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Home » Does higher-dose vitamin D reduce risk of type 2 diabetes?

Does higher-dose vitamin D reduce risk of type 2 diabetes?

Last updated on February 16th, 2023 at 12:14 pm

New peer-reviewed research shows that higher doses of vitamin D may reduce the risk of developing type 2 diabetes in patients with pre-diabetes. Although it is not a cure for type 2 diabetes nor can it prevent it in all cases, this is good evidence that it might reduce the risk.

Vitamin D seems to be the vitamin of the decade with claims that it does everything from treating COVID-19 (maybe) to cardiovascular disease (probably not) to preventing breast cancer (no). Now we have some research that vitamin D may reduce the risk of type 2 diabetes.

Let’s take a look at this new research and see if the science supports the claim.

blood sugar meter with syringe
Photo by Nataliya Vaitkevich on

What is type 2 diabetes?

Just for background, type 2 diabetes mellitus (or type 2 diabetes, T2DM) is a metabolic disorder that is characterized by high blood glucose with insulin resistance and relative insulin deficiency. In general, someone with T2DM produces low (or maybe even adequate) insulin levels, but various cells and organs become resistant to insulin, so cells don’t remove or store blood glucose.

Although the cause of T2DM is not completely understood, it results from a complex interaction between diet, obesity, genetics, age, and gender. Some of the causes of T2DM are under a person’s control, like diet and obesity, but many of the causal factors are not.

Because they are often confused, it’s important to note that T2DM has a completely different etiology and pathophysiology compared to type 1 diabetes mellitus (T1DM, once called juvenile diabetes). Type 1 diabetes results from the inability of the beta cells of the pancreas to produce insulin, as a result of an autoimmune disease. Diet and lifestyle are not related to T1DM.

There are numerous medical treatments and lifestyle changes that can moderate, or even reverse, the course of T2DM. On the other hand, there are no cures (at this time) for T1DM, and it can be a death sentence for the patient without regular daily insulin injections. Over 90-95% of diabetes in the USA is Type 2.

The consequences of both types of diabetes are almost the same. Complications of poorly managed diabetes mellitus may include cardiovascular diseasediabetic neuropathy, and diabetic retinopathy, among many other chronic conditions. 

vitamin d supplements
Chemical structure of cholecalciferol. By SbroolsOwn work, CC BY-SA 3.0, Link

What is vitamin D?

Vitamin D is a group of fat-soluble secosteroids (it’s a scientific name for steroids with a “broken” ring). The most important chemicals in this group are vitamin D3 (known as cholecalciferol) and D2 (known as ergocalciferol).

Very few foods contain either of the important types of vitamin D. However, some foods can be good sources of the vitamin:

  • Fatty fish, like tuna, mackerel, and salmon
  • Foods fortified with vitamin D, like some dairy products, orange juice, soy milk, and cereals
  • Beef liver
  • Cheese
  • Egg yolks

Many people with broad diets that include a lot of fish, eggs, and other foods can get sufficient vitamin D without supplementation. As I’ve repeated often, short of chronic malnutrition, we get plenty of vitamin D.

Moreover, vitamin D is produced by a process called dermal synthesis. That is, sunlight, specifically UV-B radiation, causes the synthesis of vitamin D in the skin. Technically, vitamin D isn’t a vitamin, because we can manufacture it, it is a hormone. For this article, we’ll just call it a vitamin, even though scientifically it is not — I hope that makes some sense.

Although we can manufacture sufficient vitamin D by sunbathing every day, the body has a feedback loop that shuts down production to prevent toxicity. Yes, excess vitamin D is quite dangerous leading to many conditions such as over-absorption of calcium to hypertension to fatigue. But it also can lead to some dangerous chronic conditions that we’ll discuss later.

Although humans can manufacture vitamin D by sitting in bright sun, there’s one major problem – the risk of skin cancer. As I’ve written before, there are very few ways to prevent cancer, but staying out of the sun is one of them.

Finally, the vitamin D we consume or produce in sunlight is not biologically active. It is generally activated by enzymatic conversion (in a process called hydroxylation) in the kidneys and liver so that the body can use it.

Vitamin D and type 2 diabetes paper

In a systematic review (considered the peak of the hierarchy of research) published on 7 February 2023 in the Annals of Internal Medicine, Anastassios G. Pittas, MD, MS, of Tufts Medical Center in Boston, and colleagues analyzed three clinical trials that examined the effect of vitamin D supplementation on the risk of type 2 diabetes.

In the three trials included in the systematic review, participants took 20,000 IU (500 mcg) per week of cholecalciferol, 4,000 IU (100 mcg) daily of cholecalciferol in the D2d study, or 0.75 mcg daily of eldecalcitol in the DPVD study

The researchers found the following results:

  • Vitamin D reduced the risk for type 2 diabetes by 15% in people with pre-diabetes, hazard ratio (HR) = 0.85. This translated to a 3.3% absolute risk reduction of type 2 diabetes over 3 years.
  • Those who took vitamin D supplementation had a 30% increased likelihood of returning to normal glucose levels, risk ratio (RR) = 1.30.

However, and this is troublesome, an accompanying editorial points out that each of the three studies included in the systematic review does not suggest that vitamin D has any link to reducing the risk of type 2 diabetes:

Even though each of the studies seems to show a risk reduction, they are not statistically significant. And even if the reduction were significant, they don’t appear to be clinically significant, meaning that in each of those studies, there isn’t a meaningful change in the risk of type 2 diabetes.

Furthermore, McKenna and Flynn warned that very high doses of vitamin D supplements may cause harm beyond the 10 to 20 mcg (400 to 800 IU) daily that is considered safe. They concluded that high doses of vitamin D do not appear to have a significant effect on the risk of type 2 diabetes, and too many individuals are taking dangerously high doses of vitamin D in the unsupported belief that if a little helps more is better.

selective focus photography of assorted color stars
Photo by Jess Bailey Designs on

My conclusion

I usually put more weight into systematic reviews than other studies, because they are supposed to be the platinum level of research. But in this case, I’m confused by the lack of evidence supporting the claims that vitamin D supplementation has an effect on the risk of type 2 diabetes.

The underlying studies just don’t support the conclusions in the systematic review, and that is problematic.

I’m giving this study only one out of five stars — it’s really weak, and it does not support the hypothesis that vitamin d supplements reduce the risk of type 2 diabetes. And I am concerned that people who read this study will think “hey, I’m going to take even more vitamin d so that I don’t get type 2 diabetes.” That could be dangerous.

For now, I am unconvinced that vitamin D lessens the risk of moving from pre-diabetes to type 2 diabetes.


Michael Simpson

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