Data from a double-blind, randomized, placebo-controlled trial indicated that long term supplementation with 420 IU of vitamin D per day was associated with 3.0 mg/dL decreases in mean glucose concentrations, while no changes were observed in the placebo group.
In addition, insulin resistance estimated using the homeostasis model (HOMA-IR) significantly decreased from 1.17 to 0.84 in the vitamin D group after one year of supplementation, while no changes were observed in the placebo group, according to data published in Nutrition Research.
Results from other studies have also reported beneficial effects of vitamin D supplements on these endpoints, but the authors of the new study argue that they did not account for the effects of daily physical activity. The new study, performed by scientists from Waseda University (Japan) and Shanghai University of Sport (China).
“[T]he results of our study showed that 1-year vitamin D supplementation increased serum 25(OH)D concentration, resulting in a beneficial effect on fasting glucose levels and insulin resistance, independent of daily physical activity,” they wrote.
The sunshine vitamin
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. Both D3 and D2 precursors are transformed in the liver and kidneys into 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D).
While our bodies do manufacture vitamin D on exposure to sunshine, the levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
The researchers randomly assigned 96 healthy Japanese adults to receive either vitamin D3 supplements (420 IU per day) or placebo for one year.
Results showed that 25(OH)D and 1,25(OH)2D levels significantly increased in the vitamin D group by 29.5 nmol/L and 7.0 pg/mL, respectively. Fasting glucose levels and HOMA-IR values also significantly decreased after on eyear of supplementation, compared to placebo.
“Some preventive effects of vitamin D on type 2 diabetes may also be related to the increase in serum 1,25(OH)2D concentration. In in vitro studies, 1,25(OH)2D, the active hormone form of vitamin D, increased the expression of insulin receptors and improved insulin resistance,” wrote the researchers.
“In the present study, the vitamin D supplementation significantly increased serum 1,25(OH)2D concentration compared with the placebo group […] Moreover, we found that changes in the serum 1,25(OH)2D concentration were positively related with changes in 25(OH)D concentration (P < .05), and inversely related with the changes in glucose concentration (P < .05).
“Thus, our data suggest that the effects of vitamin D on type 2 diabetes may, at least in part, be related to the increase in serum 1,25(OH)2D concentration.”
Source: Nutrition Research
Published online ahead of print, doi: 10.1016/j.nutres.2016.07.006
“Vitamin D supplementation reduces insulin resistance in Japanese adults: a secondary analysis of a double-blind, randomized, placebo-controlled trial”
Authors: X. Sun, et al.