Insulin resistance, whereby insufficient insulin is released to produce a normal glucose response from fat, muscle and liver cells, was significantly lower in women following high-dose vitamin D supplementation, according to results of a randomised, controlled, double-blind trial published in the British Journal of Nutrition.
The optimal effects were observed when blood vitamin D levels were in the range 80 to 119 nanomoles per litre, said the researchers, “providing further evidence for an increase in the recommended adequate levels”.
D for diabetes
This is not the first time that vitamin D has been linked to diabetes. A recent meta-analysis of data from observational studies and clinical trials in adults showed a "relatively consistent association" between low intakes of calcium, vitamin D, or dairy intake and type-2 diabetes (Journal of Clinical Endocrinology & Metabolism, Vol. 92, pp. 2017-2029).
The new study involved 81 South Asian women with insulin resistance living in New Zealand. The subjects, aged between 23 and 68, were randomly assigned to receive either 100 micrograms (4,000 IU) of vitamin D3 or placebo daily for six months.
At the end of the test period, women in the vitamin D group experienced “significant improvements” in both insulin sensitivity and resistance, said the researchers, which was also accompanied a decrease in fasting insulin levels, compared to placebo.
The greatest improvement in insulin resistance was observed when blood levels of vitamin D, measured as 25- hydroxyvitamin D (25(OH)D) – the non-active 'storage' form of the vitamin – were at least 80 nanomoles per litre.
“Improving vitamin D status in insulin resistant women resulted in improved IR and sensitivity, but no change in insulin secretion,” wrote the women. “Optimal vitamin D concentrations for reducing IR were shown to be 80 to 119 nmol/l, providing further evidence for an increase in the recommended adequate levels,” they concluded.
Shedding light on the sunshine vitamin
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
Source: British Journal of Nutrition
Published online ahead of print, doi: 10.1017/S0007114509992017
“Vitamin D supplementation reduces insulin resistance in South Asian women living in New Zealand who are insulin resistant and vitamin D deficient – a randomised, placebo-controlled trial”
Authors: P.R. von Hurst, W. Stonehouse, J. Coad