The good news is that vitamin D appears to protect against colorectal cancer. The bad news is that it doesn’t seem to do the same for other cancers, with the possible exception of breast tumours. These are the results of a recent study that assessed the connection between vitamin D levels and cancer incidence in 17,000 people (J Natl Cancer Inst 2007;99:1594-1602). Only for colon cancer did the researchers find a significantly decreased incidence when serum vitamin D levels were in the upper range. Though the frequency of breast tumours also seemed to be lower when vitamin D was high, there weren’t enough cases for this finding to reach significance. For all other cancers, the results were inconclusive.
This comes as a blow to all those who’d hoped that vitamin D might prove to be the next “miracle worker” in cancer prevention. Indeed, earlier in vitro studies had shown that it induced apoptosis in certain lines of cancer cells, including breast and prostate cancer. A number of meta-analyses have also indicated that a higher level of vitamin D goes hand in hand with overall cancer protection, particularly against breast tumours. Even the International Agency for Research into Cancer and the World Cancer Research Fund — who advise against the use of supplements in general — have made an exception for vitamin D, where the outcomes have been generally favourable.
Future studies will have to show what vitamin D can and can’t do with respect to overall cancer prevention. For now, we have to content ourselves with the knowledge that the vitamin is good for the bones — especially when combined with calcium — and that it may also help prevent colon cancer.
Wimpy winter sun
There’s an ongoing debate on how much vitamin D a person needs, where they should get it from, and how much is too much. At present, blood levels > 75 nmol/L are considered adequate. To maintain this concentration, a daily supply of the vitamin is necessary. While it’s produced in the skin upon exposure to UV-B radiation, people living at higher latitudes — especially if they have dark skin, which needs up to 10 times as much sunlight as light skin does — are unlikely to make enough vitamin D during the winter months. As a result, dietary intake and supplementation have become increasingly important.
Certain animal products are sources of vitamin D, e.g. dairy, organ meats, fish and fish products. Cod liver oil stands out from the pack, as one tablespoon contains 4 times as much of the vitamin as a small (3.5 oz) salmon steak. Indeed, it was noted in the early 19th century that the oil served as a remedy against rickets. Today, milk is fortified with vitamin D, which has substantially decreased the incidence of rickets. The Dietary Reference Intake Guidelines state that a daily vitamin D intake of 200 IU is adequate for people under 50 years of age, but several authorities suggest that 1,000 IU/day or more are optimal. For comparison: 20 minutes in the summer sun produce up to 10,000 IU. So is it time to bring back the UV lamp since we’re running out of cod? Alternatively, you can always suggest supplements at no cost to the environment.
David Jenkins, MD, PhD, FRCPC is Director of the Risk Factor Modification Centre at St. Michael’s Hospital and a professor of Medicine and Nutritional Sciences at the University of Toronto.