Because of its regulation of calcium and phosphorus metabolism, vitamin D is very important to growth in children, especially to healthy bones and teeth. It is also helpful in maintaining the nervous system, heart function, and for normal blood clotting—all of which are affected by calcium levels.

Vitamin D works together with parathyroid hormone for calcium metabolism. Functionally, vitamin D is actually more like a hormone than a vitamin; it is produced in one part of the body (the skin) and released into the blood to affect other tissues (the bones). There is a feedback system with the parathyroid to produce active vitamin D3 when the body needs it, and this "vitamin" is closely related structurally to the body hormones estrogen and cortisone.

Again, vitamin D regulates bone formation. If D is low, blood levels of calcium and phosphorus decrease, and the body pulls these minerals from the bones. This creates demineralized, weak bones, a condition called osteomalacia (loss of bone mineral), or adult rickets. Osteoporosis involves loss of bone mass (minerals and proteins together). The decreased level of calcium in the blood also affects the heart and nervous system.

Uses: Vitamin D works best with adequate calcium and phosphorus intake. It is supplied primarily to prevent or to cure rickets, the vitamin D deficiency disease. It also is used to maintain healthy bones and dentition, as D is helpful in preventing tooth decay and gum problems. Calciferol supplementation may be used to aid the healing of fractures, osteoporosis, and other bone problems.

Taking vitamin D with vitamin A has been shown in some studies to reduce the incidence of colds. It has also been used in the treatment of diabetes, cataracts, visual problems, allergies, sciatica pain, and skin problems. Some success in treating myopia (nearsightedness) and conjunctivitis has been had with high doses of vitamin D. Vitamins A and D together have helped muscle spasms, especially when related to anxiety states. A and D have been used in the treatment of asthma and arthritis as well. Menopausal symptoms such as hot flashes and depression have been helped by the use of calcium and vitamin D together. However, other than the use in menopause, these other applications of vitamin D are not very common in recent years. Medically, high vitamin D supplementation is used to treat hypocalcemia (low blood calcium) secondary to such problems as hypoparathyroidism, which may occur after thyroid surgery.

Deficiency and toxicity: There are some toxicity problems related to hypervitaminosis D. These usually occur with high doses of more than 1,000–1,500 IUs daily for a month or longer in adults, more than 400 IUs in infants, or more than 600 IUs daily in children. These are not exact numbers, of course, and may vary between individuals, time of year, and specific needs; however, it is wise to be careful with supplemental vitamin D. I personally think the combination 1,000 IU D/25,000 IU A formulas are potential trouble if taken at all regularly. However, if some people have poor fat digestion and assimilation, they may handle higher amounts of oral vitamin D.

Excessive thirst, diarrhea, nausea, weakness, and headaches are the milder symptoms of vitamin D toxicity. There are also increased levels of calcium and phosphorus in the blood and urine, and abnormal calcification of soft tissues may occur. There is some suggestion that excess vitamin D speeds the atherosclerosis process. Most symptoms decrease and clear up after excessive doses of vitamin D are discontinued. Toxic doses of vitamin D can be made by the skin through prolonged sun exposure, especially before the body has adapted through pigmentation (tanning), which protects the deeper layers where the vitamin D is synthesized. I have personally wondered if the weakness, nausea, dizziness, or headaches from sun exposure may be related to vitamin D toxicity.