Salt is one of the most ancient things of value to man, as a moment’s reflection on the origins of words – like salary (something worth working for) quickly brings to mind. Roman soldiers were paid in salt, not silver. By the 1950s, a pound of sodium chloride was one of the best bargains around, available in most supermarkets for approximately 13 cents. It was about this time that Dr. Lewis K. Dahl noticed that in populations of Sprague Dawley lab rats, there existed a sub-group of salt sensitive individuals that developed hypertension spontaneously when fed a high salt diet. With just three generations of brother-sister matings, Dahl was able to segregate the salt intolerant trait into Dahl S (for salt sensitive) animals. He also raised its counterpart, the Dahl R (for salt resistant) rat. Dahl S rats fed a diet 1% by weight sodium chloride rapidly developed sustained hypertension and died by four months of age; tellingly Dahl R rats on the same diet did not. The medical community of hypertension specialists at this time were essentially a bunch of plumbers. They were not informed by evolutionary paleoanthropology. Using crude loop diuretics like furosemide (lasix) they learned to treat high blood pressure by opening a tap in the kidney and pouring salt and water out in the urine. Ignoring the vast differences between a small furry rodent and a naked, heat-adapted endurance predator, they assumed that the Dahl S rat was a viable model of human hypertension.
The resulting emphasis on dietary salt restriction in both research and public health policy during the last 60 years cannot be exaggerated. It cannot actually be read (in any feasible amount of time) – it amounts to tens of thousands of papers, editorials and educational campaigns – it pervades and saturates. And for want of a simple Paleolithic perspective, all those trillions of dollars generated mostly hubris. A typical sample of that is the 2005 review by Meneton et al in Physiology Reviews. This paper references 414 others. By totaling the costs of less than ten of the larger of these studies one quickly surpasses $1 billion in ill-advised public spending. It states without evidence that the pre-modern human diet was the same as a chimpanzee. The author also states, relying only on modern anthropological anecdotes, that the adapted state of the human species is to ingest less than 1 gram of salt per day, an amount that modern man can eliminate by perspiration in 20 minutes. The words “perspiration” and “sweat” do not appear in this review one single time. That’s a pretty big oversight. This is because with respect to salt and water, human beings are a uniquely strange animal. We have at least two million and perhaps 40 million eccrine sweat glands, capable of pouring out 3 liters of isotonic fluid and 2.8 grams of sodium chloride per hour.
We have in essence 3 kidneys – a widely dispersed and inefficient one all over the surface of our bodies, and two additional, highly refined internal ones to clean up whatever imbalances perspiration leaves us. (There are only 800,000 to one million nephrons per kidney, so sweat glands probably outnumber them, but a nephron is a sophisticated work of bio-engineering if ever there was one. The nephrons move 800 grams – more than a pound – of pure salt into your nascent urine every day, then recycle >99.99% of that back to your blood, leaving as waste between 50 mg and 50 grams, customized to your diet and the sort of day you had.)
The sort of day humans evolved to have is that of a cunning persistence predator with no advantages of speed or weaponry. This is our African, heat adapted inheritance, which can be summarized like a panhandler’s cardboard sign: “Will run marathons for food.” We have an evolutionary bet with large ungulates that we can harass them in the open sun until they overheat and die before we do. By eating liberally of their salty tissues and drinking copiously of free water, we recharge ourselves in order to take down another of their brethren in a few days time. The physical signature of that strategy perfected over millennia is stamped onto our genes and our bodies. Humans have sported a highly derived, spring-loaded ankle and foot for more than 2 million years. We have evolved to handle sodium and water like no other animal – and certainly not in the manner of a Dahl S rat. We re-purposed the excess salts from our new diet to create profuse perspiration. We altered the physiologic essentials of blood volume and water balance. We lost our body hair, and developed arterial shunts to our surface veins, turning the dorsal hands and forearms into heat radiators. We grew a wet mop of hair on our head. It’s a lifestyle with little appeal to vegans, but the early hominids that embraced chewing roots instead left us only their enormous molars and crested skulls as fossils emblematic of an adaptive error. They’re extinct now.
When I was taught renal physiology at the University of Michigan School of Medicine in 1982, I was told that people lost 50 cc of water as perspiration per day. It occurred to me that it was not true, as it was simple to weigh myself before and after running 5 miles. I knew they were off by a factor of between 10 and 100, but medical school is replete with incorrect things. It was not so much that they didn’t conceive that such losses were possible – they just didn’t think of them as normal. At no time had Medicine with a big M sat down to reflect on what might be the evolved ecological niche of the human animal they were so intent on doctoring. More recently, as medicine has asked for hard scientific evidence to back any therapy, the basis for dietary salt restriction has literally dissolved away. The best studies over the last 10 years show that the differences in blood pressure between high salt and low salt intake groups are tiny – between 2 to 4 mm Hg. By sub-grouping, you can see that it’s mainly African-Americans who are salt sensitive. It’s what evolution would teach you to expect. Going 24 hours without a good bout of sweating is an intrinsic physiologic stress for humans. North Europeans and others who have been in temperate climates for a couple thousand generations have had a chance to adapt themselves somewhat to this stress. Salt resistant human sub-populations have evolved with modified renal hemodynamics allowing them to excrete 100% of a cold, sedentary day’s intake of salt. Blacks (and by that I mean any darkly colored person – skin pigment is an excellent proxy for generations spent living where it’s hot) have not made this adjustment, so when they cannot dump 10 grams or so in salt through their skin each day, they get hypertensive.
Nothing is more piteous than the spectacle of Clinical Standard of Practice clinging with a death grip to Public Health Policy as the last shreds of scientific footing erode out from under them. Personally, I find it insulting to be told how to behave by anyone. It’s especially annoying when those authorities are uninformed by evolution, genetics, and science in general. However, it seems certain that the low salt diet mantra will not go quietly. Paleogenomics is well positioned to provide some of the essential evidence that will allow Medicine to move on. The salt-resistant, cold dwelling urbanites that I’ve suggested above will be part of the recent 10,000-year explosion of adaptive genetic changes that Cochran and Harpending discuss in their 2009 book of the same name. Within Africa itself, there is evidence for very ancient genomic diversity that likely preserves unchanged the highly evolved water balance physiology of a million years of homo erectus ancestry. There are also indigenous groups still practicing persistence hunting, and within a small geographic area of Kenya and Ethiopia, a sub-population that represents the most highly adapted endurance runners in the world.
Longevity research scientists have combed the ranks of the world’s centenarians, looking through culture, diet, lifestyle, stress, and genetics in search of the causes of a long and healthy life. Proffered reasons range from the plausible to the absurd – mutations and other genetic factors, not being married, smoking, drinking alcohol regularly in one of its many forms. One veteran longevity researcher who had searched from Nepal to Sardinia to Peru interviewing centenarians recently remarked that no matter what country or condition they were from, the one and only thing that united healthy, very old people was that for some reason, on a daily basis, they did some kind of work that made them break into a good sweat.