We owe a lot to salt. The extraction and trade surrounding salt have contributed to shaping certain civilizations. Salt was essential for preserving food (vegetables, meats, fish). It has also been a source of deadly conflicts: tribes and countries have fought to gain access to salt. More recently, even though salt is cheap and abundant, it has sparked violent controversies among medical experts, journalists, politicians, and the agri-food industry.
Salt: the fundamentals
Common salt is also called sodium chloride. It consists of about 40% sodium and 60% chloride. Of marine origin, it is mainly harvested in salt marshes or extracted from rock salt deposits (ancient geological seas). 1 gram of salt provides 394 mg of sodium. Sodium is a nutrient, it belongs to the family of minerals. Essential for life, it cannot be produced by the body and must therefore be provided by diet. While salt is the main source of sodium in our diet, it is not the only one. Some ingredients or additives also contribute to it. Moreover, sodium is naturally present in many foods: seafood, meats, water, vegetables, milk, etc. The sodium provided by the diet is almost completely absorbed in the intestine. The surplus is eliminated through the skin (sweat) but mainly through urine. The kidney is indeed the organ that regulates the amount of sodium present in the body.
The body contains between 70 g and 100 g of sodium. Half is fixed in bones, cartilage, and tissues. About 10% is found in the blood, around 10% inside cells, and 30% in the fluids that bathe them (extracellular fluids). Sodium (along with water, chlorine, potassium, calcium, and certain hormones) ensures the proper hydration of the body and its cells. It regulates water transfers between cells, thus preventing them from swelling or losing their water. Any imbalance between sodium intake and output will result in a loss of extracellular fluid or, conversely, an increase in the volume of this fluid, to maintain constant hydration of cells and the body. Sodium also plays a major role in the transmission of nerve impulses, muscle and heart contraction, as well as in the assimilation of certain minerals and trace elements.
The human body only needs a very small amount of sodium. Together with potassium, it is essential for maintaining a good balance of fluids in and around cells. The Yanomamo, who inhabit the Amazon forests, consume only 200 mg of sodium per day (one-tenth of a teaspoon of salt) and do not experience hypertension problems, while the average American consumes 3.5 g (about 1 and a half teaspoons of salt). Before the 1970s, residents of Northern Japan consumed up to 10.3 g of salt (4 and a half teaspoons) per day. Public health policies have succeeded in considerably reducing this consumption.
The body has a complex system of checks and balances to maintain a constant amount of sodium in extracellular fluids (which bathe the cells) and in the blood. When sodium levels decrease, the kidneys and sweat glands retain water (water retention). This allows sodium not to be expelled from the body. The opposite occurs when you consume more sodium than necessary: the kidneys eliminate the excess by secreting more urine, or by excreting more sodium in the urine. In most cases, the kidneys struggle to manage excess sodium in the blood. The body then reacts by retaining water to dilute the sodium. This increases both the amount of fluids around the cells and the volume of blood in the circulation. This means more work for the heart and more pressure on blood vessels. Over time, the overwork and additional pressure can harden blood vessels, leading to high blood pressure, heart attack, or stroke. It can also lead to heart failure. It has also been observed that too much salt can damage the heart, aorta, and kidneys without increasing blood pressure, and that it can be bad for bones.
Although many risk factors contribute to it such as smoking, excess weight, reduced physical activity, stress, excess alcohol, high blood pressure is one of the major risk factors for cardiovascular diseases. It is the cause of two-thirds of all strokes and half of heart diseases. In China, high blood pressure is the leading cause of premature death and is responsible for more than one million deaths per year.
Salt: the main sources
When early humans fed on hunting and gathering, potassium was abundant and sodium rare. The Paleolithic diet provided up to 10 g of potassium per day and less than 700 mg of sodium. Historical sodium scarcity is reflected in the human body's ability to conserve it. Today, many processed foods contain a lot of sodium. According to ANSES (French Agency for Food, Environmental and Occupational Health & Safety), 80% of salt intake in heavy salt consumers comes from bread and crackers, deli meats, ready-to-eat soups, cheeses, prepared dishes, pizzas, quiches, and pastries containing salt.
If you only eat unprocessed foods, lots of fruits, vegetables, whole grains, nuts, wild fish, naturally raised beef, chicken, eggs, and milk, and prepare your own meals with a light hand on the salt shaker, you will consume much more potassium than sodium (but also many vitamins, minerals, fiber, phytonutrients, good fats, and other elements provided by a healthy diet). Unfortunately, few people eat this way. Instead, our reliance on ready-to-eat dishes tilts the balance in the other direction.
In Europe and the United States, 75% of consumed sodium comes from processed foods. And most of the time, people are unaware of the salt content of what they eat because many sodium-rich foods do not taste salty. For example, something as innocuous and apparently unsalted as a bowl of cereal can provide up to 354 mg of sodium or 24% of the recommended intake. Meals eaten at restaurants can be worse because of portion sizes (often oversized) and the fact that a menu can combine several sodium-rich foods. In Asia, especially in China and Japan, overconsumption of salt comes from the consumption of very salty condiments like soy sauce.