A friend of mine with high blood pressure was advised by his GP to reduce salt intake. That got me piqued, so I did some research. And what I found buried in all the medical research papers and scholarly advice was not what I expected.
Let’s start with the conventional medical view. The American Heart Foundation advises five lifestyle changes to help lower blood pressure. The third ‘pillar’ is salt: “Reduce sodium—Ideally, stay under 1,500 mg a day, but aim for at least a 1,000 mg per day reduction.” (http://www.heart.org/HEARTORG/Conditions/HighBloodPressure/GettheFactsAboutHighBloodPressure/Five-Simple-Steps-to-Control-Your-Blood-Pressure_UCM_301806_Article.jsp#.Winv4WXrcwc)
That’s been the conventional wisdom for many years, but what’s the theory and where did this theory come from exactly?
Ok. The idea can be expressed in a few sentences. Salt makes us thirsty so we drink more water. But salt also makes us retain water to dilute the saltiness of the blood. Water retention therefore increases blood volume and that leads to hypertension (high blood pressure). Refer: http://www.amjmed.com/article/S0002-9343(17)30508-9/fulltext (copy and paste this link if it fails on the page)
Interesting idea. Sounds logical. But keep reading that American Journal of Medicine article I just referenced:
“In 1904, two French scientists named Ambard and Beaujard further promoted the idea that salt retention was a driver of edema and hypertension. These authors were credited for inventing the Salt–Blood Pressure Hypothesis…. They studied 6 hypertensive patients (some with valvular heart disease and/or Bright's disease) with a low-salt diet consisting of 3 g of salt (1.2 g of sodium) and compared it against a high-salt diet (14 g of salt or 5.8 g of sodium). Despite the salt intake being approximately twice that compared with a normal sodium diet (ie, 5.8 vs 3.4 g of sodium), “The changes in blood pressure were not striking but tended to be downward when the low salt diet was given and upward when the higher salt intake was allowed.”
Yes, you read that correctly. The demonisation of salt began with a poorly conducted study based on just six patients. That is a mockery of the double blind placebo with replicated results tests we do today, but that is it. Other researchers misused those results and salt became the enemy. Following this antiquated research came more poor quality research, which nevertheless had a huge impact on our view of dietary salt (https://www.stat.berkeley.edu/~rice/Stat2/salt.html):
“In a seminal 1972 paper, Lewis Dahl, a physician at Brookhaven National Laboratory in Upton, New York, and the primary champion of salt reduction in this country until his death in 1975, claimed it was proven that a low-salt diet reduced blood pressure in hypertensives. When it didn't, he said, that only proved that the patient had fallen off the diet, "all protestation to the contrary, notwithstanding."
“Dahl furthered the case for a salt-blood pressure link by breeding a strain of salt-sensitive hypertensive rats. Researchers still cite this work as compelling evidence for the role of salt in human hypertension. As Simpson pointed out in 1979, however, Dahl's rats became hypertensive only if fed an amount of salt equivalent to more than 500 grams a day for an adult human--"
That is, as we say in research circles, a shitload of salt. No one consumes those quantities. Normal rats are not sensitive to salt so Dahl had to create rats that were. He genetically engineered his rats to have hypertension, then he blamed salt. What’s with that?
This link between high blood pressure and salt has been established in the public mind on the back of terrible research. South Koreans have one of the highest salt intake diets in the world. Koreans eat kimchi—salt-preserved cabbage—with every meal, and eat many dishes crammed with salt. On average their high sodium diet sees them consume 4 grams of the stuff a day (compare that with Dahl’s 500g rats). Koreans are among the world's lowest rates for hypertension, coronary heart disease and cardiovascular disease. This picture is replicated in other cultures where the local cuisine has high salt content.
So… maybe salt is not bad for your heart after all? At least restricting it in your diet has to be good… doesn’t it?
Actually, no. Salt restriction is highly detrimental to fertility (https://academic.oup.com/ndt/article/23/7/2154/1861032):
“Restriction of salt intake has major effects on procreation, gestation and lactation. Salt-induced alterations of neurophysiological functions and sexual behaviour have already been presumed by the ancient Greeks. Later on, these assumptions received scientific support.
“More than 50% of the variance in fertility is explained by nutritional factors including salt intake. A deficiency in sodium and associated excess of potassium can reduce fertility by irregular oestrus cycles, endometritis and follicular cysts….sexual attraction is associated with the dopaminergic reward system, and salt is implicated in the regulation of the dopaminergic system. Dopamine is important for motor functions and general arousal. It may have some relationship to mechanisms of ejaculation and neuroendocrine consequences of sexual activity or other processes associated with copulation..”
IMAGE: Women salting their husbands asses to increase their libido (1557)
And on it goes. Salt restriction, it appears, can also cause changes that result in insulin resistance, increased sugar cravings, enhanced appetite and “internal starvation”, which promotes weight gain. It also stimulates hormones—renin, angiotensin and aldosterone—which increase the absorption of fat.
Salt is a food we need. Restricting it in our diets has various adverse side effects, and yet, that is the advice freely dispensed by medical professionals to people with high blood pressure. That advice, it seems, needs to be taken with a grain of salt.