This article was first published in The Montreal Gazette.
There was a time when everybody was taking vitamin D to prevent everything from heart disease to cancer. But as one editorialist recently put it, “then came the randomized trials.” In fact, 2019 has not been a good year for vitamin D. A lot of the trials that came out this year have been negative.
Firstly, a meta-analysis in JAMA Cardiology of 83,000 individuals from 21 randomized trials found that vitamin D did not help reduce the risk of cardiovascular disease, heart attack stroke or death.
In terms of cancer risk, in January the large VITAL study was published in the New England Journal of Medicine and showed that vitamin D did not prevent breast, prostate or colon cancer, nor did it prevent cancer deaths overall. In April the AMATERASU randomized trial studied 417 patients with GI cancer and found that giving them vitamin D did not improve five-year survival.
Come summer, the publication of the D2d study showed that vitamin D did not reduce the risk of developing diabetes in patients who were pre-diabetic. Also, a research letter provided data on the six-year follow-up of a Danish study looking at whether giving vitamin D to pregnant women reduced the probability that their children would develop asthma. It did not. Finally, the recently published VITAL-DKD found that vitamin D did not help preserve kidney function in people with diabetes.
It would be easy to become a nihilist when reviewing all the studies published so far this year. But we should remember that vitamin D has an important role to play in our bodies. We need vitamin D in order to absorb calcium from our intestines, which in turn is necessary for proper bone strength. Vitamin D deficiency can lead to problems like rickets. In Dickensian London, rickets was a big problem. But today, with widespread vitamin D supplementation of infant formula, milk and cereals, rickets is very rare because most people have adequate levels of vitamin D in their blood.
So while vitamin D has a role to play in promoting healthy bones, the many extra-skeletal benefits of vitamin D are very doubtful. You might ask, why the research was so suggestive at first, and so negative of late. Well, many of the original studies were simply association studies that found that people with low vitamin D levels in their blood had higher rates of cancer, heart disease and so on. It was tempting and perhaps intuitive to think that vitamin D deficiency was the cause of these diseases. But it seems obvious now that confounding was at work, meaning that another third variable common to both conditions was probably at play.
For example, people with chronic illness like cancer or arthritis may have less healthy diets and therefore have lower vitamin D blood levels. They may also not go outside as much because of their illness. In scenario, which is usually referred to as reverse causation, it is not that vitamin D deficiency made people sick. It is that people who are sick tend to develop vitamin D deficiency.
The vitamin D story is a perfect example of why randomized trials are necessary to prove that something actually works before we spend millions, if not billions, of dollars on a specific treatment.
It might be a bit early for a year-end review and I doubt most people will remember 2019 as the year the vitamin D craze ended. But for those who are taking vitamin D without having been prescribed it for a valid medical reason (like osteoporosis or for PTH suppression or because of kidney failure, to name a few), maybe that it is one pill you can do without in 2020.
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