Vitamin D Cut Point Linked to Normal Glucose Metabolism

Marlene Busko

April 21, 2014

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, a new observational study suggests. 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.

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," lead author John D. Sorkin, MD, from the University of Maryland, in Baltimore, told Medscape Medical News.

"Our 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.

However, 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," Sorkin and colleagues caution. They also call for further research to determine whether their findings hold true for other racial and ethnic groups, men, and younger, older, or nonobese individuals.

The study is published in the May 2014 issue of the Journal of Nutrition.

Vitamin D Beyond Bone

The definition of normal vitamin-D levels remains controversial, and guidelines largely base their recommendations on studies of bone metabolism, the researchers explain. However, low levels of vitamin D have been recently linked with glucose intolerance, type 2 diabetes, insulin resistance, hypertension, hyperlipidemia, and cardiovascular disease.

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, Dr. 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, insulinlike 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.

Adds to Evidence, Has Some Limitations

Asked to comment, Catherine A. Peterson, PhD, from the University of Missouri in Columbia, said that "this is another little piece of evidence suggesting that vitamin-D status does matter in terms of glucose control." She previously published results from a randomized controlled trial that showed correcting vitamin-D insufficiency improved insulin sensitivity in obese adolescents.

The article has several limitations, she noted. It uses inconsistent measures for vitamin D (as opposed to just using to the standard unit, ng/mL) and describes how vitamin-D levels "cause outcomes," which cannot be established, because it was an observational study. Although Sorkin and colleagues explain that frozen samples of vitamin D have been shown to be stable for 6 to 24 years, there still might be some degradation of the 4- to 18-year-old samples in the current study, which could preclude being able to establish a precise vitamin-D threshold.

However, Dr. Peterson agrees with the authors that "the Institute of Medicine values [for vitamin-D sufficiency or insufficiency] are probably myopic. They really honed in on bone health without looking beyond bone-health measure outcomes."

The study was supported by the Baltimore Veterans Affairs Medical Center Geriatrics Research, Education, and Clinical Center and grants from the National Institute on Aging, National Institute of Diabetes and Digestive and Kidney Diseases, and National Institutes of Health. The authors declared no conflicts of interest.

J Nutr. 2014;144. Full text


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