Analysis of data from nine studies revealed that, for every 10 nanograms per milliliter increase in levels of vitamin D (25-hydroxyvitamin D) the associated risk of colorectal cancer decreased by 15 percent.
On the other hand, no association was observed between vitamin D levels and the risk of breast or prostate cancer, say results published in the International Journal of Cancer.
Shining light on the sunshine vitamin
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. Both D3 and D2 precursors are transformed in the liver and kidneys into 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
D and the big C
The link between vitamin D intake and protection from cancer dates from the 1940s when Frank Apperly demonstrated a link between latitude and deaths from cancer, and suggested that sunlight gave "a relative cancer immunity".
Since then there have been numerous studies suggesting associations between vitamin D and lower risks of certain cancers.
There is growing evidence that 1,25(OH)2D has anticancer effects, but the discovery that non-kidney cells can also hydroxylate 25(OH)D had profound implications, implying that higher 25(OH)D levels could protect against cancer in the local sites.
The new meta-analysis, led by Philippe Autier from the International Prevention Research Institute (IPRI) in Lyon, France, adds to the subject, while also showing the relationship between vitamin D and cancer is ambiguous, depending on the type of cancer.
Indeed, a recent meta-analysis published in the European Journal of Cancer (doi: 10.1016/j.ejca.2010.03.037) concluded that the association between vitamin D and breast cancer is still ambiguous.
Dr Autier and his co-workers analyzed data from 35 epidemiological studies of 25(OH)D levels and colorectal, breast and prostate cancer.
The analysis showed that for every 10 nanograms per milliliter increase in 25(OH)D levels the associated risk of colorectal cancer decreased by 15 percent, while the risk of breast cancer was associated with an 11 percent decrease. However, when the researchers restricted their analysis to prospective studies only, the breast cancer risk was decreased by only 3 percent, whereas data from case-control studies indicated a risk reduction of 17 percent.
“A non-significant decreased risk of breast cancer risk was associated with higher serum 25-hydroxyvitamin D, but results from prospective studies only did not support an association between vitamin D status and breast cancer,” said the researchers.
No association between vitamin D levels and prostate cancer were observed at all.
“If additional observational studies of vitamin D and cancer are proposed, they should adopt different designs, such as assessment of serum 25-hydroxyvitamin D colorectal at different points in time, or longer follow-up of subjects,” wrote the researcher.
“To assess whether vitamin D status is a risk factor or a risk marker for colorectal cancer, it is likely that new randomized trials will need to be organized to test whether increasing the 25-hydroxyvitamin D level changes the risk of colorectal cancer, and to determine how much of an increase is required to change the risk of cancer sufficiently to be useful as a public health measure,” they concluded.
Source: International Journal of Cancer
15 March 2011, Volume 128, Issue 6, pages 1414–1424
“Meta-analysis of observational studies of serum 25-hydroxyvitamin D levels and colorectal, breast and prostate cancer and colorectal adenoma”
Authors: S. Gandini, M. Boniol, J. Haukka, G. Byrnes, B. Cox, M.J. Sneyd, P. Mullie, P. Autier