Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol). The term vitamin D also refers to metabolites and other analogues of these substances. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation.
Vitamin D deficiency can result from inadequate intake coupled with inadequate sunlight exposure, disorders that limit its absorption, conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders, or, rarely, by a number of hereditary disorders. Deficiency results in impaired bone mineralisation, and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis. However, sunlight exposure, to avoid deficiency, carries other risks, including skin cancer; this risk is avoided with dietary absorption, either through diet or as a dietary supplement.
Sources of vitamin D in food
Very few foods are naturally rich in vitamin D, so much vitamin D intake in the industrialised world is from fortified products including milk, soy milk and breakfast cereals or supplements. Natural sources of vitamin D include:
- Fish liver oils, such as cod liver oil, 1 tablespoon. (15 mL) provides 1,360 IU (one IU equals 25 ng)
- One whole egg, provides 20 IU
- Beef liver, cooked, 3.5 ounces, provides 15 IU
The figures below are from Health Canada and are similar to the RDA for most countries. The figures basically represent the Tolerable Upper Intake Level.
The Tolerable Upper Intake Level is defined as "the highest average daily intake of a nutrient that is likely to pose no risk of adverse health effects for nearly all persons in the general population.
The recommended dietary allowances (RDA) for vitamin D are:
|Age group||RDA||Tolerable Upper Intake|
|Infants 0–6 months||400 IU*||1000 IU|
|Infants 7–12 months||400 IU*||1500 IU|
|Children 1–3 years||600 IU||2500 IU|
|Children 4–8 years||600 IU||3000 IU|
|Children and Adults 9–70 years||600 IU||4000 IU|
|Adults > 70 years||800 IU||4000 IU|
|Pregnancy & Lactation||600 IU||4000 IU|
Note*: Adequate Intake rather than Recommended Dietary Allowance.
Health effects of supplements
The effects of vitamin D supplementation on health are uncertain. A United States Institute of Medicine, (IOM) report states: "Outcomes related to cancer, cardiovascular disease and hypertension, diabetes and metabolic syndrome, falls and physical performance, immune functioning and autoimmune disorders, infections, neuropsychological functioning, and preeclampsia could not be linked reliably with calcium or vitamin D intake and were often conflicting." Some researchers claim the IOM was too definitive in its recommendations and made a mathematical mistake when calculating the blood level of vitamin D associated with bone health. Members of the IOM panel maintain that they used a "standard procedure for dietary recommendations" and that the report is solidly based on the data. Research on vitamin D supplements, including large scale clinical trials, is continuing.
Low blood levels of vitamin D are associated with increased mortality, and giving supplementary vitamin D3 to elderly women in institutional care seems to decrease the risk of death. Vitamin D2, alfacalcidol, and calcitriol do not appear to be effective. However, both an excess and a deficiency in vitamin D appear to cause abnormal functioning and premature aging. The relationship between serum calcidiol level and all-cause mortality is U-shaped, Harm from vitamin D appears to occur at a lower vitamin D level in the black population than in the white population.
Vitamin D deficiency causes osteomalacia (called rickets when it occurs in children). Beyond that, low serum vitamin D levels have been associated with falls, and low bone mineral density.
In 2012, the U.S. Preventive Services Task Force issued a draft statement recommending that there is not enough evidence to indicate that healthy postmenopausal women should use supplemental doses of calcium or vitamin D to prevent fractures. Some studies have shown that supplementation with vitamin D and calcium may improve bone mineral density slightly, as well as decreasing the risk of falls and fractures in certain groups of people, specifically those older than 65 years. This appears to apply more to people in institutions than those living independently. The quality of the evidence is, however, poor. And there does not appear to be a benefit to bone health from vitamin D without sufficient calcium.
Evidence for health effects from vitamin D supplementation for cardiovascular health is poor. Moderate to high doses may reduce cardiovascular disease risk but are of questionable clinical significance.
Low levels of vitamin D are associated with multiple sclerosis. Supplementation with vitamin D may have a protective effect but there are uncertainties and unanswered questions. "The reasons why vitamin D deficiency is thought to be a risk factor for MS are as follows: (1) MS frequency increases with increasing latitude, which is strongly inversely correlated with duration and intensity of UVB from sunlight and vitamin D concentrations; (2) prevalence of MS is lower than expected at high latitudes in populations with high consumption of vitamin-D-rich fatty fish; and (3) MS risk seems to decrease with migration from high to low latitudes." A clinical trial sponsored by Charite University in Berlin, Germany was begun in 2011, with the goal of examining the efficacy, safety and tolerability of vitamin D3 in the treatment of Multiple Sclerosis.
Low vitamin D levels are associated with some cancers and with worse outcomes in other cancers, but taking supplements does not appear to help people with prostate cancer. Currently evidence is insufficient to support supplementation in those with cancer. Results for a protective or harmful effect of vitamin D supplementation in other types of cancer are inconclusive.
Pregnant women who take an adequate amount of vitamin D during gestation, may experience positive immune effects. Pregnant women throughout the world do not take the recommended dose of vitamin D, which could negatively impact the health of the mother and her foetus.
On a personal note, a good friend of mine had been suffering acute migraines for 5 years following the start of her menopause. They were debilitating and seriously affected her life-style. She saw various doctors about this condition and even tried H.R.T. (hormone replacement therapy) - which was infective and also affected her emotionally. Finally another doctor took blood tests and noted a vitamin D deficiency. She was prescribed an 8 week course of calcium and vitamin D. Following that she has had 3 migraines in 5 months, compared to two a week before the treatment.
====Other*Information distilled from Wikipedia vitamin D article