Could Low Vitamin D Mean Higher Diabetes Risk, Thyroiditis Too?

With Cedric Garland, DrPH, and Mishaela Rubin, MD

Vitamin D isn't just necessary to keep our bones strong, for muscle movement, and to keep our immune system working.1 It appears equally important for healthy thyroid function as well as cardiovascular health and now there is clearer evidence that it may help to reduce the chances that diabetes will occur.2-5

According to findings published in PLoS One,5 it appears that the lower the level of vitamin D in your blood, the higher your risk of developing diabetes. The researchers followed 903 people over 12 years, tracking their vitamin D status and identifying which individuals were diagnosed with prediabetes or diabetes.

"There was a strong relationship between vitamin D being low and a higher incidence of diabetes," says study co-author, Cedric Garland, DrPH, an adjunct professor of family medicine and public health at the University of California San Diego School of Medicine.

Yogurt, milk, salmon, and egg yolks are the main food sources for vitamin D Foods high in vitamin D include salmon, tuna, eggs, and fortified dairy products.

How Was This Study Conducted?

The participants who were followed by Garland and his team were part of the ongoing Rancho Bernardo Study, a National Institutes of Health population-based trial of older adults, which included patients who were middle-income Caucasians living in a southern California suburb; the average age was 74 years on average at the start of the vitamin D portion of the study.5

These participants were confirmed as healthy, having no signs or symptoms of pre-diabetes or diabetes, initially.During the study, the researchers measured blood levels of vitamin D from over the first two years and gave the men and women a fasting blood glucose test every two years. If the fasting blood glucose test was high (100 milligrams/deciliter or above), an oral glucose tolerance test was done to confirm the likelihood of diabetes.

By the end of the follow-up portion of the study, 47 people were diagnosed as having developed diabetes and 337 had been identified with prediabetes. Based on these results, Garland and his team confirmed that the higher the vitamin D in the blood, the less likely they were to get diabetes.5

A Closer Look at the Relationship Between Diabetes and Vitamin D

In trying to understand the study findings, it is important to appreciate that experts disagree on what a normal blood level for vitamin D should be,6  but for the purposes of this study, the researchers identified a minimum healthy level of serum 25-hydroxyvitamin D to be 30 nanograms per milliliter (ng/ml). That level is 10 ng/ml above the level recommended by National Academy of Medicine, a health advisory group to the US government.7

"People who were low in vitamin D—below 30 ng/ml—were at five times the risk of developing diabetes as people who were at a healthier vitamin D level—50 ng/ml—over a period of 12 years," Dr. Garland tells EndocrineWeb.

Previous studies have also found a link between low vitamin D in the blood and the risk of diabetes, but as Dr. Garland and his team acknowledge, the findings have been mixed and conflicting.  In this study, the link held even when the researchers took into account such factors as obesity, lack of exercise and other known risk factors for getting diabetes.5

However, Dr. Garland stresses that his team found an association between this essential mineral and the tendency toward diabetes when the nutrient is low, but their work is not sufficient to offer definitive proof. Further investigation is needed, the researchers say, to duplicate the findings of vitamin D and diabetes risk in order to confirm the relationship.

Among the plausible explanations for why vitamin D helps keep diabetes at bay, Dr. Garland says, is that pancreatic beta cells have receptors for vitamin D, and products (metabolites) that develop when vitamin D breaks down in the body can stimulate the pancreas to produce insulin.

A Second Opinion Concurs on the Importance of Vitamin D

While the study does not demonstrate a cause-and-effect between vitamin D levels and development of diabetes, ''identifying a modifiable risk factor [for diabetes] such as vitamin D deficiency is potentially important," says Mishaela Rubin, MD, associate professor of medicine at Columbia University Medical Center. She was not involved in the study but reviewed the findings for EndocrineWeb.

The long follow-up period is a notable strength of the study, she says, as well as the researchers' taking into account other factors such as obesity and still finding the link holds up. One downside, which the researchers also mention, is that the study participants followed by Garland and his team were generally healthy and Caucasian, with unusually high average vitamin D levels, Dr. Rubin says. So the findings might not apply to the population as a whole, but certainly are useful for others who are similar to the study population.

Expert Advice on Vitamin D Levels: What’s Enough?

Based on our results,5 Dr. Gardner says, those with blood level above 50 ng/dl have the best chance at avoiding diabetes. However, an expert panel of the Endocrine Society recommends 30 ng/dl as a healthy goal,6 Dr. Garland points out, so more research is needed on the most effective level of the vitamin needed to lessen the risk of diabetes before that recommendation might be reconsidered.

Dr. Garland's advice: get a blood test to see where you stand on vitamin D, then ask your doctor for advice on how best to proceed given your overall health and diet.

Dr. Rubin's advice: Until further evidence is in, maintaining a level between 20 and 40 ng/ml ''is usually advisable for most people, including those with diabetes or prediabetes."

When you do get a blood test for vitamin D levels, your lifestyle (what you eat and other health factors) may play a role in your results.  For example, researchers found that women taking hormonal contraceptives were more likely to have their vitamin D levels overestimated.7 So your doctor may want to take that into account.  And another endocrine disease: autoimmune thyroiditis appears to be affected by low vitamin D levels, too, specifically individuals who are overweight or have obesity are at significant risk for developing this condition.8

Getting Your Daily Quota of Vitamin D

While the body makes vitamin D when the skin is exposed directly to sunlight, it is difficult for most people to get enough of the vitamin this way. Since it's important to use sunscreen to protect your skin against the risk of skin cancer, it isn't likely that you’ll make enough vitamin D to meet your body's needs.  

So how do you get enough vitamin D and how much do you need? The Food and Nutrition Board suggests 600 international units for children ages one and up, while adults need 800 IUs.9  However, that amount has been challenged by some experts as too low and not nearly sufficient to boost blood levels to where they should be.10 Yet, too much can be toxic. This is especially true if you are deficient in vitamin D, which is very, very common. The ''upper tolerable" amount of D is 4,000 IUs daily for children ages nine years through adulthood.

It’s always best to try and meet your nutrient needs through diet first. Salmon, tuna and mackerel are among the best sources of vitamin D and provide important heart-healthy fatty acids too. Dairy products, including cheese, fortified milk, yogurt, and egg yolks are good sources of vitamin D, too. Because it can be difficult to get enough vitamin D from foods alone, many must take a vitamin D supplement. However, if your doctor tells you your blood level of vitamin D, is low, ask about the amount you'll need to bring your blood level into the healthy range and what is the best way for you to get the right amount.

Neither Dr. Garland or Dr. Rubin have any relevant financial disclosures.

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