Vitamin D has become a major focus of diabetes research over the past 10 to 15 years. While study after study continues to pinpoint a connection between vitamin D and both type 1 and type 2 diabetes, there is a still a great deal of uncertainty in the research.

Which came first: the vitamin D deficiency or diabetes?

In this article, we’ll discuss what vitamin D is and how it works in the body, who’s most at risk of deficiency, signs of deficiency, research connecting vitamin D levels to type 1 and type 2 diabetes, and how to ensure you’re getting enough vitamin D on a daily basis.

What is vitamin D and how does it work in the body?

Vitamin D is a fat-soluble vitamin that plays a critical role in many aspects of how the human body functions.

Optimal levels of vitamin D in a blood test are between 25 to 80 ng/mL. This is a fairly broad window, and certain health practitioners may feel strongly about helping patients achieve the higher end of this spectrum.

The National Institute of Health reports the following functions in the human body that rely on vitamin D:

  • Maintain strong bones by ensuring calcium is absorbed
  • Critical for basic muscle function
  • Nerve function — carrying messages from the brain to other areas of the body
  • Supports a stable mood and helps to combat depression
  • Enables your immune system’s ability to fight bacteria and viruses
  • Potentially improve blood sugar levels and insulin sensitivity

While there are two main types of vitamin D — D2 and D3 — the one we need and synthesize when our skin exposed to ultraviolet B rays from the sunshine is D3.

D3 can also be found in fatty fish and food products can contain a synthetic form of the vitamin that is derived primarily from animal sources.

If you’re taking D3 in the form of a supplement or in food, it is absorbed in the small intestine and then metabolized by the liver and the kidneys before it can serve any purpose in the human body.

For this reason, patients with liver or kidney problems often have low levels of vitamin D3. Patients with gastrointestinal disorders — including celiac disease, pancreatitis, and low bile levels — can also have difficulty maintaining healthy vitamin D3 levels because their small intestine isn’t adequately absorbing it.

Groups at risk of vitamin D deficiency

In addition to having diabetes, the NIH has identified the following groups as the most at-risk for becoming deficient in vitamin D for various reasons.

Breastfed infants

Human milk alone cannot offer a baby adequate amounts of vitamin D. Many mothers can counter this by taking a vitamin D supplement themselves which will then increase the amount offered in their milk. Liquid vitamin D drops can also be added in careful doses in your baby’s bottle per your pediatrician’s advice.

Senior citizens

As you age, your body actually synthesizes vitamin D less effectively, which can contribute to brittle bones and osteoporosis. Taking a vitamin D supplement as an older adult is highly recommended. And don’t forget to spend some time in the sunshine.

People with limited sun exposure

People who are unable to leave their home regularly to get out in the sun and those who cover a great deal of their skin when outside are likely deficient in vitamin D. A supplemental source of vitamin D could be especially important for this group.

People with dark skin

The darker your skin, the more protected your skin is from ultraviolet rays. This means it will take more sun exposure for a person with darker skin to synthesize vitamin D from sunlight. Fortunately, research has shown that people with darker skin still have lower rates of bone fractures and osteoporosis compared to the Caucasian population.

People with inflammatory bowel disease or similar conditions

If your body struggles to absorb vitamins properly — especially fat-soluble vitamins — it will also struggle to maintain healthy levels of vitamin D. IBS, celiac disease, Crohn’s disease, liver disease, cystic fibrosis, and ulcerative colitis are all examples of health conditions that could impair vitamin D absorption.

People who obese or have undergone gastric bypass surgery

Research has found that those struggling with obesity often have low levels of vitamin D.

The greater amount of body fat you have, the more difficult it is for your body to circulate vitamin D synthesized from the sun.

For those who have undergone weight-loss surgery, inevitably the body will struggle to absorb vitamins as efficiently as before. Taking your daily vitamins as prescribed by your healthcare team is vital to your short-term and long-term well-being, and that includes vitamin D!

Signs of vitamin D deficiency

The NIH lists the following as signs of a vitamin D deficiency:

  • Getting sick frequently
  • Persistent feelings of tiredness and fatigue
  • Pain in your bones or back
  • Depression or loss of enthusiasm
  • Wounds that heal slowly
  • Osteoporosis and loss of bone density
  • Persistent hair-loss
  • Unexplained pain or soreness in your muscles

Vitamin D and type 1 diabetes

The connection between vitamin D and type 1 diabetes has already been pinpointed quite clearly through research, but there is still a great deal of mystery.

Let’s take a closer look at some of the most significant findings in research.

Regular doses of vitamin D early in life reduce risk of type 1 diabetes

This 2008 study in Canada found that daily doses of 2000 IU/d during early childhood reduced a person’s risk of developing type 1 diabetes by as much as 80 percent over the course of the first 30 years of their life.

The same study determined that regular vitamin D “treatment” (consuming supplements along with adequate sun exposure) improved blood sugar levels and insulin sensitivity in all types of diabetes and in non-diabetics, too.

They also found that patients with vitamin D deficiency saw a substantial improvement in their HbA1c levels after being treated with regular doses of vitamin D.

Children with type 1 diabetes often have low D3 levels

A 2017 report documenting the results of a study in the United Kingdom found that a fairly significant percentage of children and adolescents with type 1 diabetes had low concentrations of vitamin D.

The same report documented the results of a study in Finland — a country with the greatest concentration of type 1 diabetes diagnoses across the globe — found that children with a history of rickets (which is characterized by severe vitamin D deficiency) were four times as likely to develop type 1 diabetes.

One theory is that vitamin D levels are directly linked with a defect in the regulation of T-cells which play a critical role in a healthy immune system.

Rates of type 1 diabetes high in those with a family history of vitamin D deficiency

A 2009 study in Qatar found that the percentage of children with type 1 diabetes who also had a family history of vitamin D deficiency was notably higher than in non-diabetic children, 35 percent compared to 23.

The study determined that other risk factors in the group of children with type 1 diabetes included being breastfed for less than 6 months, low amounts of sun exposure, low levels of physical activity, the parents’ occupations, and a family history of diabetes.

The study concluded that vitamin D levels were clearly lower in children with type 1 diabetes, and theorized that supplementing infants with vitamin D might be a safe and effective strategy for reducing their risk of developing type 1 diabetes.

Progression of disease onset may not be related to vitamin D levels

A 2011 report from the American Diabetes Association on vitamin D and type 1 diabetes research concluded that low levels of vitamin D are common in this population.

Also significant, it’s been pinpointed that children who have multiple positive islet autoantibodies (a possible sign that type 1 diabetes) also appear to have lower levels of vitamin D in their bloodstream.

Despite this, researchers still feel strongly that it has no influence on the progression and eventual onset of the disease, directly conflicting with theories from other studies.

“Vitamin D deficiency precedes the onset of type 1 diabetes. This may be a consequence of an immune response,” explained one author. “In the case of prediabetic children, we must, therefore, be mindful of the risk of vitamin D deficiency and consider recommending vitamin D supplementation at an early stage of type 1 diabetes.”

Vitamin D and type 2 diabetes

Similar to type 1 diabetes, the research on vitamin D and type 2 diabetes often conflict with each other. Regardless, there is enough evidence to demonstrate a clear relationship between the two.

Let’s take a look at the most notable research.

Ongoing vitamin D supplementation may help blood sugar levels and prevent complications

A 2017 study from Lebanon determined that vitamin D “replacement” through synthetic supplements seems to have a positive impact on a patient’s risk of developing type 2 diabetes, how well they’re able to manage their blood sugars after diagnosis, and the development of diabetes-related complications.

Low vitamin D levels and type 2 diabetes: which came first?

A 2017 study in Italy found highly consistent data supporting the theory that vitamin D supplementation may actually reduce a person’s risk of developing type 2 diabetes.

Of course, because most studies are observational versus controlled, a clear conclusion is impossible to make. The study emphasized that they are not certain whether the low levels of vitamin D contribute to the development of type 2 diabetes, or if low levels of vitamin D are a consequence after the development of type 2 diabetes.

Vitamin D supplementation has a modest impact on A1c levels

A 2017 study from Baltimore, MD set out to determine if consistent consumption of vitamin D supplements helped improve glucose metabolism and lower patient’s A1c levels.

The study saw a modest reduction in the A1c levels of the group taking D3 supplementation compared to the placebo group, but no noticeable changes in their fasting blood sugar levels.

A link between vitamin D deficiency and diabetic retinopathy

A 2017 study in China found that vitamin D levels likely play a significant role in the development of diabetic retinopathy in patients with type 2 diabetes.

“Fifteen observational studies involving 17,664 subjects were included,” explains the report.

The patients who qualified as deficient in vitamin D had a significantly higher risk or incidence of diabetic retinopathy.

Vitamin D’s impact on insulin production and insulin resistance

A 2017 study in the Netherlands determined in animals that vitamin D plays a critical role in stimulating the secretion of insulin. The study also pinpointed an association with insulin resistance and the overall incidence of type 2 diabetes.

The first theory is focused on inflammation since vitamin D is associated with increased inflammation levels. As well as “genetic polymorphisms” of vitamin-D related genes that may increase a person’s risk of developing type 2 diabetes.

Higher body fat levels were also associated with lower vitamin D levels and increased risk for type 2 diabetes and metabolic syndrome.

However, the report adds that supplementing with vitamin D did not appear to lower blood sugar levels.

“Vitamin D deficiency needs to be prevented or cured,” explained the study’s author, “but until the results of these trials are published, high-dose vitamin D supplementation cannot be recommended for prevention or amelioration of type 2 diabetes.

How to increase your own vitamin D levels

Vitamin D is a fat-soluble vitamin — which means your body does not excrete excess amounts through your urine like water-soluble vitamins. This means, then, that it is possible to consume too much D3.

Fortunately, that’s pretty hard to do, because your body can handle and benefit from fairly large doses of vitamin D.

The bare minimum goal for your D3 levels is 25 ng/mL. Striving to increase that towards 50 is a worthwhile goal for the many benefits and roles it plays in managing your health.

Let’s take a look at how much vitamin D you should take and where it can get it from.


If vitamin D is what motivates you to spend more time outdoors (preferably doing something active versus drinking a cocktail), that’s great. The sun is absolutely your number one most powerful source of vitamin D.

It estimated that 5 to 30 minutes of sun exposure between 10 a.m. and 3 p.m. at least twice per week without sunscreen and with your face, arms, legs or back exposed is sufficient to cover your vitamin D needs.

Of course, with the sun comes the risk of sunburn and ultraviolet rays that increase our risk of skin cancer. Sunscreen is critical for preventing skin cancer, but it is a barrier to getting adequate vitamin D from the sun.

Talk to your doctor about the right amount of sun exposure for you and how you should protect your skin.

Senior citizens and people with dark skin tones will also require longer sun exposure to “initiate” vitamin D synthesis.

Keep in mind, sitting near a window with sunshine pouring in will not enable the production of vitamin D. You’ll need to be outside for the real deal.

The added benefit, hopefully, is that you’ll also be increasing your activity levels — something already well-known for its impact on blood sugar levels and insulin resistance!


While some health trends are recommending several thousand IUs of vitamin D daily to boost levels, the National Institute of Health has more modest recommendations.

  • Infants: 400 IU (10 mcg)
  • Children ages 1 to 18 yrs: 600 IU (15 mcg)
  • Adults ages 19 to 70 yrs: 600 IU / (15 mcg)
  • Over 70 yrs: 800 IU (20 mcg)

Most pharmacies and wellness stores carry vitamin D3 supplementation in pill, liquid, or chewable form. It is one of the more easily accessible and affordable supplements, making it easy to add to your daily healthcare!


The NIH recommends the following foods as a source of original vitamin D3, keeping in mind that food is not an ideal source if it’s your only source of vitamin D.

  • Salmon
  • Tuna
  • Mackerel
  • Fish liver oils
  • Beef liver
  • Cheese
  • Egg yolks

Many products are also supplemented with synthetic vitamin D3. Keep in mind that this is not the same — and likely not as powerful — as getting it from sun exposure and commercial supplements.

If you suspect you may be deficient in vitamin D, ask your healthcare team for a blood draw to test your levels.