Vitamin D(Calciferol) refers to several related fat-soluble vitamin variants, all of which are sterol (cholesterol-like) substances. D2, or activated ergo-calciferol, is the major synthetic form of provitamin D; D3, or cholecalciferol, is found in animals, mainly in fish liver oils. These are converted in the liver and kidneys to 25-hydroxycholecalciferol, and 1, 25-dihydroxylcholecalciferol, the major circulating active forms of vitamin D.
Vitamin D is also known as the “sunshine” vitamin because it is actually manufactured in the human skin when in contact with the ultraviolet light in the sun’s rays. The sunlight interacts with 7-dehydrocholesterol to form cholecalciferol, which is then transferred to the liver or kidneys and converted to active vitamin D. Wintertime, clouds, smog, and darkly pigmented skin reduce the body’s production of the “sunshine” vitamin.
This fat-soluble vitamin, when ingested, is absorbed through the intestinal walls with other fats with the aid of bile. Mineral oil binds vitamin D in the gut and reduces its absorption. From the blood, calciferol is taken mainly to the liver, where it is utilized or stored. Vitamin D is also stored in the skin, brain, spleen, and bones. Vitamin D intake must be more finely tuned in regard to the right therapeutic level than most other vitamins, and it is considered by many authorities to be the most potentially toxic vitamin.
Symptoms of vitamin D toxicity can easily occur when vitamin D is taken in large amounts or with excessive sun exposure. (It is possible that part of sun poisoning symptoms are due to vitamin D toxicity.)
Sources: Provitamin D is found mainly in animal foods. D3, or “natural” vitamin D, is found in fish liver oil, which is the traditional source of both A and D. Cod liver oil is a commonly used source. Egg yolks, butter, and liver have some D, as do the oily fish, such as mackerel, salmon, sardines, and herring. Most homogenized milk and some breakfast cereals are “fortified” with synthetic vitamin D to give children, particularly, sufficient amounts. The plant foods are fairly low in D, with mushrooms and dark leafy greens containing some. Strict vegetarians who do not get adequate exposure to sunlight need to be concerned about getting their 400 IUs of vitamin D daily.
Functions: Vitamin D helps to regulate calcium metabolism and normal calcification of the bones in the body as well as influencing our utilization of the mineral phosphorus. Calcium and phosphorus together with other minerals make up our bones. Vitamin D3 helps increase the absorption of calcium from the gut, decreases excretion from the kidneys, stimulates resorption of calcium and phosphorus from bone, helps put them into teeth, and helps to maintain normal blood levels of calcium and phosphorus. With these functions, vitamin D is closely tied to the work of the parathyroid glands. Vitamin D is most important in regulating calcium metabolism in the body. Even with adequate calcium and phosphorus intake, if our vitamin D intake is low, we will have poor calcification of our bones; whereas, with good vitamin D intake, we will have better calcification even with low calcium and phosphorus intake. This function is especially important in menopausal women, for whom many doctors prescribe straight calcium without vitamin D, which is not likely to do much good unless they are sunbathing, an activity that doctors no longer recommend. Actually, taking calcium, magnesium, and vitamin D all together is probably ideal for best bone health. Phosphorus is usually readily available in adequate amounts in most diets.
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.
Most people do not take very large amounts of supplemental vitamin D but make sufficient amounts through the skin from exposure to the sun. There is more concern with toxicity from the fortified vitamin D, especially in milk. This synthetic, irradiated ergocalciferol (D2) has decreased the incidence of rickets, but it may be contributing to calcification of the arteries, or atherosclerosis, from infancy through old age. The added 400 IUs per quart of milk is about 15 times the amount normally found in milk and may increase the amount of calcium in the circulation, which could be a problem.
Deficiency of vitamin D has not been a major problem of late. Older people are more prone to vitamin D deficiency (a blood level can be measured) since their skin production is lower, their digestion and absorption may be diminished, and their liver function may be reduced. Vitamin D may be deficient in people with gastrointestinal disease, such as ulcerative colitis. The sun’s action on the skin to produce vitamin D is inhibited by pollution, clouds, clothing, window glass, skin pigmentation, and sunscreens.
The occurrence of several of these factors together may make the development of the symptoms of rickets more likely.
The decreased absorption of calcium, along with the retention of phosphorus that usually accompanies it, leads to poor mineralization of bone and the inability of the bones to handle stress. This problem, called osteomalacia, is manifested by poor calcification and soft bones. Vitamin D deficiency in the elderly increases general bone loss and osteoporosis. Supplementing this vitamin improves calcium absorption and reduces bone loss. In children, the bone disorder from vitamin D deficiency is rickets. It is characterized by soft skull bones and fragility of other bones, with bowing of the legs, spinal curvature, and an increase in the size of the joints, such as the wrists, ankles, and knees. Muscular development may be diminished as well. Because of low calcium availability, the teeth may have poor structure, and there may be muscle spasms from a problem called tetany, which also causes tingling and weakness of the areas affected. Nearsightedness and loss of hearing may also develop from vitamin D deficiency because of the vitamin’s influence on the eye muscles and from loss of calcium in the ear bones. Furthermore, one of the current theories of multiple sclerosis is that it may be influenced by low vitamin D levels in puberty.
Requirements: Vitamin D is best utilized with vitamin A. Most of our calciferol needs are met with some vitamin D in foods and regular sunlight exposure. If we live in smoggy cities or where tall buildings block the sunlight, we may need more vitamin D. Those who have darkly pigmented skin, work nights, or cover their bodies with lots of clothes, as do members of some religious orders, probably need more vitamin D than the avid sunbather. In winter, we usually require more D from supplements or from our foods.
The RDA for vitamin D is 400 IUs, or roughly 10 mcg., per day. Infants and growing children probably need more vitamin D relative to body size than do adults. During pregnancy and lactation, more D is needed than the 400 IUs. Therapeutic doses for problems treated with vitamin D are about 1,000–1,500 IUs maximum per day, though some doctors may prescribe even more, mainly of the natural vitamin D3. In general, however, it is wise for adults to limit any supplemented vitamin D to the 400 IUs per day commonly found in multivitamins and to limit use of vitamin-D-fortified milk for a variety of reasons.