Serum levels of 25-hydroxyvitamin D [25(OH)D] are used as a clinical marker for vitamin D status. Current recommendations are that 25-hydroxyvitamin D levels should be above 40 nmols/L (16 ng/mL) for optimal health. Levels below 25 nmols/L (10 ng/mL) are likely to result in the severe deficiency symptoms of osteomalacia or rickets. In humans 25-hydroxyvitamin D is converted to 1, 25-hydroxyvitamin D which then exerts a number of physiological effects. Most tissues contain both vitamin D receptors as well as the hydroxylase enzyme necessary to convert 25-hydroxyvitamin D to 1,25-hydroxyvitamin D and therefore vitamin D has almost universal cellular effects. The best way to maintain optimal blood levels of 25-hydroxyvitamin D is to ensure adequate sun exposure. Deficiency of 25-hydroxyvitamin D has been linked to a number of diseases including cardiovascular disease, multiple sclerosis, cancer and diabetes.
Because hypovitaminosis D is associated with an increased risk of diabetes, researchers1 investigated the relationship between 25-hydroxyvitamin D concentrations and both insulin sensitivity and β-cell function in 126 healthy volunteers. β-cell function and insulin sensitivity were assessed using a hyperglycaemic clamp and the 25-hydroxyvitamin D concentrations of the subjects were tested from fasting blood samples. The results showed that 25-hydroxyvitamin D concentrations had no association with blood pressure, triglyceride levels, HDL cholesterol or waist to hip ratio. However, 25-hydroxyvitamin D plasma concentrations were inversely associated with body mass index (BMI), total and LDL cholesterol. Plasma 25-hydroxyvitamin D also showed a positive correlation with insulin sensitivity index and a negative correlation with fasting and postprandial blood glucose. Subjects with low 25-hydroxyvitamin D levels also showed decompensated β-cell function and a higher risk for metabolic syndrome.
Interestingly the results showed that 47 of the subjects had 25-hydroxyvitamin D levels below 50 nmol/L (20 ng/mL) but none showed any deficiency signs. It is generally accepted that blood levels between 25 and 40 nmol/mL are insufficient, but unlikely to show major deficiency disease. There were no sex differences in the levels of 25-hydroxyvitamin D amongst subjects, but African American, Mexican American and Asian American subjects had larger representations (54%, 41% and 47%, respectively) in the below 50 nmol/mL group, compared to white Americans (26%). Low 25-hydroxyvitamin D was associated with insulin insensitivity and improper β-cell function, which would suggest that hypovitaminosis D is a risk factor for type 2 diabetes and metabolic syndrome. This supports work by other groups. The authors extrapolated their results and suggested that increasing 25-hydroxyvitamin D levels from 25 to 75 nmol/mL could improve insulin sensitivity by 60%.
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