Vitamin D is required by human body for multiple functions, its role in calcium and phosphorous absorption and consequently in bone strengthening being a common knowledge. Our body can produce it when skin is exposed to direct sunlight. It can also be obtained from dietary sources such as egg yolks or in the form of supplements. Several food products such as milk and breakfast cereals are also fortified with vitamin D.
Recent studies indicate that vitamin D may also be associated with multiple health disorders such as hypertension, diabetes and cancers.
In their highly cited publication, Mohammed F Saad of UCLA present data showing positive association between vitamin D and insulin sensitivity. They reported that vitamin D deficiency (hypovitaminosis D) was associated with higher risk of insulin resistance, type 2 (II) diabetes.
Their result is based on a study of 126 individuals of various ethnic backgrounds living in California (73 women, 53 men, 20 – 32 years old, body mass index (BMI) 20.5 – 28.9). At enrollment to the study, all individuals had normal blood pressure (<140/90 mm Hg) and normal glucose tolerance (fasting plasma glucose: <110 mg/dL; interval plasma glucose: <200 mg/dL; and 2-h plasma glucose: <140 mg/dL).
The association between vitamin D and insulin sensitivity was independent of age, sex, ethnic background, blood pressure and BMI.
The authors, citing other animal and human studies point out evidences of vitamin D being essential for normal insulin secretion. Prior studies reported impairment of insulin secretion in pancreas deficient in vitamin D, and its improvement by dietary vitamin D supplementation. Vitamin D repletion improved glucose clearance and insulin secretion in vivo, independent of nutritional factors and prevailing plasma calcium and phosphorus concentrations. Vitamin D accelerates the conversion of proinsulin to insulin. These results suggest a positive association of vitamin D with insulin sensitivity. This is also evidenced by the observation that vitamin D supplementation reduces the concentrations of serum free fatty acids in patients with type 2 diabetes.
Saad and his co-authors suggest that a 60% improvement in insulin sensitivity, from 3.8128 to 6.1176 (µmol/L) · m–2 · min–1 · (pmol/L)–1, can be achieved by increasing vitamin D level from 10 to 30 ng/mL. They explain that the 60% improvement in insulin resistance could potentially reverse abnormal glucose tolerance. Considering there is a high incidence of vitamin D deficiency and consequent high risk of type 2 diabetes at population level, the modest effect of their suggested vitamin D treatment on insulin sensitivity in individual persons may translate into a dramatic effect in the population.
In another study by Korean researchers, led by Sung-kil Lim, based on a survey of 5787 Koreans (2453 men, 3334 women, 33 – 65 year old), low vitamin D concentration was also associated with a high risk of diabetes mellitus. They found that the lower the vitamin D concentration (<75 nmol/L 25(OH)D) the higher was the insulin resistance in their overweight or obese study subjects.
Han Seok Choi, Kyoung-Ah Kim, Chi-Yeon Lim, Sang Youl Rhee, You-Cheol Hwang, Kyoung Min Kim, Kwang Joon Kim, Yumie Rhee, and Sung-Kil Lim (2011). Low Serum Vitamin D Is Associated with High Risk of Diabetes in Korean Adults. J. Nutrition. First online published June 22, 2011, doi:10.3945/jn.111.139121
Ken C Chiu, Audrey Chu, Vay Liang W Go and Mohammed F Saad (2004). Hypovitaminosis D is associated with insulin resistance and ß cell dysfunction. American Journal of Clinical Nutrition, 79(5):820-825.