Vitamin D refers to several forms of fat-soluble vitamins found naturally in plants, fish, and dairy products. The two forms utilized in humans are ergocalciferol (D2) and the more potent cholecalciferol (D3). Sunlight can promote the synthesis of D3 in the skin. Vitamin D maintains serum calcium and phosphorus levels by regulating their absorption and excretion. It is also important for bone formation; deficiency can cause rickets and other bone disorders. Other biologic functions include its role as a potent antiproliferative agent and as a pro-differentiation hormone (1) (2).
Vitamin D was shown to have a beneficial effect on bone mineral density and the prevention of bone fractures in the elderly (3) (4) and in postmenopausal women (38). However, conflicting data from the Women's Health Initiative studies found no such effects (5).
According to the latest statement from the U.S. Preventive Services Task Force (USPSTF), current evidence is insufficient to assess the balance of the benefits and harms of combined vitamin D and calcium supplementation for the primary prevention of fractures in premenopausal women or in men. Evidence is also lacking to determine the balance of the benefits and harms of daily supplementation with greater than 400 IU of vitamin D3 and greater than 1000 mg of calcium for the primary prevention of fractures in noninstitutionalized postmenopausal women. The USPSTF recommends against daily supplementation with 400 IU or less of vitamin D3 and 1000 mg or less of calcium for the primary prevention of fractures in noninstitutionalized postmenopausal women (55).
Recent data show that calcium and vitamin D supplementation reduce may reduce weight gain (6), but does not affect cardiovascular events (7), or reduce risk of invasive breast cancer (37) in postmenopausal women. However, low levels of vitamin D were associated with upregulated circulating renin-angiotensin system which suggests that vitamin D may play a role in cardiovascular health (42).
In type 2 diabetic patients, a single, large dose of vitamin D2 improved endothelial function (8); however, vitamin D supplementation did not reduce infections or antibiotic use in an elderly population (9). Another large study found no association between low vitamin D levels and cognitive function (39). Vitamin D is effective in treating psoriasis (10), but data are inconsistent regarding its role in seasonal affective disorder (SAD) (11) (12). Low levels of Vitamin D have been associated with greater risk of mortality (18). Vitamin D supplementation reduced the risk of relapse in patients with Crohn's disease (40).
Contrary to observational data, vitamin D did not decrease the incidence or severity of upper respiratory tract infections (UTRIs) in healthy adults (54).
Vitamin D has been examined for its benefits as a preventative agent and as a treatment for many types of cancer. Vitamin D from sunlight exposure and from dietary intake may have protective effect against breast cancer (13) (14). This correlates with observations in many breast cancer survivors who were vitamin D deficient (15).
In prostate cancer patients, higher serum levels of vitamin D were correlated with greater risk of aggressive cancer (16). Vitamin D improved pain and muscle strength in patients with advanced hormone-refractory prostate cancer (17), and slowed the rate of rise of prostate specific antigen (18). There are studies on using vitamin D and its analogs singly or with chemotherapy in prostate cancer patients (19) (20).
In postmenopausal women, vitamin D and calcium supplementation may reduce the incidence of colorectal cancer, but only in women who did not use estrogen therapy concurrently (28) (29).
Preliminary results suggest vitamin D did not increase toxicity of docetaxel (21) (22) and may offer other treatment benefits (23).
Although reduced risk of cancer has been associated with increased vitamin D intake (24) (25) (26), data from a large, prospective study showed that with the exception of colorectal cancer (52), vitamin D may not protect against other cancers (27) (41). Similarly, data from the Cohort Consortium Vitamin D Pooling Project of Rarer Cancers (VDPP) epidemiological studies do not support the association between low levels of circulating vitamin D and reduced risk of non-Hodgkin lymphoma (45), ovarian (46), kidney (47), endometrial (48), or esophageal and gastric (49) cancers; however, a significant increase in the risk of pancreatic cancer was observed with high levels of circulating vitamin D (50).
According to The National Health and Nutrition Examination Survey, chronic steroid use is associated with vitamin D deficiency (53).
And the Institute of Medicine, in a report on vitamin D and calcium supplementation, recommends a higher Dietary Allowance of vitamin D at 600 IU/day with the Upper level Intake at 4,000 IU/day for bone health (51).
Patients should consult with their physicians if more vitamin D is needed for health maintenance.