In recent decades, the importance of maintaining adequate vitamin D levels has become widely known throughout the population. Sometimes called the “sunshine vitamin” because it’s produced in your body when your skin is exposed to sunlight, vitamin D is also found in certain foods — but not very many foods at levels that approach what most people need. That means at least during winter months in the United States, many people need to take supplements to maintain recommended levels of vitamin D in their blood.
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We’re constantly learning more about the role vitamin D plays in health and disease. Probably the best known role of vitamin D in the body is how it affects bone health by maintaining the balance of calcium and phosphate in the body. But vitamin D also has potential effects that are more closely tied to diabetes, such as slowing the progression of type 2 diabetes, reducing the risk of developing type 1 diabetes, and reducing the risk of neuropathic pain in diabetes.
Now, a study shows that vitamin D supplements may play a helpful role in reducing blood glucose levels in people after they have knee replacement surgery.
Glucose levels may matter in osteoarthritis
The new study, published in the journal Diabetes, Obesity and Metabolism, was designed to compare the effects of taking either 800 or 2,000 IU (20 or 50 mcg) or vitamin D daily on fasting blood glucose levels in older adults with severe osteoarthritis after single-knee replacement surgery. A total of 251 participants, with an average age of about 70, took part in the study over a number of years in Zurich, Switzerland. Among these participants, 39% had impaired glucose tolerance at the start of the study, and the average blood vitamin D level (measured as 25‐hydroxyvitamin D) was 27.48 ng/ml — not a terribly low level, but slightly below the range of 30 to 50 ng/ml that some experts recommend. Each participants was randomly assigned to take either 800 or 2,000 IU of vitamin D each day going forward.
At the start of the study, the average fasting blood glucose (FBG) level of participants was 98.8 mg/dl. While the original aim of the clinical trial was to find out how vitamin D supplementation might affect outcomes related to knee replacement, the researchers also saw an opportunity to explore how it could affect FBG in older people without diabetes — since elevated FBG is a risk factor for developing type 2 diabetes. People with diabetes who had knee replacement surgery were excluded from this study, since its aim was to look at how vitamin D supplementation could affect future diabetes risk.
After their knee replacement, participants had follow-up visits at the clinic every six months for a total of two years. Between these visits, they had a telephone check-in every two months. Based on their answers to questions during these phone calls, the compliance rate of taking vitamin D — defined as taking at least 80% of doses as recommended — was almost the same in the 800 IU and 2,000 IU groups, at 93% and 92%.
After two years, there were no significant differences in FBG between the 800 IU and 2,000 IU groups. There was also no significant difference between the two groups in a measurement of insulin resistance called HOMA-IR. But what’s notable about these findings isn’t the difference between the two groups, but the fact that FBG significantly improved over two years in both groups — something that doesn’t typically happen in older adults, particularly those with osteoarthritis, who are more likely to be overweight or obese than people without osteoarthritis. FBG fell from an average of 99.7 to 95.8 mg/dl in the 800 IU group, and from 99.0 to 97.0 in the 2,000 group.
The researchers concluded that taking a vitamin D supplement at a dose of either 800 IU or 2,000 IU may have a beneficial effect on FBG, and thus also reduce the risk of developing type 2 diabetes in older adults with osteoarthritis.
Should you take vitamin D?
This study shows that taking a vitamin D supplement may have a beneficial effect on glucose metabolism in older adults, but it doesn’t offer any helpful guidance about who should take vitamin D, or what dose to take. To figure this out for yourself, you’ll need to have your blood level of vitamin D measured, and discuss what your target level should be with your doctor.
There isn’t universal agreement about what the ideal level of vitamin D in your blood should be. But as this study shows, even in older adults whose blood level of vitamin D isn’t especially low by most traditional standards, there may be benefits from taking extra vitamin D.
Want to learn more about vitamin D and diabetes? Read “Vitamin D and Type 1 Diabetes: What the Research Says” and “Low Vitamin D Tied to Painful Diabetic Neuropathy.”