Sodium, Potassium, and Magnesium

RUTH M. LEVERTON.

SODIUM, potassium, and magnesium are essential in nutrition. They are among the most plentiful minerals in the body. Calcium and phosphorus are present in the largest amounts, and then come potassium, sulfur, sodium, chlorine, and magnesium in descending order of amounts.

A person who weighs 154 pounds has about 9 ounces of potassium, 4 ounces of sodium, and 1.3 ounces of magnesium in his body.

Sodium and potassium are similar in chemical properties but different in their location within the body. Sodium is chiefly in the fluids that circulate outside the cells, and only a small amount of it is inside the cells. Potassium is mostly inside the cells, and a much smaller amount is in the body fluids.

The interrelation between amounts of these minerals in their different locations permits substances to pass back and forth between the cells and the surrounding fluids. This process of exchange is called osmosis.

Sodium and potassium are vital in keeping a normal balance of water between the cells and the fluids. A decline in the sodium content of the fluids results in a transfer of water from the fluids into the cells. An increase in sodium causes a transfer of water from the cells into the fluids.

Sodium and potassium are essential for nerves to respond to stimulation, for the nerve impulses to travel to the muscles, and for the muscles to contract. All types of muscles, including the heart muscle, are influenced by sodium and potassium.

Sodium and potassium also work with proteins, phosphates, and carbonates to keep a proper balance between the amount of acid and alkali in the blood.

We do not have enough information to establish the exact requirements for these minerals. Dietary deficiencies are uncommon, however. Deficiencies are more likely to result from unusual losses from the body.

The body normally conserves its supply of sodium and potassium when the intake is low by reducing the amount that it excretes in the urine. Excessive sweating can cause a major loss of sodium from the body. Such a loss can be replaced easily by increasing the intake of common table salt sodium chloride.

Severe diarrhea and vomiting reduce the amount of sodium in the body and upset seriously the relationship between the sodium and potassium in the fluids and the cells. A solution of sodium chloride sometimes has to be given intravenously to correct this imbalance.

The body usually can take care of great variations in the amount of sodium taken in because it excretes promptly what it does not need. This may not be true in some types of heart disease and poor functioning of the kidneys.

The intake of sodium for persons in the United States has been estimated to be 3 to 7 grams per person per day. Salt is the main source of sodium in a person's diet, and taste and habit determine the amount of salt he eats.

Sodium is one of the nutrients that we can eat in too large amounts. An oversupply comes from the excess salt we add to our food rather than from the sodium that is present in our foods as they are grown or produced.

Sodium is present in most of our foods and in some of the materials we use in preparing and processing food. Ordinary table salt is 43 percent sodium and our most concentrated source. A large proportion of our daily intake of sodium comes from the salt we add to our food when we prepare and eat it. Baking soda is about 30 percent sodium. Ordinary baking powders contain about 10 percent.

Foods from animal sources, including meat, fish, poultry, milk, and cheese, contain more sodium than do foods from plant sources. Seafoods are higher in sodium than fish from fresh water.

Most fresh and frozen vegetables contain only small amounts of sodium, unless salt is added when they are prepared for eating. Beets, carrots, celery, chard, kale, beet and dandelion greens, and spinach are exceptions; they contain several times more sodium than other vegetables.

Canned vegetables contain more sodium than fresh or frozen ones because of the salt added during the process of canning.

All fresh, canned, and frozen fruits are low in sodium.

Breakfast cereals are low in sodium if none has been added to the grain during processing. The same is true of rice, macaroni, and spaghetti. The label usually gives this information.

Yeast breads that contain salt and other baked goods that contain salt and baking powder or soda have relatively large amounts of sodium.

Butter and margarine have a high content of sodium unless they are made without salt. The label gives this information. Shortenings and salad oils do not have salt added.

Coffee and tea contain very little sodium unless they are made with water that has a great deal of sodium.

The water supplies in some areas contain appreciable amounts of sodium. Some of the ion-exchange systems for softening water add sodium to it.

Foods contain the most sodium when salt has been added to them directly or by brining or pickling or by curing. A raw potato, for example, contains about 0.001 gram of sodium, but the same weight of potato chips may have as much as 0.340 gram of sodium. Cured ham has about 20 times more sodium than fresh pork.

Large intakes of sodium may aggravate a tendency toward high blood pressure. A moderate rather than a high intake of sodium therefore is one of the many measures advocated to prevent the development of high blood pressure. This means intakes at the lower end of the range of 3 to 7 grams of sodium a day, which is our estimated intake.

One can have a moderate intake of sodium if he does not add extra salt to foods after they are prepared and if he eats salted, pickled, and cured foods sparingly.

A person's intake of sodium can be limited to 1.5 to 2.5 grams daily if no salt is added at any time in preparing the food, and if no salted, pickled, and cured foods are used.

The daily intake of sodium sometimes is restricted to 1 gram or less as part of the treatment of diseases accompanied by high blood pressure or edema, which is the swelling of the tissues caused by retention of water. This can be done only under medical supervision.

Expert care must go into planning diets that are restricted severely in sodium, or they will be inadequate in other essential nutrients, especially high-quality protein and vitamins of the B complex.

ORDINARY DIETS of persons in the United States supply 1.4 to 6.5 grams of potassium per person per day.

The intake of potassium is related to the caloric value of the diet because this mineral is so widely distributed among different kinds of foods. We have no evidence that the healthy person needs to limit or otherwise control his intake of potassium.

MAGNESIUM is closely related to both calcium and phosphorus in its location and its functions in the body. About 70 percent of the magnesium in the body is in the bones. The rest is in the soft tissues and blood. Muscle tissue contains more magnesium than calcium. Blood contains more calcium than magnesium.

Magnesium acts as starter or catalyzes for some of the chemical reactions within the body. It also becomes a part of some of the complex molecules that are formed as the body uses food for growth and for maintenance and repair. It plays an important role as a coenzyme in the building of protein. There is some relation between magnesium and the hormone cortisone as they affect the amount of phosphate in the blood.

Animals on a diet that is deficient in magnesium become extremely nervous and give an exaggerated response to even small noises or disturbances. Such unnatural sensitiveness disappears when they are given enough magnesium. In extreme deficiencies, the blood vessels expand, the heart beats faster, and damage in the mid-brain causes such irritability that the animals die in convulsions.

A deficiency of magnesium in human nutrition is not common, but it may occur more frequently than it is diagnosed. It disturbs the calcification of bone. An excess of magnesium causes a deposition of calcium in the soft tissues.

An adult requires about 0.3 gram of magnesium a day. A child needs more magnesium in proportion to his size than an adult does, but we do not know the exact amounts. One estimate is that he requires about 0.006 gram of magnesium daily for each pound of body weight. A child who weighs 40 pounds thus would need 0.24 gram daily.

Magnesium is present in foods from both animal and plant sources meats, milk, cereals, vegetables, and fruit. They vary greatly in the amount of magnesium they contain. Nuts, legumes, and cereal grains have more magnesium than other foods. Fresh fruits contain less than other foods.

A diet that is adequate in other essential nutrients, especially protein of high quality, is likely to supply enough magnesium.

Scientists know that there is much more to be learned about the actions and purposes of magnesium in the normal functioning of the body. For example, evidence is growing that magnesium may play an important part in the body's use of fats. But we do not have the facts to prove this and other suspected relationships. Research will provide the facts in time, however, and we then can apply them to improve our diet and health.

RUTH M. LEVERTON is Associate Director of the Institute of Home Economics in the Agricultural Research Service.