Vitamin D — often called the “sunshine vitamin” because it develops from the skin’s exposure to sunlight — has long been recognized as crucial to bone health; it helps the body absorb calcium and thus maintain a healthy bone mineral density. Many of its other roles in the body, however, are not as well understood. Some evidence, though, points to a role in aiding insulin secretion in the pancreas — which means that it could potentially be important for people with Type 2 diabetes.
Many studies of vitamin D have been observational, meaning that researchers simply recorded data without measuring the effects of an intervention. This usually entails measuring the blood level of vitamin D in participants (or, in the case of supplements, recording who already takes them) and then seeing if this level corresponds to certain outcomes. For studies related to diabetes, this often means seeing who develops diabetes or, if the study involves people with diabetes, who has a higher or lower HbA1c level after a period of time. With all observational studies, however, it is possible that factors other than the one being tested — in this case, vitamin D blood level (or intake) — are responsible for the outcome being measured. For example, getting more exercise could result in being outdoors more often and thereby getting more sunlight and vitamin D — and if someone has a lower diabetes risk as a result, it could be because of the exercise and not the vitamin D.
Two recent studies attempted to get around this problem, but in different ways and measuring different outcomes. One study, published in the March issue of the journal Diabetes Care, explored the connection between blood levels of vitamin D and the incidence of Type 2 diabetes in older women. This was an observational study, following 5,140 women with an average age of 66 — who started out without diabetes — for an average of 7.3 years, after measuring their vitamin D level. According to a Reuters article on the study, it initially appeared that vitamin D had some preventive effect against developing diabetes; however, when the researchers adjusted for differences among the participants in body weight, exercise level, and dietary habits, vitamin D was no longer found to make a difference.
The other study was a randomized clinical trial, thus eliminating the uncertainty about causation inherent in observational studies. Published in the February issue of the American Journal of Clinical Nutrition, this study explored the effect of a vitamin-D-fortified yogurt drink on 90 people with Type 2 diabetes. Participants were assigned to drink either plain yogurt, yogurt with added vitamin D, or yogurt with added vitamin D plus calcium, twice a day for 12 weeks. Both of the groups that received vitamin D had a lower HbA1c level (0.4% lower), a smaller waist circumference (3.6 cm less for vitamin D; 2.9 cm less for vitamin D plus calcium), and a lower body-mass index than the group that received the plain yogurt drink. As a different Reuters article on this study notes, however, the results are complicated by the fact that HbA1c in the plain-yogurt group actually rose unexpectedly, as well as by the small size of the study.
Do you take a vitamin D supplement — or do you make an effort to get enough vitamin D from the sun, or from your diet? Do you believe vitamin D has an effect on your diabetes control? Given the conflicting evidence on optimal vitamin D levels — both in the blood, and how much to consume — whom do you trust for guidance concerning how much vitamin D to get? Leave a comment below!
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