16.8: Sodium

Most of our sodium comes as sodium chloride (ordinary table salt). We’re warned so often of the hazard of excessive salt intake that we tend to forget that sodium is an essential nutrient.

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Table salt is 40% sodium and 60% chloride. One teaspoonful has about 2,000 mg of sodium.

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Our desire for salt and its importance to us are reflected in our everyday vocabulary— such words as salary [from the Latin salarium, meaning money for salt (given as a part of a Roman soldier’s pay)], and descriptive phrases such as “above the salt” (an honored position), “salt of the earth” (fine, noble), and “worth one’s salt” (worth one’s wages).

In ancient times, people sought out salt deposits and settled near them. Inland from the salty sea, salt was a precious commodity.

Today, of course, salt is widely available and widely used. An Adequate Intake (AI) [see Dietary Reference Intake (DRI) in Chap. 18] is 1500 mg sodium (the amount in ¾ teaspoon of salt) for those over age 13.⁴

This amount is much higher than minimum requirements (less than 180 mg) because it would be hard to meet other nutrient requirements on a diet that low in sodium. Also, the Adequate Intake amounts are inadequate for people who sweat heavily, e.g., competitive athletes, manual laborers in hot weather.

The average adult intake in the U.S. is about 3400 mg/day, so the sodium in our diet is clearly excessive.⁵ The main concern with excessive sodium intake is its relation to high blood pressure (as was discussed in Chap. 9). High blood pressure increases the risk of cardiovascular disease and kidney disease.

On average, blood pressure goes up as sodium intake goes up. But there’s a lot of variation. African-Americans, older people, and people with high blood pressure, diabetes, and kidney disease are more likely to be affected, and there’s also a genetic component. Even among people with normal blood pressure, a lower sodium diet can help prevent the increases in blood pressure that occurs with aging.

Figure 16-8: Sources of Dietary Sodium.

As has been the case in discussing many nutrients, it’s hard to evaluate the effects of excessive sodium in isolation. Excessive sodium doesn’t affect blood pressure as much if the diet is low in fat and meets the recommended intake of calcium and potassium. Other factors like excess body weight raises blood pressure. To complicate the picture further, people who eat a lot of processed food and fast food tend to have diets high in sodium and fat, and low in calcium and potassium—and they tend to be overweight.

However, the strong evidence that excessive sodium intake increases blood pressure and chronic disease has led to a new DRI: a CDRR (Chronic Disease Risk Reduction Intake) for sodium of 2300 mg for those over age 13.⁴ This means that if our intake is higher than 2300 mg (which means most of us), we should cut back (read food labels!) to lower our risk of chronic disease. Even if our blood pressure isn’t salt-sensitive (most of us don’t know whether it is or not), we probably cook for, or eat with, people whose blood pressure is affected.

Of course, this is easier said than done. Salt is tasty and cheap, helps preserve food, and is calorie-free. Also, high blood pressure itself doesn’t normally make you feel bad.

Although foods in their natural state contain some sodium, most of the sodium in our diet comes from salt added during food processing (including food served at restaurants) (see Fig. 16- 8). Salt added in processing was important when there were few ways to preserve food.

Today, with refrigerators and freezers in common use, there isn’t the same need for processing meats with salts (as in ham, frankfurters, luncheon meats, etc.), storing vegetables in brine (as in pickles and kim chee), and making salty hard cheeses. But these foods are still popular for their taste and convenience.

Then there are the more obvious snack foods with added salt—chips, nuts, pretzels, crackers, and the like. Add to this the salted water often used to cook pasta or rice, the salt in most recipes, whether it be for ordinary bread, a casserole, or chocolate chip cookies, and the salt in such sauces as soy sauce, steak sauce, fish sauce, and mustard. It’s no surprise that we get more than enough sodium in our diet. There are other sources of sodium—MSG (monosodium glutamate, the flavor enhancer), baking soda (sodium bicarbonate), etc.—but these are relatively minor sources, except for those on severely restricted sodium diets.

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1 oz cheddar cheese: 174 mg sodium
1 oz low-sodium cheddar cheese: 6 mg sodium
1 cup unsalted frozen or fresh peas: 7 mg sodium
Burger King BBQ Bacon Whopper®: 1540 mg
Burger King Chicken Club Salad: 1660 mg

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Sodium is readily absorbed from the digestive tract. The amount of sodium kept in the body is controlled in the kidney; the urine carries little sodium when sodium is scarce, but excretes excesses when levels are high.

The role of the kidney in sodium balance is dramatized by patients with kidney diseases that interfere with the removal of excess sodium. Their faces, legs, and feet become swollen because the excess sodium in their tissues causes excess water to be retained (edema).

The main route of sodium loss is through urination. As was discussed in the chapter on water (Chap. 12), excess salt intake increases urine. When we eat a lot of salty foods, we feel thirsty, drink more, and urinate more.

Sodium is also lost in perspiration, the amount varying according to climate and physical activity. As was also discussed in Chapter 12, the amount of sodium and other minerals lost in perspiration is quite minimal under normal circumstances. Also, the body adapts to regular, heavy sweating. The sweat of a trained athlete contains less sodium and other electrolytes per unit volume than that of an untrained person. In the extreme situation of endurance events of 4 hours or more, athletes are advised to consume sodium-containing drinks during the event.

Sodium deficiency can occur as a result of heavy and prolonged sweating, prolonged vomiting or diarrhea, and as a result of certain kidney diseases. Sodium deficiency symptoms include muscle cramps, dizziness, and nausea. As is true of required nutrients generally, deficiencies can be fatal if severe enough.