People with type 2 diabetes who took a vitamin D supplement tended to have lower A1C levels (a measure of long-term blood glucose control) than those who didn’t take vitamin D, according to a recent study published in the journal Frontiers in Endocrinology.
As the study authors wrote, a deficiency in vitamin D could play a key role in both the development of type 2 diabetes and how the disease progresses. Previous studies suggest that vitamin D in the body may be linked to insulin secretion in the pancreas, insulin resistance in cells throughout the body, and the presence of inflammation that may increase the risk for diabetes complications. For the latest study, the researchers were interested in how taking vitamin D might affect certain diabetes-related outcomes, particularly blood glucose control but also specific measures of insulin resistance. They recruited 130 participants with type 2 diabetes, all of whom took metformin alone as a diabetes treatment. The average age of participants was 62.3 years, and the average duration of type 2 diabetes was about five years at the beginning of the study.
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Participants were randomly assigned, in equal numbers, either to take a vitamin D supplement or not to take one, in addition to continuing to take metformin. For participants who took a vitamin D supplement, the researchers followed guidelines from the Endocrine Society regarding the correct dose, which is based on blood levels of vitamin D. This means that participants who were found to have low vitamin D levels ended up taking larger doses of the vitamin as part of the study. Participants continued to take, or not take, vitamin D for the study’s duration of six months, with interim measurements taken after three months.
Participants in the vitamin D treatment group started out with somewhat lower vitamin D levels, which was purely coincidental since the groups were randomly selected — the average starting vitamin D level was 48.70 nmol/L in the vitamin D treatment group, and 58.02 nmol/L in the metformin-only group. But after three months, vitamin D levels rose to an average of 104.7 nmol/L in the treatment group but only 68.44 nmol/L in the metformin-only group. After six months, the average vitamin D level was 92.24 nmol/L in the treatment group and 51.77 nmol/L in the metformin-only group.
Larger A1C reductions seen in those taking vitamin D
At the beginning of the study, the average A1C level in the vitamin D treatment group was 6.56%, while in the metformin-only group it was 6.74% — indicating good overall blood glucose control in both groups. After three months, average A1C in the treatment group dropped to 6.32%, while it barely changed to 6.66% in the metformin-only group. After six months, the average A1c level was 6.48% in the treatment group and 6.87% in the metformin-only group. This set of measurements, the researchers wrote, “indicates that oral daily doses of vitamin D improve [A1C] levels over the 3-month and 6-month period.”
The researchers noted, though, that taking vitamin D didn’t change measures of underlying insulin resistance, which means it’s not exactly clear why taking vitamin D was linked to better overall blood glucose control. “Further investigation should consider defining the doses of vitamin D in patients with [type 2 diabetes] which may attenuate the oxidative stress risk” — a measure of damage to cells throughout the body — along with reducing “the risk of metabolic syndrome and the risk of related cardiovascular events.”
Want to learn more about A1C? Read “How to Lower A1C Levels Naturally,” “How to Lower Your A1C Levels: More Steps You Can Take” and “HbA1c: What It Is and Why It Matters.”