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Vitamin D Supplementation Improves Glucose and Insulin Function in Overweight Prediabetic Adults

James Meschino DC, MS, ROHP

A 2014 clinical trial published in the American Journal of Clinical Nutritionhas provided additional evidence that optimal vitamin D nutritional status may be important in preventing the progression of prediabetes to diabetes in prediabetic adults. (1)The insulinogenic index is a well documented test that establishes beta cell function (BF) within the pancreas. The insulinogenic index (IGI) is the ratio of insulin concentration at 30 minutes minus fasting insulin, to the difference of glucose at same time. (2)

In this study researchers examined the effect of vitamin D supplementation on insulin sensitivity and β cell function in overweight, vitamin D–deficient, non-Western immigrants who were at high risk of diabetes.  Researchers recruited a total of 130 non-Western immigrants with prediabetes, who lived in the Netherlands. Individuals qualified to be in the study if they had a fasting glucose concentration above 5.5 mmol/L (99 mg/dL) or a random glucose concentration between 7.8 to 11.1 mmol/L (140 – 200 mg/dL), as well as documented vitamin D deficiency (serum 25-hydroxycholecalciferol level below 50 nmol/L or 20 ng/mL).

During the 16-week, randomized, placebo-controlled trial subjects were randomly assigned vitamin D supplementation (1200 IU per day) or a placebo. All participants received 500 mg of calcium supplementation as calcium carbonate. Previous studies have shown that a low serum vitamin D level is associated with insulin resistance, metabolic syndrome and type 2 diabetes.

After four months of intervention each subject underwent an oral glucose tolerance test via the administration of 75 grams of oral glucose. The results showed that mean serum vitamin D concentrations increased significantly in the vitamin D treated group compared to the placebo group, with the vitamin D treated group showing an average increase of 38nmol/L (15ng/mL) compared to the placebo group. When patients who had diabetes at the outset of the studywere excludedfrom the final data,a significant increase in the insulinogenic index was observed in prediabetic subjects who obtained a vitamin D concentration ≥60 nmol/L (24ng/mL). Researchers concluded that improvement in the insulinogenic index was observed in prediabetic subjects who obtained a vitamin D concentration ≥60 nmol/L (24ng/ml) (2).

It is worth noting that ina previous study A. Belenchia et al, tested the efficacy and safety of 4000 IU vitamin D per day in a group of obese (average BMI above 39.8) diabetic adolescents.  Compared to the control group, the vitamin D supplemented group showed significant increase in serum 25 hydroxycholecalciferol levels, fasting insulin and improved insulin sensitivity. The researchers concluded that the correction of sub-optimal vitamin D status through dietary supplementation may be an effective addition to the standard treatment of obesity and its associated insulin resistance (3).

Based on emerging data it appears to be prudent to suggest that overweight patients with diabetes or prediabetes undergo a blood test to determine their serum level of 25-hydroxycholecalciferol. If their vitamin D blood level is below 60nmol/L (24ng/mL) then it seems prudent to recommend sufficient vitamin D supplementation to raise their vitamin D blood levels into the more desirable range. Some studies suggest that the optimal vitamin D serum concentration of 25 hydroxycholecalciferol for reducing insulin resistance is between 80 – 119 nmol/L (32 – 48 ng/mL) (4). To achieve this level very often vitamin D supplementation between 1200 and 4000 IU per day is required. (1,4,5) Thus, in addition to exercise, a lower glycemic diet, avoiding over eating, intake of supplements known to improve insulin sensitivity and thermogenesis, the addition of vitamin D supplementation is another simple, cost-effective intervention that appears to be highly beneficial for a select group of diabetic and prediabetic patients who exhibit sub-optimal serum levels of 25-hydroxycholecalciferol.


  1. Oosterwerff MO, Eekhoff EMW, Van Schoor NM, Boeke AJP, Nanayakkara P et al. Effect of moderate-dose vitamin D supplementation on insulin sensitivity in vitamin D-deficient non-Western immigrants in the Netherlands: a randomized placebo-controlled trial. Am J Clin Nutr. 100;1:152-160. 2014. Diabetes Association – Diabetes Pro – Professional Resources on-line)
  2. Belanchia, AM, Tosh AK, Hillman LS and Peterson CA. Correcting vitamin D insufficiency improves insulin sensitivity in obese adolescents: a randomized controlled trial. Am J Clin Nutr. 2013. 050013
  3. Sung CC, Liao MT, Lu KC, Wu CC. Role of Vitamin D in insulin resistance: Review article. Journal of Biomedicine and Biotechnology. 2012. (Hindawi Publishing Corporation)
  4. Smith SM, Gardner KK, Locke J, Zwart SR. Vitamin D supplementation during antarctic winter Am J Clin NutrApril 2009vol. 89 no. 4 1092-1098
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