Vitamin D is the best-selling single-ingredient nutritional supplement in the US, second only to multivitamins in popularity, and with good reason.
The majority of Americans have a suboptimal blood level of vitamin D, placing them at an increased risk of a wide range health problems—including diabetes.
How Diabetes and Low Vitamin D Are Related
Multiple studies have found that vitamin D status, as measured by blood concentrations of 25-hydroxyvitamin D, is inversely correlated with risk of type 1 and type 2 diabetes. The lower the vitamin D blood level, the greater the susceptibility to diabetes.
The reason is that low vitamin D status is associated with several impairments that affect blood sugar metabolism:
- Increased vulnerability to, and severity of, autoimmune diseases
- Decreased insulin production in the pancreas
- Insulin resistance in cells throughout the body
- Obesity, particularly in the abdominal area
Each of these factors increases the likelihood that someone will develop either type 1 or type 2 diabetes.
Why Vitamin D Deficiency is Linked to Type 1 Diabetes
Type 1 diabetes is an autoimmune disease, caused by an immune system attack on the body’s own tissues—in this case on insulin-secreting beta cells in the pancreas. Insulin is the hormone that carries glucose out of your bloodstream and into your cells where it’s burned for energy.
As the disease progresses, beta cells lose their ability to produce insulin, blood sugar remains elevated, and patients develop a lifelong dependence on insulin therapy.
Autoimmune diseases are believed to result from two factors—a genetic predisposition to developing it, along with environmental factors that trigger the disease. One of the genetic “triggers” linked with type 1 diabetes is an impairment in the gene that’s critical for vitamin D metabolism.
So, could getting too little vitamin D—coupled with trouble metabolizing vitamin D—be a trigger for developing type 1 diabetes? Studies show that blood levels do tend to be low in people with a recent diagnosis. Furthermore, type 1 diabetes is more prevalent in northern latitudes with limited year-round sunshine and lower average vitamin D concentrations.
There is also an inverse association between vitamin D levels and other autoimmune diseases, providing further support for an underlying link.
Can Vitamin D Supplements Reduce the Risk?
Although vitamin D is best known for its role in calcium absorption and bone health, it has significant anti-inflammatory and immune-modulating properties—properties that underscore supplemental vitamin D’s protective effects against type 1 diabetes.
In a study conducted in Finland, which has one of the world’s highest rates of type 1 diabetes, children born in 1966 were followed for 30 years. Those who were supplemented with 2,000 IU of vitamin D during their first year of life had an 88 percent reduced risk of developing this disease.
A more recent study, published in Diabetologia in 2020, found that in genetically susceptible children, early vitamin D supplementation protected against the development of type 1 diabetes.
Limited research suggests that risk may also be increased in genetically prone children whose mothers have low vitamin D levels while pregnant—and that risk may be reduced by supplementing with vitamin D during pregnancy. This research is not definitive, and studies are ongoing. Nevertheless, taking vitamin D3 supplements during pregnancy as well as infancy and childhood is prudent.
Type 2 Diabetes, Prediabetes & Metabolic Syndrome
More 34 million people in the US have diabetes, and in 90–95 percent of cases it’s type 2. Another 88 million have prediabetes, meaning their blood sugars are higher than normal. And as many as one in three Americans has metabolic syndrome, a cluster of conditions that includes high blood pressure, abdominal obesity, and abnormal blood lipids along with high-normal blood sugar.
Vitamin D insufficiency has been linked with all these conditions. Plus, studies suggest that among individuals who have diabetes, those with higher vitamin D levels are less likely to develop diabetic complications.
Adequate vitamin D stores also slow disease progression. Swedish researchers followed 2,378 men and women with prediabetes for eight to 10 years and found that those with the highest vitamin D concentrations were 60–62 percent less likely to progress to type 2 diabetes compared to those with the lowest levels—and the higher the vitamin D level, the lower the risk.
To Prevent Type 2 Diabetes, Vitamin D3 Is Critical
For all of my patients with diabetes, prediabetes, or metabolic syndrome, I recommended optimizing their vitamin D levels with vitamin D3 supplements for three reasons:
- To improve insulin sensitivity. The common underlying abnormality in all three of these conditions is insulin resistance. The cells do not respond properly to insulin’s signals to let glucose in, and blood sugar remains elevated—which wreaks all kinds of havoc. Vitamin D improves insulin sensitivity.
- To enhance insulin production. In response to elevated blood sugar, the beta cells churn out more and more insulin, which overworks the pancreas and, over time, decreases its ability to produce insulin. Vitamin D supports the beta cells and pancreas and enhances insulin secretion.
- To help fight obesity. Most people with type 2 diabetes, prediabetes, and metabolic syndrome are obese or overweight, usually in the abdominal area. Vitamin D levels are consistently lower in people who are obese compared to those of normal weight—all the more reason to take vitamin D3.
How Much Vitamin D Should You Take?
As a general recommendation, I recommend 5,000 IU of vitamin D3 per day. This is the dose Canadian researchers used in a 2019 double-blind placebo-controlled study, which concluded: “In individuals at high risk of diabetes or with newly diagnosed type 2 diabetes, vitamin D supplementation for six months significantly increased peripheral insulin sensitivity and beta-cell function, suggesting that it may slow metabolic deterioration in this population.”
The best way to determine the specific dose that’s right for you is to have your 25-hydroxyvitamin D blood level tested. If it is in the optimal range of 50–80 ng/mL, stick with what you’re doing, whether it’s spending time in the sun or taking supplements. If not, start with a daily dose of 5,000 IU of vitamin D3, retest in a few months, and adjust your dose as needed.