Why Are So Many People With Diabetes Also Low in Vitamin D?

Low vitamin D levels are surprisingly common in people with diabetes. Here’s what science currently says about the connection between vitamin D, insulin resistance, and metabolic health.

I used to think of vitamin D as the “bone vitamin”.

The thing your GP mentions in winter, the supplement hidden somewhere in the kitchen drawer, the nutrient associated with brittle bones, grey skies, and elderly patients.

Then I started noticing something interesting.

Again and again, studies looking at diabetes, especially type 2 diabetes, kept running into vitamin D.

Low levels. Poor glycaemic control. Insulin resistance. Metabolic dysfunction. The same pattern appearing across different populations and age groups.

And suddenly, vitamin D stopped looking like a side character, instead it started looking more like a quiet metabolic co-star.

The relationship between diabetes and vitamin D is still being investigated, and science has not given us all the answers yet, but what we do know is fascinating enough to deserve attention.

Vitamin D Is Much More Than a Bone Nutrient

Vitamin D is essential for calcium absorption and bone health, yes. But that’s only part of the story.

It also plays a role in:

• immune regulation
• neuromuscular function
• inflammatory responses
• cellular signalling
• pancreatic beta-cell activity

Which means it quietly influences systems that extend far beyond the skeleton. That matters because diabetes, especially type 2 diabetes, is not simply “a blood sugar problem”. It’s a complex metabolic condition involving inflammation, insulin signalling, hormonal communication, and cellular stress.

And vitamin D appears to interact with several of those pathways.

Low vitamin D levels are extremely common worldwide, particularly in older adults, people living in northern climates, individuals with darker skin tones, and those spending little time outdoors. Researchers have also repeatedly observed lower vitamin D levels in people living with diabetes.

The big question is: does vitamin D deficiency contribute to diabetes, or does diabetes somehow contribute to low vitamin D?

The answer may be: a bit of both.

Vitamin D and Type 1 Diabetes

Type 1 diabetes is an autoimmune condition in which the immune system attacks the insulin-producing beta cells of the pancreas.

Because vitamin D has important effects on immune regulation, researchers have long wondered whether deficiency could influence the development or progression of the disease.

Some early studies suggest that vitamin D3 supplementation may help improve insulin secretion and support glycaemic control in certain people with type 1 diabetes. Other research has explored whether adequate vitamin D levels could slow disease progression in children recently diagnosed with type 1 diabetes.

There are also epidemiological studies showing that lower vitamin D levels may be associated with a higher risk of developing type 1 diabetes in the first place.

But this is where scientific nuance matters. Association does not automatically mean causation.

At the moment, evidence is still considered preliminary. We cannot say with certainty that taking vitamin D supplements prevents type 1 diabetes or dramatically changes its course. However, maintaining adequate vitamin D levels is still considered an important part of overall health, particularly in people already living with autoimmune conditions.

Type 2 Diabetes, Prediabetes, and Insulin Resistance

The relationship becomes even more interesting when we look at type 2 diabetes.

People with obesity, metabolic syndrome, prediabetes, and type 2 diabetes are significantly more likely to have low vitamin D levels compared to metabolically healthy individuals.

Researchers have also found associations between vitamin D status and:

• insulin sensitivity
• beta-cell function
• glucose metabolism
• inflammatory markers

Over the years, multiple studies have investigated whether vitamin D supplementation could improve diabetic outcomes.

The results are not dramatic, but they are intriguing.

Some meta-analyses have shown modest reductions in HbA1c levels, particularly in people who were vitamin D deficient before supplementation. Others found that higher vitamin D levels were associated with a lower risk of developing type 2 diabetes over time.

The effect size is usually described as “small but potentially meaningful”.

And honestly, that phrase describes many things in preventive medicine. Sleep. Movement. Fibre intake. Stress reduction. Sunlight exposure. None of them are magic individually. But together, they shape metabolic resilience.

Why Vitamin D Deficiency Is So Common

This is the part that fascinates me most as a pharmacist.

Vitamin D deficiency is incredibly widespread, despite vitamin D technically being something our own bodies can produce.

When ultraviolet B (UVB) rays hit the skin, the body synthesises vitamin D naturally. But modern life is surprisingly bad at supporting that process.

We spend more time indoors. We live in northern latitudes. We wear sunscreen (for very good reasons). We work under artificial lighting. Winters are long. Skin pigmentation affects synthesis. Age reduces efficiency.

And suddenly, many people end up running chronically low without realising it.

Food sources help (oily fish, egg yolks, fortified foods) but diet alone is often insufficient; which is why supplementation is sometimes recommended, particularly in countries with limited sunlight exposure.

Should Everyone With Diabetes Take Vitamin D?

Not necessarily.

This is where internet wellness culture tends to oversimplify things.

Vitamin D is important, but more is not always better. Supplementation should ideally be guided by individual risk factors, symptoms, blood levels, and medical advice.

That said, if someone has diabetes or prediabetes, ensuring adequate vitamin D status is probably a sensible part of a broader health strategy.

Not because vitamin D is a miracle cure, but because metabolism is rarely influenced by one single thing. It’s shaped by hundreds of tiny biological conversations happening at the same time.

And vitamin D appears to participate in more of those conversations than we once realised. Sometimes the most interesting nutrients are the ones quietly working in the background the entire time.

Thank you for reading! 🍫🌻💜

1. *https://bmcendocrdisord.biomedcentral.com/articles/10.1186/s12902-022-01209-x*
2. *https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9671203/*
3. *https://www.diabetes.org.uk/about_us/news_landing_page/can-vitamin-d-help-prevent-type-2-diabetes*
4. *https://pubmed.ncbi.nlm.nih.gov/27932304*
5. *https://diabetesjournals.org/spectrum/article/24/2/113/32324/Vitamin-D-and-Diabetes*