Vitamin D is both a vitamin and a hormone. It's a vitamin because your body cannot absorb calcium without it; it's a hormone because your body manufactures it in response to your skin's exposure to sunlight.
There are two major forms of vitamin D, and both have the word calciferol in their names. In Latin, calciferol means "calcium carrier." Vitamin D2 (ergocalciferol) and D3 (cholecalciferol) have similar actions in the body and can both be found in fortified food and supplements.
Strong evidence tells us that the combination of vitamin D and calcium supplements can be quite helpful for preventing and treating osteoporosis. Other potential uses of vitamin D have little supporting evidence.
Dosages of vitamin D are often expressed in terms of international units (IU) rather than milligrams. The Institute of Medicine's guidelines for Recommended Dietary Allowance are:108
These recommendations have recently been increased based on the latest evidence. For example, in a study of military personnel in submarines, use of 400 IU of vitamin D daily was inadequate to maintain bone health, while six days of sun exposure proved capable of supplying enough vitamin D for 49 sunless days.89 A study of veiled Islamic women living in Denmark found that 600 IU of vitamin D daily was insufficient to raise vitamin D levels in the blood to normal levels.1 The authors of this study recommend that sun-deprived individuals should receive 1,000 IU of vitamin D daily.
There is very little vitamin D found naturally in the foods we eat (the best sources are coldwater fish). In many countries, vitamin D is added to milk and other foods like breakfast cereals and margarine, contributing to our daily intake.
As indicated by the study of submarine personnel noted above, by far the best source of vitamin D is sunlight. However, current recommendations which stress sun avoidance and the use of sunblock may have the unintended effect of increasing the prevalence of vitamin D deficiency. Severe vitamin D deficiency was common in England in the 1800s due to coal smoke obscuring the sun. During that time, cod liver oil, which is high in vitamin D, became popular as a supplement for children to help prevent rickets. (Rickets is a disease caused by vitamin D deficiency in which developing bones soften and curve because they aren't receiving enough calcium.)
Vitamin D deficiency is known to occur today in the elderly (who often receive less sun exposure) as well as in people who live in northern latitudes and don't drink vitamin D-enriched milk.5,7 The consequences of this deficiency may be increased risk of hypertension, osteoporosis, and several forms of cancer.8
Additionally, phenytoin (Dilantin), primidone (Mysoline), and phenobarbital for seizures; corticosteroids; cimetidine (Tagamet) for ulcers; the blood-thinning drug heparin; and the antituberculosis drugs isoniazid (INH) and rifampin may interfere with vitamin D absorption or activity.9-27
For therapeutic purposes, vitamin D is taken at the nutritional doses described in Requirements/Sources (and sometimes in even higher amounts). If you wish to exceed nutritional levels of vitamin D intake, physician supervision is recommended (see Safety Issues).
Without question, if you are concerned about osteoporosis, you should take calcium and vitamin D. The combination appears to help prevent bone loss.28,29 This is true even if you are taking other treatments for osteoporosis. After all, you can't build bone without calcium, and you can't properly absorb and utilize calcium without adequate intake of vitamin D.
Vitamin D may also help prevent the falls that lead to osteoporotic fractures. In a review of 26 randomized trials, researchers found that vitamin D along with calcium lowered the risk of falls in high-risk elderly people.111
Some evidence suggests that getting adequate vitamin D may help prevent cancer of the breast, colon, pancreas, prostate, and skin, but the research on this question has yielded mixed results.30-49,90,105 One study suggests that combined use of calcium plus vitamin D, but not either supplement separately, can help reduce risk of colon cancer.84 However, an extremely large study involving over 36,000 post-menopausal women found that supplementing the diet with 1,000 mg of calcium plus 400 IU of vitamin D daily did not lower the risk of breast cancer over a period of 7 years.106 Based on the results of this placebo-controlled study, there does not appear to be a connection between vitamin D and breast cancer risk.
Weak evidence hints that adequate vitamin D intake might reduce the risk of hypertension50-53,98 and diabetes.74-76 A very large randomized, placebo-controlled trial of over 36,000 postmenopausal women found daily supplementation with 1,000 mg of calcium plus 400 IU of vitamin D did not reduce or prevent hypertension during 7 years of follow-up. These results are possibly limited by non-study calcium use.107
A review of 4 randomized trials with 242 patients found vitamin D supplements decreased fasting glucose levels in patients with prediabetes and type 2 diabetes. Vitamin D was also associated with improved sensitivity to insulin in 6 trials with 316 patients. However, there were no significant differences in long-term glucose control in 4 trials with 184 patients or progression to diabetes in 2 trials.112
One preliminary study suggests that supplementation with vitamin D and calcium may be helpful for women with polycystic ovary syndrome.54
A meta-analysis (formal statistical review) of published studies found some evidence that use of vitamin D at recommended levels may reduce overall mortality.102 This article suggested, but did not attempt to establish, just how vitamin D might accomplish this. Similarly, an extensive 2011 review of 50 randomized trials (including 94,148 people) found that vitamin D 3 (but not other forms of the vitamin) reduced the risk of mortality among elderly women at risk for vitamin D deficiency.109
Vitamin D is sometimes mentioned as a treatment for psoriasis. However, this recommendation is based on Danish studies using calcipotriol, a variation of vitamin D 3 that is used externally (applied to the skin).55 Calcipotriol does not affect your body's absorption of calcium, so it is a very different substance from the vitamin D you can purchase at a store.
It has been suggested that since vitamin D levels in the body drop in the wintertime, vitamin D supplements might be helpful for seasonal affective disorder ("winter blues"). A small double-blind, placebo-controlled trial conducted during winter on 44 people found that vitamin D supplements produced improvements in various measures of mood.79 However, a double-blind, placebo-controlled study of 2,217 women over 70 failed to find benefit.91 It has been hypothesized that light therapy (used successfully for SAD) works by raising vitamin D levels, but there is some evidence that this is not the case.92
A randomized, placebo-controlled trial involving 164 men (aged 18-28) found that taking vitamin D 3 (400 units daily for 6 months) may prevent respiratory infections.110 But, there was no evidence that the vitamin decreased sick days or severity of cold symptoms.
Vitamin D supplements also do not appear to help enhance growth in healthy children.80
Individuals with severe osteoporosis often have low levels of vitamin D.56,58 Supplementing with vitamin D alone is probably no more than minimally helpful, at best,59,60,81,99,100 but the combination of calcium and vitamin D is probably more effective. (See the Calcium article for more information.)
Supplementation with vitamin D plus calcium may aid healing after a fracture has occurred.88
Interestingly, vitamin D may offer another benefit for osteoporosis in seniors: most (though not all) studies have found that vitamin D supplementation improves balance in seniors (especially female seniors) and reduces risk of falling.77,78,82,83,85-87,93-97,99,103,104 However, this was not the case in a randomized trial of 409 women who lived at home with a history of one or more falls in the past year. Vitamin D without exercise was not associated with a reduced risk of falls when compared to placebo combined with exercise. In this case, strength and balance exercises were the only interventions that improved physical functioning.115
In two reviews of 13 randomized trials with 4,394 patients, vitamin D 3 supplements (oral or intramuscular injection) were not effective in reducing depressive symptoms when compared to placebo. However, some benefit was found in an analysis of 2 trials with patients who had more severe, clinically significant depression.113,114
When taken at recommended dosages, vitamin D appears to be safe. However, when used at considerable excess, vitamin D can build up in the body and cause toxic symptoms. At an intake level of about 40,000 IU daily (about 100 times the recommended daily intake) vitamin D can cause dangerous elevations in blood calcium levels.101 Doses five times higher than this were consumed by a few individuals due to a manufacturing error; the resulting toxicity was severe and may have caused death in one individual.101
However, short of these vastly excessive dosages, it is not clear at what level vitamin D becomes toxic. The safe upper limits for vitamin D daily intake are as follows:108
Note, however, that some authorities believe these upper limits have been set a bit too low.66,68 Their arguments closely parallel those discussed in the Requirements/Sources section regarding nutritional needs.
There is no disagreement that people with sarcoidosis or hyperparathyroidism should never take vitamin D without first consulting a physician.
Taking vitamin D and calcium supplements might interfere with some of the effects of drugs in the calcium-channel blocker family.69 It is very important that you consult your physician before trying this combination.
The combination of calcium, vitamin D, and thiazide diuretics could potentially lead to excessive calcium levels in the body.70,72 If you are taking thiazide diuretics, you should consult with a physician about the right doses of vitamin D and calcium for you.
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