Dear Dr. Cannell,
I am worried about the studies that show increased risk of cancer with both high and low levels of vitamin D. What should I do?
Join the club, especially since a group of good scientists (FNB vitamin D Board) has recently said that vitamin D levels of 50 ng/ml (levels I recommend) may be dangerous. They based their warning on about a dozen studies that show a U-shaped curve, that is, increased risk with both lower and higher vitamin D blood levels. The studies that show this risk are almost all the same type of studies. Scientists take frozen blood samples drawn decades ago and test them for vitamin D in a group of subjects who doctors have followed closely, comparing them to a similar group who did not develop the disease.
However, in a very recent study, a meta-analysis of all such studies done on colon cancer, scientists showed what most studies suggest: there’s a decreased risk with higher vitamin D levels, as the authors put it, “in a linear dose-response manner.” That’s important because it suggests levels of 40 ng/ml are better than levels of 30. However, not enough people have levels high enough to answer the next logical question, “Are levels of 50 better than levels of 40?”
Touvier M et al. Meta-analyses of vitamin d intake, 25-hydroxyvitamin d status, vitamin d receptor polymorphisms, and colorectal cancer risk. Cancer Epidemiol Biomarkers Prev. 2011 May; 20(5): 1003-16.
The studies that show a U-shaped risk (increased risk with low and high vitamin D levels) share several similarities. Many, but not all, were conducted in Scandinavian countries, where cod liver oil consumption is high and vitamin A toxicity will run hand in hand with high vitamin D levels. Virtually all were conducted at fairly high latitudes, where a steep fall-off of vitamin D levels occurs in the autumn, a decline that may – according to Professor Reinhold Vieth – cause repeated yearly episodes of intracellular deficiencies of vitamin D. Finally, virtually all the studies share the similarity that scientists measured the vitamin D levels in blood taken during the 1980s and ’90s that had been frozen for at least a decade.
Most, but not all, of the studies in question are cancer studies, especially prostate and pancreatic cancer. If higher vitamin D levels are riskier, then perhaps those who develop cancer will die sooner if their vitamin D levels are high? The exact opposite is true. Studies show that the higher your vitamin D levels at the time of a cancer diagnosis, the longer you live. That is, higher vitamin D levels have a treatment effect in cancer.
Such studies exist for breast, colon, melanoma, lung, and prostate cancers. The higher the vitamin D level at the time of a cancer diagnosis, the longer you live. Similar findings were recently announced for a leukemia that is currently “incurable,” chronic lymphocytic leukemia (CLL). To quote the authors, “the association between 25(OH)D and survival increased consistently as 25(OH)D increased.” The authors added, “these findings suggest that vitamin D insufficiency may be the first potentially modifiable host factor associated with prognosis in newly diagnosed CLL.” In other words, vitamin D may be the first effective treatment for CLL. Way to go vitamin D!
Shanafelt TD et al. Vitamin D insufficiency and prognosis in chronic lymphocytic leukemia. Blood. 2011 Feb 3; 117(5): 1492-8.
Please note one other thing. These studies clearly show that people with high vitamin D levels still can get cancer. That is, vitamin D only reduces the risk of getting and dying from cancer; it does not prevent it. This is important because we all know, or will know, someone who took vitamin D and died from cancer anyway. Humans being who they are, friends and relatives of such cancer victims will become dispirited; silently hoping vitamin D is a sure cure. Vitamin D is not that. As I say when I speak, everyone who takes vitamin D will die.