Should We Prescribe Vitamin D to Prevent Type 2 Diabetes?

Ronan Roussel, MD, PhD; Chantal Mathieu, MD, PhD


July 10, 2015

Editor's Note:
The following is an edited, translated transcript of a conversation taped in June 2015 between Ronan Roussel, MD, PhD, professor of endocrinology at Bichat Hospital, Paris, France, and Chantal Mathieu, MD, PhD, chair of the Division of Clinical Endocrinology at the Catholic University of Leuven, Belgium.

Prof Roussel: Hello and welcome to Medscape. Today we're going to look at the relationship between vitamin D deficiency and diabetes risk. For this, I am joined by Prof Chantal Mathieu from the University of Leuven, in Belgium. Hello, Chantal.

Prof Mathieu: Hello.

Prof Roussel: This relationship between vitamin D deficiency and diabetes risk is no doubt quite common. How close is this link?

Prof Mathieu: There are epidemiologic studies showing, in fact, that vitamin D deficiency or even vitamin D insufficiency—that is, fairly low levels but short of an actual deficiency—causes an increased risk of developing diabetes, especially type 2.

Prof Roussel: In the case of vitamin D deficiency, is the risk increased by 10%? Is it doubled?

Prof Mathieu: There are studies showing that, in certain populations, the risk is doubled. However, it is very difficult to interpret these epidemiologic studies. This is because vitamin D deficiency is always accompanied by a specific profile. Most of these vitamin D–deficient people are also obese, older, and very often of non-European origin. Therefore, several risk factors accompany vitamin D deficiency in type 2 diabetes.

Prof Roussel: So you're suggesting that there are confounding factors and that, in the end, this relationship is not one of causality.

Prof Mathieu: There are definitely some confounding factors, but when they are adjusted for (obesity, age, etc), there's still an increased risk, and it varies according to the population. Some studies report [diabetes risk] figures that are twice as high in patients with [vitamin D] deficiency.

Prof Roussel: Do we have an idea of the biological plausibility of the mechanisms involved?

Prof Mathieu: There are vitamin D receptors in beta cells, insulin target cells, the liver, muscle, and adipose tissues. It's thought that vitamin D might play a physiologic role in these tissues [and therefore in the pathophysiology of type 2 diabetes]. Vitamin D-deficient patients have below-normal beta-cell function—that is, beta-cell dysfunction together with insulin resistance. Therefore, people with vitamin D deficiency have both diminished beta-cell function and increased insulin resistance.

Prof Roussel: Are there any measures that could be taken in animals or humans that might impact not necessarily type 2 diabetes—I think that if vitamin D prevented type 2 diabetes, we'd know—but intermediate factors, such as insulin secretion or insulin sensitivity? Is insulin sensitivity increased when vitamin D is taken?

Prof Mathieu: Studies were carried out several years ago in populations with severe vitamin D deficiency, such as dialysis patients. When these high-risk individuals were given vitamin D [supplements], their insulin sensitivity improved and their risk for type 2 diabetes decreased. However, these are fairly old studies, and they weren't randomized and are therefore very difficult to interpret. There are more recent studies, such as the SUNNY trial,[1] in which subjects who already had type 2 diabetes were given 50,000 IU of vitamin D by month. There was no difference in terms of the glycosylated hemoglobin concentration, etc. Therefore, it's very difficult to say if taking more vitamin D has an impact in people who already have type 2 diabetes.

Prof Roussel: So, in type 2 diabetes, vitamin D is not a modality for glycemic control or, in any event, for modifying this metabolic control. But are there any trials under way that are looking at diabetes prevention?


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