Vitamin D is a fat-soluble vitamin. Very few foods naturally contain vitamin D, so humans mainly obtain it from exposure to the sun, often with additional supplements and vitamin D-fortified foods. Sunlight triggers the skin to synthesize the vitamin. Regardless of how the body gets vitamin D, the vitamin must be converted, first into calcidiol by the liver, and then into calcitriol by the kidneys. It is calcitriol that is the physiologically active form of vitamin D.
There are two major forms of vitamin D that are important in humans: D2 or ergocalciferol, which is synthesized by plants, and D3 or cholecalciferol, which is synthesized by human skin when it is exposed to sunlight. Vitamin D-fortified foods and vitamin supplements may be of either form or both. The physiologically active form of vitamin D (calcitriol) is a hormone. It regulates the body’s absorption and use of the minerals calcium and phosphorus. Vitamin D is essential for strong teeth and bones, and with calcium helps to maintain proper bone mineral density, which can prevent fractures.
It is also important to the body’s many calcium-driven neurologic and cellular functions, as well as normal growth and development. In addition, it assists the immune system by playing a part in the production of monocytes, which are a type of white blood cell. White blood cells help fight infection as part of the normal immune response. In addition, vitamin D is believed to help people avoid osteoporosis, high blood pressure (hypertension), cancer, and a number of autoimmune diseases.
The Food and Nutrition Board at the Institute of Medicine of The National Academies has listed the recommended dietary allowances (RDA) of vitamin D that are sufficient to maintain bone health and calcium metabolism in a healthy person. RDA is measured in International Units (IU), and there are 40 IU in a microgram (mcg). RDA by age is:
- individuals aged 1–70 years: 15 mcg or 600 International Units (IU)
- individuals 70 years old and older: 20 mcg or 800 IU
- infants up to 12 months: 10 mcg or 400 IU
One of the major uses of vitamin D is to prevent and treat Osteoporosis. This disease is essentially the result of depleted calcium. Calcium supplements alone will not prevent it, however, because vitamin D is required to properly absorb and utilize calcium. Taking vitamin D without calcium is also ineffective. A combination of vitamin D and calcium is used to increase bone density in postmenopausal women, who are most susceptible to bone loss and complications such as fractures.
Osteomalacia and rickets
Other disorders that vitamin D supplementation can help to prevent and treat are osteomalacia and rickets. Both of these conditions are caused primarily by deficiencies in vitaminD and result in a softening of the bones. Osteomalacia occurs in adults. Rickets affects children.
Research studies suggest that vitamin D may be associated with a reduced risk of some cancers, especially colorectal cancer, but with a greater risk for others. The protective effect of vitamin D with colorectal cancer has been noted when it is used in conjunction with calcium. Because calcium and vitamin D function in concert, this has led to some difficulty in determining the contribution of vitamin D alone.
Epidemiologic studies also indicate that vitamin D is linked to a decreased risk for advanced colon cancer. It is problematic in observational epidemiologic studies to determine definitively whether vitamin D is the reason for the decreased risk, or whether some other unknown factor may be behind the decline.
Additional studies have shown that vitamin D levels have no effect on six less common types of cancer: endometrial, esophageal, gastric, kidney, non-Hodgkin’s lymphoma, and ovarian. Studies do, however, indicate that high levels of vitamin D may be linked to an increased risk of developing pancreatic cancer, and again heightened levels of calcium may be an extenuating factor in that increase.
Some metabolic diseases are responsive to treatment with specific doses and forms of vitamin D. These include Fanconi syndrome and familial hypophosphatemia, both of which result in low levels of phosphate, among other things. Phosphate is an important bone mineral. For these conditions, vitamin D is given in conjunction with a phosphate supplement, because vitamin D aids in the body’s absorption of phosphate.
A topical form of calcitriol is available and can be helpful in the treatment of psoriasis vulgaris, the most common form of psoriasis. Applied to the affected skin, this cream is not thought to affect internal calcium and phosphorus levels.
Oral supplements of vitamin D are also effective. Oral vitamin D can cause hypercalcemia (too much calcium in the blood), but with proper dosing and monitoring, this side effect can be avoided.
Insomnia and daytime sleepiness
Vitamin D is sometimes suggested to treat insomnia, but the correlation between the vitamin and insomnia is not clear cut.
A study published in the 2012 issue of the Journal of Clinical Sleep Medicine showed that vitamin D has differing effects depending on the patient population. The study suggested that among those individuals with normal vitaminD levels, with increasingly lower amounts of vitamin D experienced higher daytime sleepiness.
In addition, the study showed that the opposite was true among vitamin D–deficient individuals, but only in African Americans; among black participants with vitamin D deficiency, daytime sleepiness rose with increasing vitamin D levels.
Other health uses
Numerous studies have attempted to ascertain whether vitamin D has other healthful benefits. Various studies have suggested that vitamin D may help fight such illnesses as the flu, heart disease, and Parkinson’s disease, but large, rigorous research studies are needed to verify its role.
- Natural sources
Exposure to sunlight is the primary method of obtaining vitamin D. In clear summer weather, less than 30 minutes per day in the sun generally produces adequate amounts. The amount varies with an individual’s complexion, the time of day, the geographic location (distance from the equator and altitude can both affect vitamin D production), and amount and thickness of clothing. In the winter, when the sun is not as strong, that time is increased.
Many people get insufficient sunlight exposure in the winter but are able to utilize the vitamin that was stored during extra sun exposure over the summer.
Since it is fatsoluble, extra can be stored in the liver and fatty tissues for future use. This does not mean that sunscreen should be avoided. Sunscreen blocks the ability of the sun to produce vitamin D but should be applied as soon as the minimum exposure requirement has passed in order to reduce the risk of skin cancer.
Vitamin D is naturally found in fish liver oils, butter, and eggs in the form of vitamin D2.
Fortified milk products are the main dietary source for most people. Some other dairy products are not a good supply of vitamin D, as they are made from unfortified milk.
Plant foods are also poor sources of vitamin D.
- Supplemental sources
Most oral supplements of vitamin D are in the form of ergocalciferol. It is also available in topical (calcitriol or calcipotriene), intravenous (calcitriol), or intramuscular (ergocalciferol) formulations by prescription only.
As with all supplements, vitamin D should be stored in a cool, dry place; away from direct light; and out of the reach of children.
In adults, a mild deficiency of vitamin D may be manifested as loss of appetite and weight, difficulty sleeping, and diarrhea.
A more major deficiency causes osteomalacia and muscle spasm. The bones become soft, fragile, and painful as a result of the calcium depletion. This is due to an inability to properly absorb and utilize calcium in the absence of vitamin D.
In children, a severe lack of vitamin D causes rickets.
Risk factors for deficiency
- The most likely cause of vitamin D deficiency is inadequate exposure to sunlight. This can occur with people who do not go outside much, or people who live in areas of the world where pollution blocks ultraviolet (UV) light or in areas where the weather prohibits spending much time outdoors. Glass filters out the rays necessary for vitamin D formation, as does sunscreen.
- Skin pigmentation and geography are also factors. Those with dark skin may absorb smaller amounts of the UV light necessary to effect conversion of the vitamin. In climates far to the north, the angle of the sun in winter may not allow adequate UV penetration of the atmosphere to create D3. Getting enough sun in the summer, along with dietary sources, should supply enough vitamin D to last through the winter.
- People who do not consume dairy products may become vitamin D–deficient, especially if they get too little sun. The elderly, who have a decreased ability to synthesize vitamin D, are also at risk for deficiency. Babies are usually born with about a nine-month supply of the vitamin, but breast milk is a poor source. Those born prematurely are at an increased risk for deficiency of vitamin D and calcium, and may be prone to tetany. Infants older than approximately nine months who are not getting vitamin D–fortified milk or adequate sun exposure are at risk of deficiency. People with certain intestinal, liver, and kidney diseases may not be able to convert vitamin D3 to active forms and may need an activated type of supplemental vitamin D.
- Supplements may be required for those taking certain medications, including anticonvulsants, corticosteroids, or the cholesterol-lowering medications cholestyramine or colestipol. Therefore, people who are on medication for arthritis, asthma, allergies, autoimmune conditions, high cholesterol, epilepsy, or other seizure problems should consult with a healthcare practitioner about taking supplemental vitamin D. The abuse of alcohol also has a negative effect. In the case of vitamin D, chronic overuse of alcohol products diminishes the ability to absorb and store the vitamin.
- Populations with poor nutritional status may tend to be low on vitamin D, as well as other vitamins. This can result from poor sun exposure, poor intake, or poor absorption. A decreased ability to absorb oral forms of vitamin D may result from cystic fibrosis or removal of portions of the digestive tract. Other groups who may need higher than average amounts of vitamin D include those who have recently had surgery, major injuries, or burns. High levels of stress and chronic wasting illnesses also tend to increase vitamin D requirements.
The body will not make too much vitamin D from overexposure to sun, but since vitamin D is stored in fat, toxicity from supplemental overdose is a possibility.
Symptoms are largely those of hypercalcemia and may include high blood pressure, headache, weakness, fatigue, heart arrhythmia, loss of appetite, nausea, vomiting, diarrhea, constipation, dizziness, irritability, seizures, kidney damage, poor growth, premature hardening of the arteries, and pain in the abdomen, muscles, and bones.
If the toxicity progresses, itching and symptoms referable to renal disease may develop, such as thirst, frequent urination, proteinuria, and inability to concentrate urine.
Overdoses during pregnancy may cause fetal abnormalities. Problems in the infant can include tetany, seizures, heart valve malformation, retinal damage, growth suppression, and intellectual disability. Pregnant women and individuals with hypercalcemia, sarcoidosis, or hypoparathyroidism should consult with their doctors about a proper amount and form of vitamin D supplementation for them.
Minor side effects may include poor appetite, constipation, dry mouth, increased thirst, metallic taste, or fatigue. Other reactions, which should prompt a call to a healthcare provider, include headache, nausea, vomiting, diarrhea, confusion, and depression.
The absorption of vitamin D is improved by calcium, choline, fats, phosphorus, and vitamins A and C. Supplements should be taken with a meal to optimize absorption.
A number of medications can interfere with vitamin D levels, absorption, and metabolism. These include such medications as Rifampin, H2 blockers, barbiturates, heparin, isoniazid, colestipol, cholestyramine, carbamazepine, phenytoin, fosphenytoin, and phenobarbital. Anyone who is on medication for epilepsy or another seizure disorder should check with a healthcare provider to see whether it is advisable to take supplements of vitamin D. Overuse of mineral oil, Olestra, and stimulant laxatives may also deplete vitamin D. Osteoporosis and hypocalcemia can result from long-term use of corticosteroids. It may be necessary to take supplements of calcium and vitamin D together with this medication.
The use of thiazide diuretics in conjunction with vitamin D can cause hypercalcemia in individuals with hypoparathyroidism. Concomitant use of digoxin or other cardiac glycosides with vitamin D supplements may lead to hypercalcemia and heart irregularities. The same caution should be used with herbs containing cardiac glycosides, including black hellebore, Canadian hemp, digitalis, hedge mustard, figwort, lily of the valley, motherwort, oleander, pheasant’s eye, pleurisy, squill, and strophanthus.
Overall, all patients who are taking other medications, including alternative-medicine substances, or who have health problems should consult with their physicians to determine the proper level and form of vitamin D for them.