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Could vitamin D supplements prevent type 2 diabetes?

Vitamin D deficiency is a growing problem, particularly in countries in the northern hemisphere. Similarly, so is type 2 diabetes. This has led some researchers to think there might be a link between the two.

The proposed mechanism behind this theory is that vitamin D is required for insulin secretion to be at a healthy level, so a lack of it could not only put people at risk of type 2 diabetes but also make insulin resistance worse in people who do have type 2 diabetes.

However, researchers have found it difficult to prove that supplementation with vitamin D could lower the risk of type 2 diabetes, even in those who have been diagnosed as pre-diabetic.

A meta-analysis published in Diabetes Care in 2020 found a small reduction in risk of around 11% from vitamin D supplementation in people with prediabetes after some high profile studies failed to show a significant effect.

The latest study, carried out in Japan and published in the BMJ, has also found no significant effect.

Mixed results

In this multi-center, double-blind randomized placebo control trial involving 1,256 participants, researchers sought to measure the effects of daily vitamin D on diabetes risk.

They looked at whether 630 participants classed as pre-diabetic were less likely to develop type 2 diabetes when given 0.75μg of eldecalcitol—an active vitamin D analogue—daily, when compared to a group of 626 participants given a placebo.

They followed up on the participants for an average of 2.9 years, comparing their fasting plasma glucose measurements at the start of the trial and every three months, as well as measurements from glucose tolerance tests carried out at the start and then annually. The researchers also looked at the participants’ bone density measurements annually

Dr. Tetsuya Kawahara, lead author of the study from the University of Occupational and Environmental Health, Kitakyushu, Japan told Medical News Today that their results were mixed.

The group given eldecalcitol showed a decrease of 13% in their risk of developing type 2 diabetes over the study period but the researchers found it was not significant.

“Despite the fact that eldecalcitol has been shown to prevent type 2 diabetes development in our pilot study, treatment with it did not show a preventive effect on the incidence of type 2 diabetes, nor a beneficial effect on the rate of regression to normoglycaemia in this current study,” -Dr. Kawahara

However, they found that a certain group seemed to get better results

“After adjusting for 11 potentially influential factors, including age, sex, blood pressure, body mass index, and family history of diabetes, the results suggested that eldecalcitol might prevent type 2 diabetes in prediabetic patients with insufficient insulin secretion,” - Dr. Kawahara

He said he thinks the reason behind these mixed results could be that the study was underpowered.

A small, but potentially significant effect

Dr. James Brown from Aston Research Centre for Healthy Ageing at Aston University in Birmingham, United Kingdom, who studies type 2 diabetes and metabolism, highlighted the same issue.

“The study protocol which was published in 2016, and included a sample size calculation for the primary outcome of diabetes diagnosis which was 750 participants, based upon an estimated 36% lower risk, although in the final data the risk was reported as 13%,” -Dr. Brown

Dr. Tatiana Christides, from the Queen Mary University of London who wrote an accompanying editorial for the study in the BMJ said that even a small difference in risk for developing type 2 diabetes could be important on a population level as so many people have prediabetes.

Depending on which literature you read, there is a risk of 5-10% of progression amongst people with prediabetes,” she told MNT in an interview.

She went on to explain with an example:

Let’s say there are 100 people with prediabetes, and every year 10 of them are going to go on to develop full-blown diabetes. That means you’d be stopping 1 of those 10 people from going on to get full-blown diabetes. That’s, at a public health level, significant,” she said.

However, Dr. Christides also said she had concerns that advising people to take vitamin D to prevent developing type 2 diabetes. Without robust evidence, this might deter people from trying interventions to reduce risk, such as weight loss, which there is much stronger evidence for, she added.


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