Feb 8, 2015

Our salt intake

While there are many different chemicals falling into the category termed "salts" by chemists, to laypeople "salt" means sodium chloride. That's the salt that we crave, season our food with, consume too much of, and get sick from. Today, salt comes from a salt-shaker on every dining table and ultimately from a supermarket, is cheap, and is available in essentially unlimited quantities. Our bodies' main problem with salt is to get rid of it, which we do copiously in our urine and in our sweat. The average daily salt consumption around the world is about 9 to 12 grams, with a range mostly between 6 and 20 grams (higher in Asia than elsewhere).

Traditionally, though, salt didn't come from salt-shakers but had somehow to be extracted from the environment. Imagine what the world used to be like before salt-shakers became ubiquitous. Our main problem with salt then was to acquire it rather than to get rid of it. That's because most plants contain very little sodium, yet animals require sodium at high concentrations in all their extracellular fluids. As a result, while carnivores readily obtain their needed sodium by eating herbivores full of extracellular sodium, herbivores themselves face problems in obtaining that sodium. That's why the animals that you see coming to salt licks are deer and antelope, not lions and tigers. Human hunter-gatherers who consumed much meat, such as the Inuit and San, thus met their salt requirement readily, though even their total salt intake was only 1 or 2 grams per day because much of their prey's sodium-rich blood and other extracellular fluids became lost in the course of butchering and and cooking. Among traditional hunter-gatherers and farmers consuming a diet high in plant food and with limited meat, those living on the seacoast or near inland salt deposits also have easy access to salt. For instance, average daily salt consumption is around 10 grams among the Lau people of the Solomon Islands, who live on the coast and use salt water for cooking, and also among Iran's Qashqa'i nomadic herders, whose homeland has natural salt deposits on the surface.

However, for dozens of other traditional hunter-gatherers and farmers whose daily salt intake has been calculated, it falls below 3 grams. The lowest recorded value is for Brazil's Yanomamo Indians, whose staple food is low-sodium bananas, and who excrete on the average only 50 grams of salt daily: about 1/200 of the salt excretion of the typical Americans. A single Big Mac hamburger analyzed by Consumer Reports contained 1.5 grams (1,500 milligrams) of salt, representing one month's salt intake for a Yanomamo, while one can of chicken noodle soup (containing 2.8 grams of salt) represents nearly two months of Yanomamo salt consumption. A possible record was set by a Chinese-American restaurant near my home in Los Angeles. Its double pan-fried noodles combo dish was reportedly analyzed as containing one year and three days' worth of Yanomamo salt intake: 18.4 grams.

Hence traditional peoples crave salt and go to great lengths to obtain it. (We, too, crave salt: just try eating nothing but fresh, unprocessed, unsalted food for one day, and then see how wonderful salt tastes when you finally sprinkle some on your food.) New Guinea Eastern Highlanders with whom I have worked, and whose diet consists up to 90% of low-sodium sweet potatoes, told me of the efforts to which they used to go to make salt a few decades ago, before Europeans brought it as trade goods. They gathered leaves of certain plant species, burned them, scraped up the ash, percolated water through it to dissolve the solids, and finally evaporated the water to obtain small amounts of bitter salt. The Dugum Dani people of the Western New Guinea Highlands made salt from the only two natural brine pools in their valley, by plunging a spongy piece of banana trunk into a pool to soak up brine, removing the piece and drying it in the sun, burning it to ash, and then sprinkling water on the ash and kneading the moist mass into cakes to be consumed or traded. After all that traditional effort to obtain small quantities of impure bitter-tasting salt, it's no wonder that New Guineans eating in Western-style cafeterias can't resist grabbing the salt-shaker on the dining table and letting the stream of pure salt run out onto their steaks and salads at every meal.

With the rise of state governments, salt became widely available and produced on an industrial scale (as it still is today) from salt-water drying pans, salt mines, or surface deposits. To its use as a seasoning was added its use, reportedly discovered in China around 5,000 years ago, to preserve food for storage over the winter. Salt cod and salt herring became fixtures of the European diet, and salt became the most traded and most taxed commodity in the world. Roman soldiers were paid in salt, so that our word "salary" for pay is derived not from the Latin root for "money" or "coins" but from the Latin root for "salt" (sal). Wars were fought over salt revolutions; revolutions broke out over salt taxes; and Mahatma Gandhi rallied Indians against the perceived injustice of British colonial rule by walking for one month to the ocean, violating British laws by illegally making salt for himself on the beach from the freely available salt water, and refusing to pay the British salt tax.

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. 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. 
(Jared Diamond, ‘The World until Yesterday)