Niha Zubair, Arivale Clinical Research Scientist, PhD
It’s no secret that not getting enough vitamin D is associated with poor muscle and bone health. However, the evidence has been mixed on whether having a low level of vitamin D is associated with increased risk of cancer, in particular, colorectal cancer.
In 2011, the Institute of Medicine (now the National Academy of Medicine) established that there was insufficient evidence on vitamin D and cancer, so it created its dietary intake recommendations for vitamin D based only on bone health.
However, additional research on vitamin D and colorectal cancer has been conducted since then, including a large international study published this month in the Journal of the National Cancer Institute and covered by NPR that found a link between what the Institute of Medicine considers “deficient” levels of vitamin D and an increased risk of colorectal cancer.
In the collaborative study, researchers pooled together data from 17 other cohort studies to perform an analysis on over 5,000 colorectal cancer participants and over 7,000 control (cancer-free) participants.
Researchers found that men and women with “deficient” levels of 25-hydroxyvitamin D – or 25OHD, the major circulating form of vitamin D – had a 31 percent increased risk of colorectal cancer compared to people in the lower range of sufficiency for 25OHD.
Interestingly, researchers found that people with “beyond sufficient” levels of 25OHD had a 19% to 27% lower risk of colorectal cancer.
(As per the Institute of Medicine, a “deficient” level of 25OHD is below 12 nanograms per milliliter; “insufficient” is between 12 and 20 ng/mL; “sufficient” is 20 to 30 ng/mL; and “beyond sufficient” is 30 ng/mL and above.)
Researchers conducted a second analysis stratified by sex. The found that each 10 ng/mL increase in circulating 25OHD levels was associated with an approximately 19 percent decrease in colorectal cancer risk in women and 7 percent in men. Only the association in women was statistically significant.
Risk factors for colorectal cancer that might impact 25OHD levels, such as body mass index, physical activity, and diet, minimally affected these results.
Overall, these results indicate that the optimal 25OHD concentration for reducing colorectal cancer risk is between 30 and 40 ng/mL – which would be considered “beyond sufficient” by the Institute of Medicine recommendations currently in place.
Despite the findings, this study isn’t a pass to overdo it on the vitamin D supplements! Excess vitamin D in the body can be toxic and is associated with an increased risk of kidney stones, falls, and all-cause mortality.
It’s necessary to understand what one’s current vitamin D status is via a blood draw. Based on this and other vitamin D-influencing factors – such as race, location of residence, sunlight exposure, and genetics – one can make an informed decision – with the help of a medical professional, dietician, etc. – on whether to supplement with vitamin D and, if so, how much to supplement with.
Importantly, there were some limitations to this study.
First, as mentioned by NPR, studies like this do not prove cause and effect but rather demonstrate that there may be a link between circulating levels of vitamin D and colorectal cancer.
Second, the results are based on a single blood draw at a median of 5.5 years before colorectal cancer diagnosis, which may be an imperfect measure of long-term 25OHD status. This limitation was not addressed in the NPR coverage of the study. In addition, the study didn’t – or rather couldn’t – evaluate whether adding vitamin D through food, sunlight, or supplements made a difference in cancer risk.
Lastly, despite the large size of the study, researchers had limited participants with 25OHD concentrations greater than 40 ng/mL; therefore, risk estimates for this group were imprecise, potentially explaining why no significant associations with colorectal cancer were found at that level of 25OHD concentrations. In addition, there were limited racial/ethnic subgroups represented in the study, so these results cannot be generalized to non-Caucasians. This limitation was also not mentioned by NPR.
Note: The Institute of Medicine and this study reported vitamin D in nmol/L, but we’ve converted it to ng/mL, which is how Arivale reports vitamin D levels.
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[Arivale Hot Topics address health stories currently in the news. The Arivale Clinical Team’s commentary on these news articles is not a review of the scientific evidence, nor an endorsement of a specific study, and is not meant as official medical opinion.]