As a result of the relatively recent adoption of a high-salt diet by our still largely traditional bodies adapted to a low-salt diet, high salt intake is a risk factor for almost all of our modern non-communicable diseases. Many of these damaging effects of salt are mediated by its role in raising blood pressure, which I’ll discuss below. High blood pressure (alias hypertension) is among the major risk factors for cardiovascular diseases in general, and for strokes, congestive heart disease, coronary artery disease, and myocardial infarcts in particular, as well as for Type-2 diabetes and kidney disease. Salt intake also has unhealthy effects independent of its role in raising blood pressure, by thickening and stiffening our arteries, increasing platelet aggregation, and increasing the mass of the heart’s left ventricle, all of which contribute to the risk of cardiovascular diseases. Still other effects of salt intake independent of blood pressure are on the risks of stroke and stomach cancer. Finally, salt intake contributes indirectly but significantly to obesity (in turn a further risk factor for many non-communicable diseases) by increasing our thirst, which many people satisfy in part by consuming sugary high-calorie soft drinks.
Salt and blood pressure
Let’s now pause for a quick crash course on blood pressure and hypertension, to help you understand what those numbers mean when your doctor inflates a rubber cuff about your arm, listens, deflates the cuff, and finally pronounces, “Your blood pressure is 120 over 80.” Blood pressure is expressed in units of millimeters of mercury: the height to which your blood pressure would force up a column of mercury in case, God forbid, your artery were suddenly connected to a vertical mercury column. Naturally, your blood pressure changes throughout each heart stroke cycle: it rises as the heart squeezes, and it falls as the heart relaxes. Hence your physician measures a first number and then a second number (e.g., 120 and 80 millimeters of mercury), referring respectively to the peak pressure at each heartbeat (called systolic pressure) and to the minimum pressure between beats (termed diastolic pressure). Blood pressure varies somewhat with your position, activity, and anxiety level, so the measurement is usually made while you are resting flat on your back and supposedly calm. Under those conditions, 120 over 80 is an average reading for Americans. There is no magic cut-off between normal blood pressure and high blood pressure. Instead, the higher your blood pressure, the more likely you are to die of a heart attack, a stroke, kidney failure, or a ruptured aorta. Usually, a pressure reading higher than 140 over 90 is arbitrarily defined as constituting hypertension, but some people with lower readings will die of a stroke at age 50, while others with higher readings will die of a car accident in otherwise good health at age 90.
In the short run, your blood pressure increases with your anxiety level and with vigorous exercise. In the long run, though, it increases with other factors, especially with salt intake (for reasons discussed below) and (in us Westernized moderns but not in traditional peoples) with age. The relationship between salt intake and blood pressure was noted more than 2,000 years ago in the Chinese medical text Huangdi neijing suwen, which says, “Therefore if large amounts of salt are taken, the pulse will stiffen and harden.” In recent experiments on captive chimpanzees, our closest animal relatives, their blood pressure while consuming a Purina Monkey Chow diet providing 6 to 12 grams of salt per day (like most modern humans eating a Western diet) was a pleasingly healthy 120 over 50, but it increased with age (also like modern humans on a Western diet). After a year and seven months on a high-salt diet of up to about 25 grams per day, the chimps’ blood pressure rose to about 155 over 60, qualifying them to be called hypertensive by human standards, at least as judged by their systolic blood pressure.
For us humans it’s clear that salt intake does influence blood pressure, at least at the opposite extremes of very low and very high salt intake. The international INTERSALT project of the 1980s used a uniform methodology to measure salt intake and blood pressure in 52 populations around the world. The population that I already mentioned as having the world’s lowest recorded salt intake, Brazil’s Yanomamo Indians, also had the world’s lowest average blood pressure, an astonishingly low 96 over 61. The two populations with the next two lowest salt intakes, Brazil’s Xingu Indians and Papua New Guinea Highlanders of the Asaro Valley, had the next two lowest blood pressures (100 over 62, and 108 over 63). These three populations, and several dozen other populations around the world with traditional lifestyles and low salt intakes, showed no increase in blood pressure with age, in contrast to the rise with age in Americans and all other Westernized populations.
At the opposite extreme, doctors regard Japan as the “land of apoplexy” because of the high frequency of fatal strokes (Japan’s leading cause of death, five times more frequent than in the United States), linked to high blood pressure and notoriously salty food. Within Japan these factors reach their extremes in northern Japan’s Akita Prefecture, famous for its tasty rice, which Akita farmers flavor with salt, wash down with salty miso soup, and alternate with salt pickles between meals. Of 300 Akita adults studied, not one consumed less than 5 grams of salt daily (three months of consumption for a Yanomamo Indian), the average Akita consumption was 27 grams, and the most salt-loving individual consumed an incredible 61 grams—enough to devour the contents of the usual 26-ounce supermarket salt container in a mere 12 days. That record-breaking Akita man consumed daily as much salt as an average Yanomamo Indian in three years and three months. The average blood pressure in Akita by age 50 was 151 over 93, making hypertension the norm. Not surprisingly, Akita’s frequency of death by stroke was more than double even the Japanese average, and in some Akita villages 99% of the population died before 70.
The evidence is thus striking that extreme variations in salt intake have big effects on blood pressure: very low salt intake results in very low blood pressure, and very high salt intake results in very high blood pressure. However, most of us will never follow a diet as extreme as that of a Yanomamo Indian or an Akita farmer. Instead, we would like to know whether more modest variations in salt intake, within the middle of the range of world salt intakes, have at least some modest effects on blood pressure. For several reasons, it really isn’t surprising that there is still some controversy about effects of variation within this middle range. The middle range encompasses only a narrow spread of salt intake: for instance, 48 of the 52 populations in the INTERSALT study (all populations except the Yanomamo and the three other low-salt outliers) had mean salt intakes falling between 6 and 14 grams per day. Individual variation in salt intake and blood pressure within most populations is large and tends to obscure average differences between populations. Salt intake itself is notoriously difficult to measure consistently unless one confines people in a hospital metabolic ward for a week and measures salt levels in all of their foods consumed and urine produced. That’s completely impossible to do for Yanomamo Indians in the jungle, as well as for most of us city-dwellers wanting to lead normal lives outside metabolic wards. Instead, salt intake is commonly estimated from 24-hour urine collections, but those values are subject to huge variation from day to day, depending on whether one happens to eat a Big Mac or a can of chicken noodle soup on some particular day.