Combined assessment of parathyroid hormone along with vitamin D may be needed to assess the impact of vitamin D status on glucose metabolism, according to Toronto researchers. Their study was published online in the journal Diabetes on May 29, 2014.
The new findings might explain why studies of vitamin D alone have been conflicting and why clinical trials of vitamin D supplementation to improve diabetes have been disappointing, said principal investigator Dr. Ravi Retnakaran, a clinician-scientist at the Lunenfeld-Tanenbaum Research Institute at Mount Sinai Hospital in Toronto.
In response to low vitamin D levels, the body increases secretion of parathyroid hormone (PTH), which is its upstream regulator, according to the study. Retnakaran and co-authors show that glucose levels are affected only when low vitamin D is accompanied by increased PTH.
About the study
Retnakaran’s team evaluated the prospective associations of 25-OH-D and PTH in 494 postpartum women undergoing serial metabolic characterization.
Thirty-two percent of those with pre-diabetes or diabetes at 12 months postpartum had both vitamin D deficiency and PTH in the highest tertile at three months postpartum. Similarly, on multiple linear regression analyses, vitamin D deficiency or insufficiency with PTH in the highest tertile at three months independently predicted poorer beta-cell function and insulin sensitivity, and increased fasting and two-hour glucose at 12 months postpartum. In contrast, vitamin D deficiency or insufficiency with lower PTH did not predict these outcomes.
These findings support an effect of vitamin D status on glucose homeostasis by demonstrating an independent role of the PTH-vitamin D axis in the development of dysglycemia, insulin resistance, and beta-cell dysfunction, according to the study authors.
These data highlight the need for assessment of the entire PTH/25-OH-D axis when studying the effect of vitamin D on glucose metabolism. This concept may partly explain the conflicted literature to date and has implications for future clinical trials examining the impact of vitamin D supplementation on pre-diabetes and diabetes, according to the authors.
Not all patients with low vitamin D behave in the same manner, the researchers said. Only those also showing increased PTH appear to be at risk for pre-diabetes and diabetes. For that reason, research on vitamin D in diabetes should focus on patients with low vitamin D and increased PTH.
“Speaking clinically, with these results we can’t say that vitamin D supplements should be given to everyone at risk of diabetes who has low vitamin D levels,” says Dr. Retnakaran. “We can assess risks associated with low vitamin D only in the context of the person’s PTH levels as well.”