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This cartoon shows vitamin D killing cancer. It doesn’t. It doesn’t prevent diabetes either.

Low vitamin D levels appear to be a risk factor for diabetes. Observational studies have found a correlation between a low blood level of Vitamin D (25-hydroxyvitamin D) and the risk of developing type 2 diabetes. There is biological plausibility, and supplementation has been shown to improve pancreatic beta-cell function. But does vitamin D supplementation lower the risk of diabetes? A new study just published in The New England Journal of Medicine set out to answer that question. The answer was “No.” But questions remain.

The study

It was a gold-standard trial: a large, well-designed, rigorous, multiauthor, multicenter randomized double-blind placebo-controlled clinical trial sponsored by the National Institute of Diabetes and Digestive and Kidney Diseases, independently monitored, with no input from manufacturers. The 2,423 participants were at high risk of developing diabetes: they met at least two out of three glycemic criteria for prediabetes. 44.8% were women, 33.3% were non-white, mean age was 60 years, and mean BMI was 32.1. They were randomized to get either a daily soft-gel pill of vitamin D3 (4000 IU of cholecalciferol) or a matching placebo. They were followed for an average of 2.5 years.

Optimal vitamin D levels are still controversial. Values from 20 to 50 ng/ml are generally considered adequate. In this study, 21.7% of participants had levels below 20ng/ml. Average baseline serum level of vitamin D was 28 ng/ml; this rose to 54.3 ng/ml in the vitamin D group but remained unchanged at 28.8 ng/ml in the placebo group. 293 people in the vitamin D group developed diabetes compared to 323 in the placebo group. The difference was not statistically significant.

The study’s results were consistent with the results of other studies. Giving everyone supplemental vitamin D is not likely to decrease the incidence of diabetes. But what if we targeted people with unusually low levels? This study did not ask or answer whether supplementation could prevent diabetes in people with very low serum levels of vitamin D. That would be nice to know.

Vitamin D deficiency and correlation vs. causation

Some people don’t get enough sun or a good enough diet to prevent vitamin D deficiency. The elderly, breastfed infants, and certain other high-risk groups are particularly susceptible. Deficiency may cause rickets or osteoporosis. In many cases, supplemental vitamin D is indicated. But in many other cases, supplemental vitamin D has been recommended without any real evidence that it does any good.

In addition to diabetes, low levels of vitamin D have also been linked to a long list of other health problems, including an increased risk of cardiovascular disease, cancer, pre-eclampsia, hypertension, multiple sclerosis, autism, Alzheimer’s disease, rheumatoid arthritis, inflammatory bowel disease, asthma severity, the flu, and even all-cause mortality. These links are only correlations. Correlation is not the same as causation. If A is correlated with B, that could mean that A causes B, that B causes A, that some other factor causes both A and B, or it could be a spurious correlation, like the correlation between civil engineering doctorates awarded and the per capita consumption of mozzarella cheese. We need to know whether the low level of vitamin D causes the increased health risk. If we determine that it is a cause, it’s only natural to assume that raising the vitamin D level with dietary supplements will reduce the risk. But assumptions can be wrong, and the only way to know is to test the assumptions.

Common sense can be wrong

This new study of diabetes is one of several studies that have recently shown that dietary supplementation is not the answer. Here are just a couple of examples:

This is part of a pattern. Whenever a low level of any nutrient is linked to a disease, people tend to assume that raising the level with supplements will prevent the disease. But testing often proves otherwise. For instance, a high level of homocysteine is a risk factor for heart disease, and it’s associated with low levels of vitamins B6, B12, and folate. Vitamin supplements effectively lower the level of homocysteine, but they do not reduce the risk of heart disease.

More is not always better

If vitamin D is a good thing, that doesn’t mean that more is necessarily better. In some cases, supplements are just a waste of money. What’s worse, too much vitamin D can be harmful. A Canadian man who was prescribed high doses by a naturopath developed kidney damage. And Gary Null sued the manufacturer of “Gary Null’s Ultimate Power Meal” when a bad batch of his eponymous dietary supplement nearly killed him. He thought he was getting 2000 IU of vitamin D per day (the recommended upper limit) but he was actually getting 2 million IU. Oops!

Bottom line: Test everything

We can’t rely on logic or common sense. Assumptions may seem very convincing, but they must be tested with controlled scientific studies. More research is needed to clarify what level of vitamin D is optimal and when raising low levels with supplements is beneficial. We’ll have to stay tuned.

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Posted by Harriet Hall

Harriet Hall, MD also known as The SkepDoc, is a retired family physician who writes about pseudoscience and questionable medical practices. She received her BA and MD from the University of Washington, did her internship in the Air Force (the second female ever to do so),  and was the first female graduate of the Air Force family practice residency at Eglin Air Force Base. During a long career as an Air Force physician, she held various positions from flight surgeon to DBMS (Director of Base Medical Services) and did everything from delivering babies to taking the controls of a B-52. She retired with the rank of Colonel.  In 2008 she published her memoirs, Women Aren't Supposed to Fly.