The short answer to this question is: We don’t know!
Some studies have shown that individuals with vitamin D deficiency had a 31% higher risk for colorectal cancer. Other studies have shown that postmenopausal women with vitamin D levels higher than 60 ng/ml had an 80% lower risk for breast cancer compared to postmenopausal women who had vitamin D levels below 20 ng/ml. These studies were retrospective and population-based, and although they suggest a correlation between low vitamin D levels and cancer risk, we don’t know if increasing vitamin D levels through supplements will lower that risk. None of the studies had a rigorous confirmatory randomized design so we cannot use them to recommend Vitamin D for the prevention or treatment of cancer.
Although many healthcare providers and guidelines recommend that the minimum level of vitamin D should be 20 or 30 ng/ml, there is controversy on what a sufficient level of vitamin D should be. Most endocrinologists recommend that levels of 25-hydroxyvitamin D, 25 (OH)D3, the main circulating form of vitamin D, should be 20 ng/ml for bone health and 30 ng/ml for anticancer effects.
The “pro-vitamin D” healthcare providers believe that the current retrospective and observational studies suggest that the level of vitamin D influences cancer and the outcomes associated with cancer therapy. They recommend that vitamin D and 25(OH)D3 should be in the range of 30-100 ng/mL and suggest that vitamin D intake of 4,000 IU daily is safe. On the other hand, the “anti-vitamin D” healthcare providers believe that these studies fail to account for possible confounding factors –such as obesity, inactivity, or poor diet. They also believe that while there are no well designed confirmatory studies on the relationship between vitamin D levels and cancer, they do not object to taking up to 1,000 IU of vitamin D daily and maintaining vitamin D levels at 20 ng/mL.
The bottom line…
Although there are a number of diseases in which vitamin D appears to have a positive effect, we lack confirmatory data on a cause-and-effect relationship. There is an ongoing randomized trial of vitamin D and Omega-3 fatty acid supplementation that includes almost 26,000 men and women. This clinical trial is investigating whether taking daily dietary supplements of vitamin D at 2,000 IU or 1 g of omega-3 fatty acids reduces the risk for cancer, heart disease, stroke, and fractures among individuals without a prior history of these diseases. Another aim of the study is to identify a vitamin D blood level that could be protective against cancer.
In my mind, until the results of this study are published, we should check our blood 25(OH)D3 levels and take vitamin D supplement if the levels are below 30 ng/mL.