Food and You
Dallas E. Boggs,PhD
Seven elements make up three fourths of the mineral salts found in the human body. They are calcium, phosphorus, sodium, potassium, magnesium, chlorine and sulfur. The remainder includes smaller amounts of aluminum, bromine, cobalt, copper, fluorine, iodine, iron, manganese, nickel and zinc. In fact, it is possible to detect in the body most of the common elements found in the earth's crust. The mere presence of an element in the body may not mean that it has a special function. It may simply have been absorbed accidentally with the foodstuffs. It was pointed out in an earlier chapter that the intestine does not exercise much discretion in what it absorbs.
Utilization of the Minerals
THE MINERALS play an important part in the vital processes of the body, and they differ from other foodstuffs in that they are not burned or otherwise used up. Mineral salts are taken in with the diet, used for a while and then discarded in the excreta. In this sense, these molecules are only borrowed by the body for a short time and then returned unchanged to the earth, from whence they came.
The mineral elements combine among themselves to form various salts. For example sodium and chlorine combine to form sodium chloride. The free elements, that is metallic sodium and gaseous chlorine, are highly reactive and poisonous; but as sodium chloride they form one of the most useful and harmless mineral salts of the body. In water this salt splits into sodium and chloride ions. Iron, on the other hand, is inert and harmless as the metal. The body uses it mainly in combination with organic compounds to form hemoglobin and other pigments. The mineral requirements are spoken of in terms of the elements; but in actual use in the body they are combined with each other to form mineral salts or with the other foodstuffs—proteins, fats, carbohydrates and the vitamins.
Calcium salts account for most of the hard material in bones and teeth. The absorption of calcium (the most abundant mineral element in the body) from the intestine is aided by the presence of a proper amount of vitamin D. All cells need calcium, and it is especially important in the nervous system. If blood calcium falls below a certain concentration, the nerves become very sensitive to stimulation. In this condition, an animal or a child may go into convulsions from a very mild stimulus such as stroking of the skin or a noise that normally would only cause a turning of the head. The regulation of blood calcium is therefore very important, and the parathyroid hormone seems to have this job as its sole function.
The body is always losing calcium through the excreta. Most of it passes into the large intestine and is discarded in the feces—a smaller amount escapes in the urine. In order to maintain "calcium balance" enough of this element must be obtained from the food to make up for these unavoidable losses. For the child or pregnant woman—in whom new bone is being formed—the calcium intake must be much greater than for the adult man. The bones and teeth of adults were long considered to be fixed and lifeless materials; but later work disproved this old idea. With the aid of radioactive calcium and phosphorus it was demonstrated that these elements move in and out of bones and teeth with considerable speed.
In this exchange of old calcium for new, there is a certain amount of loss. It is important, therefore, even for the grown man, to give some thought to the daily mineral intake. The skeleton of a mature adult is not a fixed structure; it is changing every day.
It has been known for a long time that a pregnant woman will supply calcium for the bones in her unborn baby—even if she does not get enough in her diet. The only supply available in this situation is the woman's own bones. The expression, "A tooth for every baby," was very common before the days of scientific nutrition. This does not mean that the calcium required for the child is taken from the mother's teeth. The whole skeleton probably contributes calcium; and the jaw bone, among others, can be weakened so much that one or more teeth may drop out. It is now possible to adjust the diet so that a woman can have a baby without weakening her bones or losing her teeth.
Calcium also enters into the highly complicated process of blood clotting. Drawn blood will not clot without calcium ions. When the Red Cross bleeds its volunteers to supply the blood bank, it is necessary to prevent clotting. This is done by placing some citrate in the bottle; it binds blood calcium and prevents clotting.
Calcium is the one mineral element most likely to be low in the average diet; and, according to dietary surveys, about half of the American population gets less calcium than it needs for good health. Milk and its products, such as cheese and ice cream, are the best dietary sources of calcium; and among the vegetables, carrots are a good source. Some of the cereals, such as whole wheat and oats, contain considerable calcium; but it is not all available. These cereals contain phytic acid, which combines with calcium so firmly that the digestive system of man is unable to split the combination. Calcium combined in this way is not available to the body—the insoluble compound of calcium and phytic acid is carried through the digestive tract and excreted unchanged.
Phosphorus ranks next to calcium in amount present in the body. It is part of the calcium phosphate salt that accounts for most of the hard structure of bones and teeth. Phosphorus is also widely distributed throughout other tissues of the body. The amount in the blood stream is quite closely regulated; but if changes occur they are usually opposite to changes in blood calcium, i.e., if calcium goes up phosphorus goes down and vice versa.
Phosphorus is probably the most versatile of the mineral elements. It may combine with most other foodstuffs, and some of these combinations are necessary to assure complete utilization in the body. Casein, one of the proteins of milk, is combined with phosphorus. It will be recalled that the phospholipids are important in the absorption and transport of fat in the body. Phosphorus is extremely important in the utilization of carbohydrate—it is involved from the time sugar is absorbed from the intestine until it is stored as glycogen or burned. This element seems to combine easily with sugar as well as with many of its breakdown products. Furthermore, the enzyme systems that control the breakdown of sugar require other compounds of phosphorus for their proper operation.
Sodium and Chlorine
Sodium chloride is the salt most abundant in the fluids of the body. It is particularly important in the blood and tissue fluids because it provides a "salt solution" in which the cells feel at home. For instance, when the red blood cells are separated from the blood plasma and placed in water, they swell and burst; but when they are placed in a solution that contains 0.9% of sodium chloride, they retain their original shape and survive for a long time. This is also true for other cells. A piece of fresh tissue removed from an animal will survive for some time in an "isotonic saline solution" (0.9% NaCl) —but its cells will swell and burst in plain water. One of the main functions of sodium chloride then is to provide the fluids of the body with enough salt so that the individual cells will retain their shape and normal water content. Under average conditions, more than enough table salt is consumed to supply the body's requirement. Any excess is easily disposed of in the urine. In extremely hot weather, when a great deal of water is drunk, the losses of salt may be excessive both through the kidneys and through the skin in the sweat. Under these conditions it is wise to use more salt in order to make up for the excessive losses. This is best done by extra salting of food rather than by eating salt pills. The pills do not agree with many people and may cause vomiting and thus aggravate the condition they are supposed to cure!
Potassium, Magnesium, Sulfur
Potassium is concentrated within the cells and tends to remain there except in times of stress. If a muscle is exercised very strenuously, some of the potassium in the muscle cells leaks out and finds its way into the blood plasma. In recovery from exercise, the potassium finds its way back again to restore the normal condition. The potassium of the blood is found within the red cells. It combines easily with the hemoglobin, and this compound is important in the transport of the two respiratory gases, oxygen and carbon dioxide. The possibility of developing a potassium deficiency is quite unlikely for most people; potassium is present in adequate amounts in both plant and animal foods. The content of this element is much higher in plant than in animal foods. On a vegetarian diet there is a chance that the excess of potassium will drive out of the body enough sodium to cause a sodium deficiency. In such instances a "salt hunger" develops; the human vegetarian puts more salt on his food, and the herbivorous animals hunt out the natural salt licks. The modern farmer finds it necessary to provide blocks of salt for his cattle.
Magnesium is related chemically to calcium. Like calcium it depresses the activity of nerves; but it is present in much smaller quantities in the body as a whole. Magnesium plays an important part in the utilization of carbohydrate as an essential part of an enzyme that couples sugar with phosphorus.
Sulfur is usually not eaten as such or as a salt. It is supplied to the body as part of two amino acids—namely, cystine and methionine—which are found in most dietary proteins. When these amino acids are broken down in the body, the sulfur is partially oxidized and appears in the urine as inorganic sulfate.
Iron, Copper, Iodine
The iron requirement has been discussed to some extent elsewhere in this book. It is only necessary to emphasize that an adult man requires very little iron in his diet. The growing child and the menstruating or pregnant woman, however, need more than the adult male; and it is possible for them to develop a nutritional anemia due to lack of iron. The normal child is born with a supply of iron in his liver that will last less than a year—by the end of this period he must have a dietary source of iron. Milk contains only a very small trace of iron, and therefore it is necessary to supplement the infant's diet—first with vegetables and then with meat. The chief purpose of iron in the body is for the formation of new hemoglobin; and whenever blood is lost, iron is needed in making good the loss.
Need for copper is so small that there is little danger of running short of this mineral element. It seems to be necessary for the utilization of iron in hemoglobin formation. Copper is not built into the hemoglobin molecule; it appears likely that it is part of an enzyme system that brings the iron and hemoglobin together.
Iodine is used by the thyroid gland in manufacturing the hormone thyroxin. No other mineral element can be substituted for iodine. In many parts of the world this element is present in very small quantities so that the water and food do not contain enough to supply the daily need. This fact was recognized many years ago and the deficiency was corrected by adding iodine to table salt.
The rest of the minerals are found in the body only in very tiny amounts; and hence they are often called the "trace" elements or minerals. Some of them have important functions, while others seem to be present only because they incidentally happen to be occur in some foods.