Vitamin D is a fat-soluble vitamin, and it can be acquired either by ingestion or by exposure to sunlight. It is known as the "sunshine" vitamin because the action of the sun's ultraviolet rays activates a form of cholesterol, which is present in the skin, converting it to vitamin D.
The provitamins D are found in both plant and animal tissue. Vitamin D2 is known as calciferol, a synthetic; vitamin D3 is the natural form as it occurs in fish-liver oils. D3 can be made synthetically by ultraviolet irradiation of 7-dehydrocholesterol, a derivative of cholesterol.
Vitamin D aids in the absorption of calcium from the intestinal tract and the breakdown and assimilation of phosphorus, which is required for bone formation. It helps synthesize those enzymes in the mucous membranes which are involved in the active transport of available calcium. Vitamin D is necessary for normal growth in children, for without it bones and teeth do not calcify properly.
Adults also benefit from vitamin D. It is valuable in maintaining a stable nervous system, normal heart action, and normal blood clotting because all these functions are related to the body's supply and utilization of calcium and phosphorus. Vitamin D is best utilized when taken with vitamin A. Fish-liver oils are the best natural source of vitamins A and D.
Ingested vitamin D is absorbed with the fats through the intestinal walls with the aid of bile. Vitamin D from dehydrocholesterol by sun radiaion is formed in the skin and absorbed into the circulatory system. Pigmentation is a factor in the absorption of ultraviolet rays. The more pigment there is in the skin, the less vitamin D is produced in the body by irradiation.
After absorption from the intestine or formation in the skin, vitamin D is transported to the liver for storage; other deposits are found in the skin, brain, spleen, and bones. The body can store sizable reserves of vitamin D. Mineral oil can destroy the vitamin D already stored in the intestinal tract.
Most of the body's needs for vitamin D can be met by sufficient exposure to sunlight and from the inges-tion of small amounts of food, but the sun's action on the skin can be inhibited by such factors as air pollution, clouds, window glass, or clothing. The National Research Council sets the dietary allowance of vitamin D at 400 IU per day to meet the requirements of practically all healthy individuals who have little or no exposure to ultraviolet light. This same dosage is recommended for infants, provided the calcium consumption at the same time is adequate. During pregnancy and lactation women need to include extra vitamin D in their diets. According to the National Research Council, there are no vitamin D recommendations for adults over twenty-two years of age since there are no data available upon which to base such a recommendation.
The adult rate of calcium and phosphorus loss from the skeletal system is thought to be less rapid than that of the growing organism.
It must be emphasized that good will result from the provision of adequate vitamin D only when the calcium and phosphorus requirements are met. No extra benefit is obtained from taking more than 400 IU daily except for therapeutic reasons; then dosages may range from 1500 to 2800 IU daily for several months. Increased heart activity requires increased calcium, which is not supplied unless there is enough vitamin D in the system.
It is known that "hypervitaminosis D" can occur and can cause pathological changes in the body. Excessive amounts may cause high levels of calcium and phosphorus in the blood and excessive excretion of calcium in the urine; this leads to calcification of soft tissues and of the walls of the blood vessels and kidney tubules, which is hypercalcemia.
Symptoms of acute overdosage are increased frequency of urination, loss of appetite, nausea, vomiting, diarrhea, muscular weakness, dizziness, weariness and calcification of the soft tissues of the heart, blood vessels, and lungs. These symptoms will disappear within a few days when the overdosage is terminated.
Some infants react hyperactively to the amounts of vitamin D found in fortified milk. This reaction to the vitamin could result in further medical complications. Hypercalcemia that has developed in children ingesting average supplementation of D may be an indication of hyperreactivity to the vitamin.
Large doses of vitamin D given to rheumatoid arthritic patients resulted in deposition of calcium in the arteries. This condition can cause kidney dysfunction and high blood pressure.
Deficiency Effects and Symptoms
A deficiency of vitamin D leads to inadequate absorption of calcium from the intestinal tract and retention of phosphorus in the kidney, leading to faulty mineralization of bone structures. The inability of the soft bones to withstand the stress of weight results in skeletal malformations. Rickets, a bone disorder in children, is a direct result of vitamin D deficiency. Signs of rickets are softening of the skull; softening of the fragile bones with bowing of the legs and spinal curvature; enlargement of the wrist, knee, and ankle joints; poorly developed muscles; and nervous irritability.
A deficiency is most commonly seen in premature babies or children who have too little exposure to sunshine. "Adult rickets," called osteomalacia, may also occur.
It is believed that vitamin D and parathyroid hormones work together to regulate the transport of calcium. A deficiency may cause tetany, a condition characterized by muscular numbness, tingling, and spasm.
Dr. Arthur A. Knapp, an ophthalmologist, reported tests indicating that a vitamin D deficiency may cause myopia, or nearsightedness. An imbalance in calcium is the root of this disorder (see Animal and Human Tests). A vitamin D deficiency may also lead to faulty development of tooth structure.
The severe low calcium levels found in kidney disease have been attributed to the body's inability to properly metabolize vitamin D. Significant improvement in absorption has been observed in patients receiving 100 IU of supplemental vitamin D.
Beneficial Effect on Ailments
Vitamin D helps prevent and cure rickets, a disease resulting from insufficient calcium, phosphorus, or vitamin D. It also aids in repairing osteomalacia in adults.
Vitamin D plays an important role in dentition. Besides being necessary for proper tooth eruption and linear growth, it continually strengthens the teeth. According to Adelle Davis, vitamin D helps in preventing tooth decay and pyorrhea, an inflammation of the sockets of the teeth.
Vitamins D and A have been beneficial in reducing the incidences of colds. The two vitamins taken along with vitamin C act as a preventive measure. Researchers have reported that the acidity of gastric juices is affected by the amount of vitamin D in the diet. These juices are named as a cause of stomach ulcers. Therefore an ulcer patient should be checked to see whether his diet has a sufficient supply of vitamin D.