A million study subjects is my personal best; the most I have ever seen in a meta-analysis. From the Sixth People’s Hospital in Shangai, China, Dr. Yanlei Ma and five of her colleagues looked at all the existing studies on vitamin D and colon cancer, finding 18 studies with one million subjects, 7,000 of which with colon cancer.
They found no U shaped curve, no evidence of a limit on the effectiveness of higher vitamin D levels on the prevention of colon and rectal cancer. In fact, for every 10 ng/ml increase in blood vitamin D, risk was cut by 25%.
Even with one million people, they did not have enough people with levels in the 40s and 50s to know for sure, but if one simply extrapolates their findings, a level of 40 ng/ml may mean a 75% reduction in colon cancer. I doubt it goes that high but it might.
Dr. Ma has done her due diligence, praising Professor Cedric Garland as being the first scientist to discover that vitamin D may prevent colon cancer. She also praised Professor Lappe for conducting the first randomized controlled trial showing that vitamin D appears to prevent many different deadly cancers.
What we don’t know is, what will vitamin D do for people who already have cancer? Studies have found that higher vitamin D levels at the time of diagnosis of breast, colon, lung, prostate, and blood cancers, improve prognosis. However, I’d like to point out that this means some people with the highest levels still got cancer. That said, those with the highest levels tended to live longer.
We do know that some cancers, like some breast cancers, block vitamin D very early in their evolution, it is as if the cancer knows it can’t win unless it defeats vitamin D first.