A Population Genetics (But Not Racial) Approach to the Etiology of Salt-Sensitive Hypertension in African-Americans
African-Americans suffer one of the highest rates of hypertension in the world. Psychosocial stressors such as racial discrimination and financial strain have been identified by some researchers, while others have sought the answer in biological traits. This article targets genetics as the primary factor for this condition, while at the same time repudiating antiquated “racial” classification schemes. The ancestors of African-Americans came from West Africa. However, it was not from the coast, awash in ocean salt, but rather from sodium-deficient regions as much as 1,000 miles inland.
This population group had lived for millennia in the sweltering heat of the tropics without the benefit of dietary salt to supplement their diets. They had, in essence, evolved “super-kidneys,” which were able to process the minimal levels of salt in the environment with greater efficiency than was the case with other populations, including members of their own ethnic groups, who lived in coastal regions. But an evolutionary advantage in the mineral-poor interior of West Africa has proven medically disastrous for the descendants of this population. American Blacks have today become acculturated into a food supply, whose salt saturation hovers close to toxic levels as far as their sodium-conserving bodies are concerned.
This population group had lived for millennia in the sweltering heat of the tropics without the benefit of dietary salt to supplement their diets. They had, in essence, evolved “super-kidneys,” which were able to process the minimal levels of salt in the environment with greater efficiency than was the case with other populations, including members of their own ethnic groups, who lived in coastal regions. But an evolutionary advantage in the mineral-poor interior of West Africa has proven medically disastrous for the descendants of this population. American Blacks have today become acculturated into a food supply, whose salt saturation hovers close to toxic levels as far as their sodium-conserving bodies are concerned.
Hypertension among African-Americans has reached a crisis level, developing at an earlier age and progressing with greater disease severity. As a consequence, Blacks suffer a 1.8 times greater rate of fatal stroke than Whites, 1.5 times greater rate of heart disease death, 4.2 times greater rate of end-stage renal failure and a 50% higher frequency of heart failure. (Ferdinand and Armani) While progress has been made in the treatment of hypertension among most Americans, such does not appear to be the case with Blacks, who suffer disproportionately from the salt-sensitive form of the disease. As a consequence, racial disparities seem, if anything, to be growing.([1]-[2]
Over the course of the last several decades, the differential impact of hypertension on Blacks, has drawn considerable attention among medical researchers, spawning several hypotheses and theories that rose to prominence then slipped from grace. As early as 1973, Professor Lillian Gleiberman postulated a theory relating sodium retention in blacks to an evolutionary adaptation of their ancestors to a salt-deficient environment.[3] In 1986, T.W. Wilson elaborated on this theory. He proposed that a geological deficiency of salt in certain regions of West Africa might have created, through natural selection, the renal characteristics, which allowed this population to retain sodium and thus live on minimal supplies of the mineral. ([4]) However, by 1991, two events had overshadowed this sodium-deficiency hypothesis. Firstly, Wilson began a collaboration with Clarence Grim on a new theory, which soon overshadowed earlier arguments, which came to be known as the “Slavery Hypertension Hypothesis.”[5] It explained the high rate of salt-sensitive hypertension in African-Americans as a function of a biological bottleneck created by the arduous trans-Atlantic slave journey. Given the brutality of the transatlantic slave trade, the theory that slaves possessing a higher natural ability to retain salt were less sensitive to dehydration, and diarrhea (“fluxes”) was plausible. This hypothesis took hold within the medical community and captured the public imagination. However, another reason the earlier Gleibermann and Wilson sodium-deficiency arguments faded from view was the fact that one of America’s foremost experts on the slave trade, Philip Curtin, of Johns Hopkins University, issued a scathing rebuttal of them. He argued:
From the historical point of view, the slavery hypothesis to explain African-American hypertension not only lacks supporting evidence but also runs counter to what evidence we do have. West Africans had plenty of salt through most of their history. ([6])
It is regrettable, perhaps even tragic, that more African historians did not involve themselves in this debate at that time because the stakes were high then and remain so today. Understanding the source of salt-sensitive hypertension among blacks will be a critical factor in finding approaches to alleviate this health crisis. And it now appears that historian Philip Curtin may have inadvertently assigned his cogent analysis to the wrong West African group.
Salt was a luxury import in the areas of West Africa inhabited by the ancestors of African-Americans. Sophisticated trading networks crisscrossed the region, in which West African gold was traded for rock salt mined in the Southern Sahara. Ironically, the forest and savannah regions were so geologically deficient in salt that the elites willingly traded their gold pound for pound for Saharan rock salt or halite. ([7]) Even today, according to the International Council for the Control of Iodine Deficiency Disorders (ICCIDD): “There is almost no salt production in the entire Central and West African region except for Ghana and Senegal.” ([8]) During the period of the transatlantic slave trade the importation of rock salt into this region made this commodity literally worth its weight in gold. The only problem was that the ancestors of African-Americans were low-status peasant farmers, who could afford neither gold nor salt.
The history books overlooked the economic conditions of these subsistence farmers living in decentralized or stateless societies because of an unspoken bias favoring so-called great civilizations and written records, which dominated the field of African History until recently. ([9]) When Dr. Curtin published The Atlantic Slave Trade: a Census, considerable effort had been made to scour slave ship records and bills of sale to determine slave origins. Such geographic locales as the Windward Coast, the Gold Coast, the Bight of Biafra, and the Bight of Benin were provided alongside estimates of the number of slaves shipped from each of these areas. ([10]) However, in truth, these were coastal catchment areas for slaves sold and loaded onto European ships. No written records existed as to how far into the interior they had been captured and marched to the coast. But when efforts were made by slavers to determine actual origins by the language the captives spoke, it became clear that a substantial number had exchanged hands four or five times since their capture, as they were forcibly led vast distances in leg irons from the interior to the slave ships anchored along the coast. ([11])
The ancestors of African-Americans were not the wealthy, literate inhabitants of the great sub-Saharan empires like ancient Ghana, Mali and Songhay, whose dinner tables did not lack for rock salt imported from Mauritanian salt mines. Nor were they citizens of the coastal states of Dahomey, Ashanti, the Congo, or Angola, who possessed salt aplenty being on the Atlantic seaboard. These were the slave states whose denizens had captured them in the first place. The soon-to-be enslaved farmers inhabited the forests and savannahs outside the boundaries of the large states. Without the professional armies that full-fledged states possess, this population was especially vulnerable to being kidnapped and sold to slave traders. They were not the sort of people most African historians have in the past studied. And they are certainly not the people Prof. Curtin referred to when he insisted that salt was available in abundance in the interior of West Africa.
Clarifying the historical origins of Black Americans with greater geographic and socio-economic specificity might contribute to putting what has appeared to be an enigma to rest. This population did indeed emanate from West Africa. But it was not from the Atlantic coast, where dietary salt was inexhaustible, either being boiled from ocean brine or processed through solar evaporation. Sea salt quickly dissolved in the humidity and spoiled on account of organic impurities remaining in it and thus could not be transported into the interior for purposes of trade. During the early years of the transatlantic slave trade, slaves did come from near the coast. However, as those areas became depopulated, the trade moved farther and farther inland. European accounts agree that by the 1700s, slaves were captured in the far interior. Just how far inland these regions were can be illustrated by a passage in the logbook of a slave ship called the Sandown, which sailed from London to Sierra Leone in 1793. The ship’s captain, Samuel Gamble wrote: “They [slaves] sometimes come upwards of one thousand miles out of the interior part of the country. ([12]) Another observer, Dr. Alexander Falconbridge, served as the surgeon aboard a number of slave ships that plied their trade between the West African coast and the Caribbean in the late 1700s. He described his experiences in a popular book published in 1788. More importantly, he explained why so little was known by Europeans as to the actual geographic origins of the slaves themselves. Dr. Falconbridge commented:
"There is great reason to believe, that most of the Negroes shipped off from the coast of Africa, are kidnapped. But the extreme care taken by the black traders to prevent the Europeans from gaining any intelligence of their modes of proceeding; the great distance inland from whence the Negroes are brought; and our ignorance of their language (with which, very frequently, the black traders themselves are equally unacquainted), prevent our obtaining such information on this head as we could wish. I have, however, by means of occasional inquiries, made through interpreters, procured some intelligence relative to the point, and such, as I think, puts the matter beyond a doubt. ([13])
The regions inhabited by the subsistence farmers fell within one of the two most sodium-deficient geological zones on the globe, with the other being the Amazon Basin. ([14]) Because very little geological mapping has been conducted in Africa, the scarcity of salt is best illustrated by historical accounts of a salt trade so lucrative that three empires emerged from the 10th through the 16th century based on it. Taking advantage of its scarcity, the merchants of Ghana, Mali and Songhay traded rock salt mined in the Southern Sahara for gold panned by the decentralized populations on their borders. In 988 A.D. the Arab traveler and chronicler Ibn Hawqal astonished the Islamic world with news about the wealth of ancient Ghana, when he described seeing a promissory note for 42,000 dinars for one gold-salt transaction. ([15])
However, the pivotal issue here is not whether salt was available to anybody in that region, but whether it was available to the subsistence farmers. Salt was the most valuable, luxury commodity traded in this interior portion of West Africa. While the more privileged members of these societies benefited from the gold trade in order to obtain salt as an expensive delicacy, such was not the case for the lower rungs of society, that is, the subsistence farmers. But the latter were precisely the people who were most vulnerable to being kidnapped, enslaved and sent to the Americas. It was not the chiefs, their families and retainers, who traded their gold for salt and sometimes even collaborated with the slave traders. The eighteenth century Scottish explorer, Mungo Park, found these class differences based on access to salt, intriguing. He observed:
It would appear strange to an European, to see a child suck a piece of rock-salt, as if it were sugar. This, however, I have frequently seen; although, in the inland part s, the poorer class of inhabitants are so very rarely indulged with this precious article, that to say a man eats salt with his victuals, is the same as saying, he is a rich man. ([16]) When higher-status individuals found themselves caught up in the slavers’ nets, the common practice was for the family to pay a ransom in order to procure their freedom. The French researcher, Marc Henri Piault, has even suggested that nobles were at times deliberately targeted with the expectation that their families would pay a ransom to win their return. ([17]) This was not an option for hard scrabble farmers, whose nutrient-poor lands only produced enough to feed themselves and their families. Nevertheless these peasants toiled in tropical temperatures that hovered around 100 degrees for months at a time, without the advantage of beasts of burden and with a minimal amount of supplementary salt in their diets.
In fact, so intrigued and flabbergasted was one eighteenth century European traveler named John Matthews at the exorbitant price paid for salt in the West African interior that he remarked:
“The best information I have been able to collect is, that great numbers are prisoners taken in war, and are brought down, fifty or a hundred together, by the black slave merchants; that many are sold for witchcraft, and other real, or imputed, crimes; and are purchased in the country with European goods and salt; which is an article so highly valued, and so eagerly sought after, by the natives, that they will part with their wives and children, and everything dear to them, to obtain it, when they have not slaves to dispose of; and it always makes a part of the merchandize for the purchase of slaves in the interior country.” ([18])
So what might all this mean in biological terms? The adaptive mechanisms that ensured the survival of populations inhabiting the sodium-deficient West African interior became maladaptive in the salt-saturated environments of the Americas. And yet, two decades of medical research have not confirmed this fact because of the use of imprecise and outmoded “racial” classifications. In multi-ethnic research it becomes vital to identify the discrete, genetically relevant, population groups under discussion rather than employ imprecise sociological terminology to prize out genetic differences. Use of such terms as “Blacks” and “African-Americans” can be useful shorthands. But they are certain to confound any research searching for the etiology of diseases or traits, which may be characteristic of a geographically and genetically distinct population group. Ghanaians who emigrate to the United States can be termed African-Americans. But few of them come from the sodium-deficient far interior that the previously enslaved ancestors of Black Americans once inhabited. And secondly, Black Americans are an admixed population, whose 20% European ancestry also becomes part of the genetic calculation. In short, the three common racial categories of Whites, Blacks and Asians should be employed with care in medical research because they are amorphous sociological terms, which can and have compromised research data. What is of value for purposes of medical research and genetic studies, however, is to recognize the importance of human population groups, which number in the thousands. Such groups represent the inhabitants of particular geographical locales, who share certain genetic or evolutionary adaptations. But groupings can also be fluid, depending on what biological characteristics are being studied.
The ancestors of African-Americans inhabited environments that had only minimal access to sodium. While it is possible that their kidneys adapted to these conditions, it is just as likely that they inherited this trait from humankind’s Paleolithic forebears. This would mean that the evolutionary adaptation to high sodium environments so familiar to the rest of the world is more of a global evolutionary adaptation that bypassed the African groups under discussion.
In fact, one of the original proponents of the sodium-deficiency, Thomas W. Wilson remarked in a 1986 edition of Lancet:
For instance, in Senegal and Gambia salt production has been extensive since ancient times, whereas in ancient Nigeria the population had to depend on local vegetables salts or meagre imports of the mineral. Reports that blood-pressures in Senegalese and Gambians are lower than those in Nigerians may be a reflection of historical differences in salt supply. (3)
A wealth of historical material exists on the West African salt-gold trade, even though geological surveys of rock salt or halite deposits are scant for that continent. The region of the tropical world with a sodium-deficiency most similar to that of inland West Africa is the Amazonian rainforest of Brazil. Sodium concentrations decline monotonically with increasing distance from the Atlantic Ocean. ([19]) In a study entitled “Lust for Salt in the Western Amazon,” zoologists Robert Dudley, Michael Kaspari and Stephen Yanoviak describe the use of mineral licks by animals for purposes of sodium supplementation in the absence of that mineral. ([20]) Although this research does not target human populations, its assessment of the region is relevant to establishing the reasons for the Yanomami Indians of that region having evolved a virtually salt-free diet. ([21]) The global INTERSALT Study included an extensive series of hypertension tests, which were carried out on the unacculturated Yanomami tribe of the Amazon rain forest between 1989 and 1992. The study found not a single case of hypertension in the entire tribe. The Yanomami showed: a very low urinary sodium excretion (0.9mmol/24h); mean systolic and diastolic BP levels of 95.4mmHg and 61.4 mmHg, respectively; and no cases of hypertension or obesity. ([22]) Three other isolated populations were also identified by INTERSALT as exhibiting exceedingly low risk factors for hypertension and a low-sodium diet. They were the Xingu Indians of the Amazon Basin and the rural populations of Papua, New Guinea and Kenya. These are also non-coastal populations, who may as a consequence have little access to salt. But these possible correlations will need to be studied further.
However, what may be of greater relevance to the current discussion is what happened when members of the Yanomami people began leaving the tribal areas and resettling in typical Brazilian towns. In 2010, the principal researchers on the original study returned for a follow-up study of those Yanomami women, who had become acculturated. While they had previously suffered from anemia and stunting, those disorders had disappeared and been replaced with hypertension and obesity. The principal investigators wrote in Hypertension:
Very few Amazon tribes remain uninfluenced by Brazilian civilization. Moreover, there are no longer completely unacculturated ethnic groups outside the Amazon basin, although, by definition, these Indian tribes keep speaking primarily their original . . . . However, if acculturation seems to protect against anemia and stunting, it also increases by ≈3.3 and 3.8 times the odds for obesity and hypertension, respectively P<0.001 for both). ([23])
A host of other reasons, ranging from lifestyle stresses to lack of exercise, could also have caused the change and the subject bears further study. It will be of vital importance for future research to track hypertension rates among these populations over time, assuming that they follow the trajectory of other such groups, who have begun de-isolating themselves and moving into the cities. A limitation of this study is the fact that the focus has been restricted to salt and sodium-deficiency, topics that are commonly tied to questions of dietary iodine sufficiency. However, this latter issue, while of vital importance to human health, lies beyond the scope of this article.
So what has this article shown? It is that the humans who inhabit geological zones that are deficient in sodium sustain or evolve salt-retention characteristics. When such populations migrate to what would be considered a more “normal” sodium environment, they are at risk of developing unusually high rates of salt-sensitive hypertension and end-stage renal failure. And “race” is far too imprecise a term to use for the identification of such population groups.
We should also remember that salt sensitive high blood pressure in African-Americans, while it does lead to kidney disease and premature death, is not caused by the pathological functioning of their kidneys. That is, their renal function is perfectly attuned to their ancestral environments. But alas, they no longer live there. The problem is rather the sodium toxicity of the environment into which this population was injected, involuntarily, several centuries ago as slaves. The general level of kidney functioning among the West African ancestors of blacks represented a rare gift of nature and evolutionary adaptation to a tropical, but sodium-deficient ecology rather than a disease susceptibility. As such the American descendants of those slaves deserve to be protected from the toxic sodium environment in which they today find themselves, or at the very least, “warned.”
Nearly three decades ago, one of the original proponents of the sodium-deficiency hypothesis, Thomas Wilson, cautioned that increased salt consumption among previously salt-deprived West Africans and their Black American descendants was a “serious public health hazard.”16 That message was ignored. Today, a growing number of American nutritionists have even gone so far as to dismiss the dangers of sodium overconsumption as alarmist and unsubstantiated. ([24]-[25]) So, which side is right? The error is in attempting to universalize nutritional requirements, which should be aligned to the genetic history of specific population groups. Healthy African-Americans who consume the Federal Food and Drug standard of 2400mg/day sodium may not be healthy for long, putting themselves at risk of developing salt-sensitive hypertension and worse.
But there is another message underlying this study on the etiology of salt-sensitive hypertension in blacks. Our medical ingenuity is no match for nature’s. Nor can we humans re-write biological health standards, merely because we would feel more comfortable if we could simplify it to a one-size-fits-all standard. So what options might there be in approaching salt-sensitive hypertension in African-Americans? This study provides no new directions, pharmacologically. But it does offer a suggestion. Warning an individual that he must deprive himself of sodium-laden food to avoid the symptoms of renal pathology is a different message from telling that individual that he has inherited a unique ancestral gift, which he must learn to accept, honor and -- if it is not too late -- protect.
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