MILICENT L. HATHAWAY AND RUTH M. LEVERTON.
CALCIUM, the most abundant mineral in the body, comprises 1.5 to 2.0 percent of the weight of an adult's body. It usually is associated with phosphorus, which is 0.8 to 1.1 percent of the body weight. A person who weighs 154 pounds would have 2.3 to 3.1 pounds of calcium and 1.2 to 1.7 pounds of phosphorus in his body.
About 99 percent of the calcium and 80 to 90 percent of the phosphorus are in the bones and the teeth. The rest is in the soft tissues and body fluids and is highly important to their normal functioning.
Calcium is essential for the clotting of blood, the action of certain enzymes, and the control of the passage of fluids through the cell walls. The right proportion of calcium in the blood is responsible for the alternate contraction and relaxation of the heart muscle.
The irritability of the nerves is increased when the amount of calcium in the blood is below normal.
Calcium in a complex combination with phosphorus gives rigidity and hardness to the bones and teeth.
Phosphorus is an essential part of every living cell. It takes part in the chemical reactions with proteins, fats,and carbohydrates to give the body energy and vital materials for growth and repair. It helps the blood neutralize acid and alkali.
Both calcium and phosphorus are essential for the work of the muscles and for the normal response of nerves to stimulation.
The human embryo at 12 weeks contains about 0.2 gram of calcium and 0.1 gram of phosphorus. (There are 28.4 grams in an ounce.) The values are 5.5 and 3.4 grams, respectively, for these two minerals by the 28th week, and 11 and 7 grams by the 34th week. The most rapid increase in the calcium and phosphorus content of the unborn child occurs from the 34th week to the 40th week.
One-half of the total calcium and more than one-third of the total phosphorus in the baby's body at birth are deposited during the last 6 weeks. The baby's body contains about 23 grams of calcium and 13 grams of phosphorus at birth.
The calcium content of the body increases faster in relation to size during the first year of life than at any other time. About 60 grams of calcium are added. A child is depositing only about 20 grams a year when he is 4 or 5 years old and weighs about 40 pounds. He may be depositing as much as 90 grams a year when he is 13 to 14 years old and weighs 110 pounds. He will deposit more if he weighs more.
All these gains in calcium content depend on an adequate supply of calcium in the diet and the ability of the body to use it for normal growth.
The percentage as well as the total amount of calcium and phosphorus increases during growth. The infant's body is about 0.8 percent calcium and the adult's is about twice as much-1.5 to 2 percent. The phosphorus content of the body increases from 0.4 percent at birth to 0.8 to 1.1 percent in adulthood.
Bone is composed of tiny, complex crystals of calcium and phosphorus, which are set in honeycomb fashion around a framework of softer protein material, called the organic matrix.
The crystals contain about twice as much calcium as phosphorus. They also contain oxygen and small amounts of hydrogen and other minerals.
The honeycomb structure gives strength and an enormous surface area to a small amount of bone material as much as 3,100 square yards to 1 ounce. Connecting canals containing blood and lymph vessels, nerves, and bone marrow pass throughout the matrix and bone crystals. Intercellular fluid surrounds the crystals and keeps them supplied with the materials for repair.
Crystals of the same kind are deposited to make the enamel and dentin of the teeth. The crystals are larger, however, than those in bone. That may be the reason why enamel and dentin are harder than bone.
Phosphorus and calcium are of equal importance in the bones. Phosphorus is involved in ossification or calcification just as much as calcium. When bone is formed, phosphorus is deposited with the calcium. When the bone loses calcium (by decalcification), it also loses phosphorus. They are closely associated in blood and foods.
Phosphorus is included, therefore, even though it is not named each time that calcium is mentioned.
The major change during growth is in the size and the compactness of the bone material. The shape of the bones in a young child is much the same as it will be when he is an adult. The bones in an infant are like firm cartilage and have a low content of calcium and phosphorus. They become firmer as these minerals are deposited in and around the cartilage. This process of bone building is called ossification or calcification. The bones and teeth are said to ossify or to calcify.
Certain bones in the wrist and ankle and the permanent teeth do not begin to calcify until after birth. Groups of specialized cells that are present at birth have the ability to deposit calcium and phosphorus around them and thus become bones and teeth. They are called ossification centers and tooth buds.
Eight small bones in the wrist are mere ossification centers at birth. Two of them usually are calcified in the 1st year, one in the 3d year, two in the 5th year, one each in the 6th and 8th years, and the last one in the 12th year. The exact time of ossification varies among children. Girls often are a little ahead of boys in this process.
The tooth buds of the first teeth begin to form in the human embryo at about 4 to 6 weeks and begin to calcify at about 20 weeks. The upper and lower first molars of the permanent teeth begin to calcify very soon after birth. Others begin at 3 months to 3 years. The wisdom teeth may not begin to calcify until sometime between the 8th and loth year.
Bones and teeth calcify more slowly in children who have diets deficient in calcium and phosphorus and other essential nutrients. Severe deficiencies can cause permanent stunting of size or malformations of bones and teeth.
Changes occur on both the inside and outside of a bone as it grows larger. New bone is deposited around the outside of the shaft of a long bone. Bone on the inside of the shaft is absorbed at the same time and used elsewhere. Thus the cavity that contains the bone marrow is widened. Bone is added also to the outside of each end of the shaft and then taken from the outside of the area just beneath it.
Adding material to the outside of bone and subtracting it from the inside gives the skeleton size and strength without unnecessary weight. If bones grew only by adding material to the outside and none were subtracted from the inside, the skeleton would weigh so much that the muscles could not move it around.
The intricate process of bone building requires many nutrients besides calcium and phosphorus. Vitamin D is essential for absorption from the intestinal tract and the orderly deposition of the bone material. Protein is needed for the framework and for part of every cell and circulating fluid. Vitamin A aids in the deposition of the minerals. Vitamin C is required for the cementing material between the cells and the firmness of the walls of the blood vessels.
Bones can accumulate a reserve supply of calcium and phosphorus at any age if the diet provides enough for the growth and repair and some is left over for storage.
When the intake is generous, the minerals are stored inside the ends of the bones in long, needlelike crystals, called trabeculae. This reserve can be used in times of stress to meet the body's increased calcium needs if the food does not supply enough.
When there is no reserve to use, the calcium has to be taken from the bone structure itself--usually first from the spine and pelvic bones. The dentin and enamel of the teeth do not give up their calcium when the body must provide what the diet lacks.
If the calcium that is withdrawn in times of increased need is not replaced, the bone becomes deficient in calcium and subnormal in composition. From 10 to 40 percent of the normal amount of calcium may be withdrawn from mature bone before the deficiency will show on an X-ray film. Height may be reduced as much as 2 inches because of fractures of the vertebrae, which are caused by pressure and result in rounding of the back. Such fractures may occur with relatively minor jolts or twists of the body and may not be recognized at the time they happen.
Bones with a low content of calcium are weaker and break more easily than bones well stocked with calcium. Breaks in older persons often are related directly to the thinness and brittleness of the bones and are difficult to treat. The bones may be too weak to hold pins or other means of internal repair, and healing may be slow because of the low activity of bone-forming cells.
The calcium and phosphorus and other minerals in our food are dissolved as the food is digested. Then they are absorbed from the gastrointestinal tract into the blood stream. The blood carries them to the different parts of the body where they are used for growth and upkeep.
Calcium as it is present in food dissolves best in an acid solution. It begins to dissolve in the gastric juice of the stomach. The calcium is absorbed when the contents of the stomach move into the small intestine. Farther along in the intestine, the contents change from an acid to alkaline reaction, which does not favor the absorption of calcium.
Usually 10 to 50 percent of the calcium eaten is not absorbed but is excreted in the feces. A small portion of the excreted calcium comes from the intestinal fluids.
The calcium that is absorbed travels in the blood to places where it is needed, particularly the bones. If any of the absorbed calcium is not needed, it is excreted by the kidneys into the urine. Normal functioning of the kidneys is essential for the normal metabolism of calcium and other minerals.
Vitamin D is essential for the absorption of calcium from the gastrointestinal tract. Vitamin D does not occur naturally in many foods. Egg yolk, butter, fortified margarine, and certain fish oils are the chief sources. To a few foods, notably milk and cereals, some vitamin D is added.
A special substance, cholesterol, is present in the skin and is changed to vitamin D by the ultraviolet rays of the sun. We cannot be sure that enough of the rays reach the skin and produce vitamin D in all seasons of the year and in all parts of the country to insure normal growth. Most infants and young children therefore are given daily a concentrated source of vitamin D, such as cod-liver oil or some other fish oil. Adults normally do not need more vitamin D than they customarily get from food and exposure of the skin to the effective rays of sunshine.