What It Is:Vitamin D is a fat-soluble vitamin. There are two major forms of vitamin D D2 (ergocalciferol) and D3 (cholecalciferol). Both vitamin D2 and D3 and appear to be absorbed with equal efficiency and, at moderate doses, are equally able to raise levels of 25-hydroxyvitamin D the hormonally active form of vitamin D and a clinical measure of vitamin D status. However, at very high doses, D3, may be more efficient at raising 25-hydroxyvitamin D levels.Vitamin D2 (ergocalciferol) is made by the conversion of a sterol found in plants and yeast. Vitamin D2 is used in some dietary supplements.Vitamin D3 is produced naturally in human skin exposed to ultraviolet B light and occurs in some animal products, such as cod liver oil, and, in smaller amounts, in other fatty fish such as herrings, mackerel, sardines, and tuna. Vitamin D3 is the most common form used in dietary supplements and is the form generally used to fortify foods such as milk (which naturally contains a small amount of vitamin D3). Vitamin D3 is made by the conversion of cholesterol compounds, such as 7-dehydroxycholesterol from lanolin found in sheep’s wool.BoneVitamin D regulates the amount of calcium and phosphorus in the body, partly by controlling their levels of absorption. Vitamin D treats and prevents rickets in children and osteomalacia (bone softening) in adults. Given to breast-fed infants, vitamin D may help increase bone density.
Taken with calcium, vitamin D can help decrease post-menopausal bone loss and prevent osteoporosis (loss of bone density), as well as improve tooth retention in the elderly. In girls ages 9 to 13, regular supplementation with calcium and vitamin D has been shown to significantly increase bone density and bone strength (measured in arms and legs) compared to placebo (Greene, Osteoporosis Int 2011).Other effects:A study found that older women (69 years and older) whose vitamin D levels were not between 20 and 29.9 ng/mL had a greater risk of being frail.18 Frail individuals were those experiencing at least three of the following criteria: weight loss, weakness, exhaustion, slowness, and low physical activity. The risk of frailty was increased by 47% among those with vitamin D levels below 15 ng/mL, 24% among those with levels below 20 ng/mL, and 32% among those with levels above 29.9 ng/mL. An average of 4.5 years after these measurements were made, those originally not frail but whose blood levels had been below 20 ng/mL were 21% more likely to have become frail or died. These findings correspond with the 2010 report from the Institute of Medicine (IOM) indicating that 20 ng/mL is a sufficient level for vitamin D and that levels above 30 ng/mL may be associated with certain risks.17
Research has found that men with low levels of vitamin D in the blood (15 ng/mL and lower) were at increased risk for heart attack compared to those with sufficient levels (30 ng/mL and higher) even after adjusting for other risk factors and physical activity. This may contribute to the higher rate of cardiovascular mortality among black Americans compared to white Americans, as blacks tend to have lower vitamin D levels.1 More recently, an analysis of two large studies showed that men who consumed 600 IU or more per day of vitamin D from foods and supplements were 16% less likely to have cardiovascular disease and stroke over a period of approximately 20 years compared to men consuming less than 100 IU per day. The same association was not seen among women; the reason for this is unclear but one possible explanation given is that women may need higher intake of vitamin D because they tend to have a higher percentage of body fat than men and vitamin D is fat soluble. In addition, vitamin D intake during the study period, which ended in 2006, may have been too low to produce meaningful differences.21 A large trial giving 2,000 IU per day as a supplement is underway and may yield additional insights (principal investigator is J.E. Manson).
Lower levels are also associated with a higher risk and severity of depression. A study in Italy, for example, showed that older women with low vitamin D levels (below 20 ng/mL) were twice as likely to develop depressive mood as those with higher levels. Older men with low levels were 60% more likely to develop depressive mood.9 Data from the same study showed that those who were severely vitamin D deficient (below 10 ng/mL) were approximately 60% more likely than those who were vitamin D sufficient (above 30 ng/mL) to experience substantial cognitive decline, although there was no such association attention level.13A study from Finland suggested that high vitamin D status provides protection against Parkinson’s disease. People with the highest vitamin D levels (above 20 ng/mL) had a 65% lower risk of developing Parkinson disease than those with the lowest vitamin D levels (below 10 ng/mL). A limitation of the study was that none of the groups had sufficient vitamin D levels (due to limited sun exposure in Finland).14 It is possible that greater risk reduction would have been observed in people with sufficient vitamin D levels.Low levels of vitamin D are also associated with a higher risk in women of developing rheumatoid arthritis. There is conflicting evidence about whether vitamin D helps reduce the overall risk of dying from cancer, although studies have consistently shown that higher vitamin D serum levels were associated with decreased risk of death from gastrointestinal cancers.Studies suggest that vitamin D may also improve balance and reduce the risk of falls in older adults, for reasons that aren’t clear. However, a recent study in women aged 70 and older who were at-risk for bone fracture showed an increase in falls and fractures among those given an extremely high, single, annual dose (500,000 IU) of vitamin D3.10 This unexpected finding may have resulted from unusual effects of the extreme dose.11 Much weaker evidence hints that giving vitamin D supplements to infants might decrease the risk of type 1 diabetes later in life, and that if women avoid vitamin D deficiency it might reduce their risk of multiple sclerosis.A study in post-menopausal women showed 400 IU of vitamin D3 and 1,000 mg of calcium daily were less likely to gain small to moderate amounts of weight compared to women taking placebo.Researchers in Japan studied the effect of vitamin D3 supplements (1,200 IU per day from December through March) on the incidence of seasonal influenza A in school children. Influenza A infection occurred in 18.6% of children in a placebo group versus 10.8% of children who received the supplement a 42% reduction in risk among those taking the supplement. The reduction was more prominent among children who had not been taking other vitamin D supplements. Influenza infection was not reduced among a subgroup of asthmatic children but those who became infected were significantly less likely to have an asthmatic attack if they received vitamin D than if they had not. Supplementation did not affect the incidence of influenza B (which is less common than influenza A and is not seasonal).14
A review of medical studies published from 1950 to 2009 that looked at, among other variables, vitamin D intake and asthma, suggested that vitamin D deficiency may be linked to airway inflammation, decreased lung function and poor asthma control. The researchers conducting the review hypothesized that vitamin D supplementation may lead to improved asthma control, although this cannot be established as many of the studies were not specifically designed to test the effects of vitamin D supplementation on patients with asthma.15
Higher serum vitamin D levels are associated with a reduced risk of allergy in children and adolescents. A review of data from a nationwide study of over 6,000 individuals showed that allergic sensitization was more common in those with serum vitamin D of less than 15 ng/mL compared to those with 30 ng/mL or greater for 11 out of 17 allergens. Results were adjusted for potentially confounding factors like time spent on indoor activities. The strongest associations were for allergy to oak (5 times the risk), peanut (2.4 times the risk), and ragweed (1.8 times the risk). There was also increased risk of allergy to dog, cockroach, mite, shrimp, ryegrass, Bermuda grass, birch and thistle. In adults, there was no consistent association between allergy and vitamin D levels.19