Chapter 2. Why Do We Crave Salt?

Why Do We Crave Salt?

Read the article below. Then answer the questions that follow.

A seasoning for all seasons

Pass the ionic compound of sodium and chloride, please.

Most people know that a high salt diet is associated with hypertension (also called high blood pressure), which is why public health campaigns advise us to limit our salt intake. This can be difficult, since most of us have a strong taste for salt, and many recipes call for it. Grasping for a saltshaker when something doesn’t taste quite right is something we seem to do without even thinking.

Do humans eat salt only because we like the way it makes our food taste, or have our body systems evolved to need it? How can an excess of salt in our diets lead to hypertension, while a deficiency can kill us? Before we discuss these questions, let’s learn about salt—starting with its chemical description—and also learn a bit about its history with humans.

A salty tale

Table salt is an ionic compound that consists of sodium and chloride. Gourmet salts may have a different crystal structure and moisture content, and they may contain additional ingredients. But even mineral-rich French sea salts, and Hiwa Kai—a traditional Hawaiian salt that contains small amounts of activated charcoal—are both still primarily made of sodium and chloride.

Salt was crucial to humans before refrigeration because it could preserve food that would otherwise quickly rot. At times salt was as valuable a commodity as gold. Roman soldiers were sometimes paid in salt, and cutting off enemy salt supplies was used as a wartime tactic. Describing someone as “worth his salt” and “the salt of the earth” were considered high praise and are reminders of salt’s exalted status in our history.

Have we evolved to crave salt?

Some researchers suggest our cravings for sodium may be hardwired by evolution and physiology. Early in evolution, life transitioned from the salty sea to land. Researchers suggest this required a way to keep cells surrounded by a salt solution similar to seawater. As the earliest cells evolved into more complex organisms, salt was a critical ingredient of life.

Later on, our hominid ancestors evolved in an environment where salt was not readily available. The climate was hot, and while the primary diet of seeds, nuts, and veggies was high in potassium, it was deficient in sodium. This is why the kidneys evolved to conserve sodium and excrete potassium.

This may explain why our salt cravings seem primal at times—yet while this mechanism served our ancestors, it doesn’t help members of western culture, who have far too much salt in their diet. A single serving of ketchup can contain 200 mg of sodium, for example, and as our western diet has few veggies, we don’t get much potassium.

Sodium and high blood pressure

Doctors tell us to limit salt intake as one way to avoid the dangers of high blood pressure.

In humans, a low potassium diet causes sodium retention. This is because the proteins that carry sodium out of the cell, and calcium and potassium in, begin to function differently when the ratios of these ions change.

Increasing sodium levels inside the cell opens flood gates for calcium, which in turn cause vascular cells to contract. Low potassium levels exacerbate this, resulting in even more calcium inside the cell. This is why a diet high in potassium is thought to dilate blood vessels, while a depletion causes the vessels to contract (or close). Drugs that inhibit sodium-calcium exchanges are also being explored to treat hypertension.

High salt also causes hypertension at the physiological level. According to the rule of osmosis—the movement of water across a semi-permeable membrane towards a higher solute concentration—salt results in higher circulatory volumes. When water flows from tissue towards higher salt concentrations in the blood, the extra fluid pressure stresses the vessels. They react by thickening and narrowing, requiring more pressure to move blood through the system. This in turn stresses the heart.

Why humans need salt

Doctors recommend you eat less than 1 teaspoon of table salt—about 2.4 grams of sodium—a day. This leaves many scrutinizing the ingredient lists on the back of ketchup bottles and other foods that contain a lot of sodium. Even so, completely eliminating salt from our diets is not an option. There are many vital functions that cannot happen without salt.

Salt controls the body’s ability to regulate fluid levels at the cellular level, and our normal growth requires the ingestion and retention of sodium. Without dietary salt, animals and humans grow more slowly, they can’t reproduce, and sometimes even die. People that eat sodium-free diets cease to mineralize bone, and tissue growth stops. Short-term sodium deficiency can cause a range of problems, including muscle cramps, loss of appetite, nausea, as well as the disruption of glucose metabolism and blood viscosity.

While salt is critical for body functions, it has been—as we have discussed—a scarce commodity through history. Because not everyone could afford to indulge their salt cravings, hypertension didn’t cause as much trouble historically. Yet now that modern refinement processes have made salt available to the masses, high blood pressure is on the rise.

© 2015 WH Freeman and Company.

2.1 Comprehension Questions

Answer the following questions to demonstrate your understanding of the article.