By Perry Holman, Executive Director, Vitamin D Society

Based on recent media reports you may be questioning or doubting if vitamin D really works. Does it really prevent cancer?

You may have seen these headlines:

High Doses of Vitamin D Fail to Cut Cancer Risk

Vitamin D pills may not protect against cancer after all

These were based on a new study published in JAMA by Lappe et al. It was a randomized controlled trial of 2,303 women which studied the effects of 2,000 IU/d of vitamin D3 and 1500 mg/d of calcium supplement vs placebo on cancer outcomes.

But it did reduce cancer. A new diagnosis of cancer was confirmed in 109 participants, 45 (3.89%) in the Vitamin D and calcium group and 64 (5.58%) in the placebo group. Tell me which group you would want to be in? I think that this is a great result and proves that vitamin D could help prevent cancer.

Let’s have a closer look at the details.

The study seems to have had a good result with a reported Hazard Ratio of 0.70 (95% CI, 0.47 to 1.02). This means that there was a 30% reduced risk in cancer outcomes for the group that took vitamin D and calcium. So why did the news reports say that high doses of vitamin D fail to cut cancer risk? Well it comes down to those confidence intervals (CI). Based on the numbers there is a 95% chance that the Hazard Ratio will be between 0.47 and 1.02. So there is a chance, in this case a very small one, that the result could be above 1.00 and therefore not a reduced risk of cancer. This is called significance and in this case the results were not statistically significant. So the researcher’s had to conclude that vitamin D and calcium compared to placebo did not result in a significantly lower risk of cancer. 

Were the cards stacked against vitamin D working in this study. First, the starting vitamin D blood levels in the group were 32.8 ng/ml or 82 nmol/L. So these participants all started off with fairly high levels of vitamin D. For comparison, the mean average for Canadians is 61 nmol/L and 35% of Canadians are under 50 nmol/L. We know that the greatest benefit in disease prevention is for people that are deficient in vitamin D who receive a dose that drastically increases their vitamin D level. But in this case they took people with pretty good vitamin D levels and gave them a bit more and studied the difference on cancer.

Secondly, participants in both groups were allowed to take up to 800 IU/d in vitamin D supplementation in addition to their study pills. So the placebo group was taking vitamin D and were not deficient. In fact they took 869 IU a day in vitamin D supplement vs the treatment group who took 740 IU/d. This is part of the reason why they all had values well over the recommendations by Health Canada and the Institute of Medicine (50 nmol/L) of 79 nmol/L. This is always going to be a problem with vitamin D studies. It is deemed ethnically irresponsible if the participants are denied any vitamin D. In addition, participants sun exposure can also affect the study results.

Importantly, the study did a post hoc analysis to go back and see if they could isolate whether those participants with lower vitamin D levels had a higher cancer risk than those with higher vitamin D levels. Compared with 25(OH)D levels below 75 nmol/L (30 ng/ml) those with levels greater than 75 nmol/L had a 35% reduction in cancer risk HR 0.65 (95% CI, 0.44 to 0.97).

So yes, this proves that higher vitamin D levels can help prevent and reduce your risk of cancer.