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Adequate Vitamin D Levels Linked to Normal Glucose Metabolism

Low levels of vitamin D have been recently linked with glucose intolerance, type 2 diabetes, insulin resistance, hypertension, hyperlipidemia, and CV disease…. 

The definition of normal vitamin D levels remains controversial and guidelines largely base their recommendations on studies of bone metabolism, the researchers explain. A new observational study suggests that a 25-hydroxyvitamin D (vitamin D) level of about 26 ng/mL is needed for normal glucose metabolism in both black and white obese, postmenopausal women. Women with a blood vitamin D concentration at or above this threshold had lower body fat and blood glucose, insulin, and triglyceride levels than women with lower levels of vitamin D.

Lead author John D. Sorkin, MD, from the University of Maryland, in Baltimore stated that the cutoff is below the minimal vitamin D level recommended by the Endocrine Society (30 ng/mL) but above that recommended by the Institute of Medicine (20 ng/mL); however, both guidelines are based on studies of bone health. The current study implies that "if you want to think about defining cut-points for vitamin D, you need to think about things other than bone."

"The results suggest that the Institute of Medicine recommendation of 20 ng/mL is probably too low." Importantly, the data also indicate that the cut-point for sufficient vitamin D is the same for black and white women, he said.

Sorkin and colleagues caution that, this was a retrospective, observational study with inherent limitations. "A large, prospective interventional study in black and white women will be needed to confirm that increasing 25(OH)D concentration above [around 26 ng/mL] improves glucose homeostasis and insulin sensitivity with little improvement above this value,". They also call for further research to determine if their findings hold true for other racial and ethnic groups, men, and younger, older, or nonobese individuals.

The researchers aimed to study the effect of vitamin D on glucose tolerance, insulin resistance, and other cardiovascular risk factors. The study population included obese individuals and blacks, who are more likely to have a vitamin D deficiency, Sorkin explained.

They performed a cross-sectional study of 83 black and 156 white overweight or obese, sedentary, postmenopausal women without diabetes who had participated in studies at their center between June 1995 and July 2009 and had a fasting blood glucose and 2-hour oral glucose tolerance test. Other assessments included insulin resistance, insulin-like growth factor 1, parathyroid hormone (PTH), aerobic fitness, body composition (using dual X-ray absorptiometry), subcutaneous abdominal and visceral fat, and blood pressure.

Vitamin D was inversely related to fasting glucose, fasting insulin, 2-hour insulin, insulin resistance, visceral abdominal fat, percentage fat, PTH, and triglycerides. There was no relationship between vitamin D and blood pressure, lipids (other than triglycerides), or fitness.

Practice Pearls:
  • The definition of normal vitamin D levels remains controversial.
  • The study suggests that a 25-hydroxyvitamin D (vitamin D) level of about 26 ng/mL is needed for normal glucose metabolism.
  • There was no relationship between vitamin D and blood pressure, lipids (other than triglycerides), or fitness.

J Nutr. 2014;144. Full text