2007/06/more_on_the_hei.html
http://econlog.econlib.org/archives/2007/06/the_height_of_b_1.html
good discussion. nothing like a little challenge to draw out latent parts of the argument — eg, that it may have been the paleo-climate and environment of Africa, not contemporary climate, that is important. dicussions on this blog are sometimes in a category of their own.
TGGP, I draw your attention to the wag who said it was the European love of cheese that explains it, with the implication that everyone has focussed on the wrong part of “cheese-eating surrender monkeys”. Since a medical man has seriously argued that the difference might be caused by Yurp’s young slurping milk while Murrken youngsters swill down sodas, the wag may feel half vindicated.
Hey Razib -
I don’t have anything to add to the hypertension debate, but since TNR won’t let me comment on the articles there (I’m not a subscriber), I thought I’d just leave a general comment on the exchange here – to wit: I’m sure Messers. Chowkwanyn and Shubow are nice young fellows. They certainly seem bright and articulate enoguh, but really, what sort of standing do a law student and a history grad student have to be arguing these points in a national publication? I mean, hell, why not pull some random cab driver and farmer out of the population to debate genetics and race? They’d be just as qualified as those two. I’m not sure I’m getting what TNR is trying to achieve here with this discussion.
Good call, Alan. Why these 2 cats? Basically some law student reads Mismeasure of Man and feels qualified to tell me how great it was via an important political forum. No wonder he sounds like such a douche.
Where are the scientists, the geneticists? Seriously, let Lewontin debate Crow, or even Marks debate Risch or Leroi. Then maybe we can finally get some brass tacks commentary out there. I’m not a pro scientist, but these talking points are weak – I could write a much better article.
Seriously, let Lewontin debate Crow, or even Marks debate Risch or Leroi. Then maybe we can finally get some brass tacks commentary out there. I’m not a pro scientist, but these talking points are weak – I could write a much better article.
amen to that.
TGGP:
There’s Komlos again trying to prove how bad living conditions in the US are because of our lack of a nationalized healthcare system and “unequal income distribution.” Komlos’ documentation of historical changes in height as a response to changes in nutrition and living standards is interesting, but I’ve always found his conclusions rather suspect (and politically motivated). He also seems to make a mountain out of a molehill. If you look at the NHANES data he uses in his paper, non-hispanic white US men born between 1975 and 1983 are about 179cm. Harping on about how Northern Europeans (i.e., Germans, Scandinavians, Dutch, etc.) born in those same years are 1 or 2 cm (or in the case of the Dutch, about 4cm) taller doesn’t convice me that genetics doesn’t play a role in the difference. The US White population is not drawn exclusively from Northern European countries. Looking at EC Household Panel data and Eurostat, English and Greeks born in the same period are about 178cm, Italians, Irish, and French are 177cm, and Spaniards are 176cm. Americans seem to be intermediate in stature between South-West and North-East Europeans, as one would expect from a population deriving from a mixture of them. To be convincing that economic and health care differences are responsible for the difference, Komlos would have to present data that German and Scandinavian Americans are shorter than Northern Europeans. Also, Komlos own work show that US heights are normally distributed, just like Northern European heights, so there isn’t a long left tail of poor people dragging the average down.
what sort of standing do a law student and a history grad student have to be arguing these points in a national publication?
TNR just recently changed hands and has a new owner — The Asper family, owner of CanWest. I think maybe they just wanted to clear their name from any associations with Andrew Sullivan having published large sections of “The Bell Curve”. There’s absolutely nothin’ new in what these guys are talking about.
see recent Eric Alterman piece from prospect.org my marty peretz problem and ours.