Lower levels of vitamin D were tied to higher glucose levels in a study of Brazilian women.
In the cross-sectional analysis, more than 65% of women had hypovitaminosis D — considered to be a serum 25(OH)D level under 30 ng/mL — which was associated with higher odds of having a blood glucose level of 100 mg/dL or higher (OR 1.29, 95% CI 1.05-1.57, P=0.015).
This relationship was also seen among women with even lower serum levels of vitamin D, Tânia Valladares, MSc, of the University of Sao Paulo School of Public Health in Brazil, and colleagues wrote in Menopause: The Journal of The North American Menopause Society.
Among the quarter of women with a serum 25(OH)D level <20 ng/mL, they also had significantly higher odds of having a blood glucose level of 100 mg/dL or higher, although the magnitude was no greater (OR 1.25, 95% CI 1.04-1.50). Self-reported use of supplements was not associated with blood glucose levels.
“It has been postulated that vitamin D plays a role in pancreatic beta cell function, improving the insulin response to an increase in the blood glucose level,” Valladares’ group explained. “Those data underscore a potential benefit of a higher 25(OH)D level in individuals who still have a minimal beta cell population, improving the insulin response and perhaps preventing the progression of insulin dependence.”
The findings were consistent with the literature, the researchers said, although intervention studies haven’t shown any significant benefit to vitamin D supplements. Consequently, they recommended future studies should aim to differentiate whether or not serum vitamin D “can actually improve blood glucose levels or if its importance is restricted to its function as a marker of ill health.”
In a statement, JoAnn Pinkerton, MD, executive director of the North American Menopause Society, who was not involved with the study, underscored the importance of the relationship between low vitamin D levels and type 2 diabetes. “Vitamin D supplementation may help improve blood sugar control, but intervention studies are still needed,” she said. However, Pinkerton also stressed that no causal relationship between these two variables has been proven yet.
When determining vitamin D deficiency in patients, the American Association of Clinical Endocrinologists (AACE) and the American College of Endocrinology (ACE) note that “defining ‘vitamin D inadequacy’ is extremely controversial.”
“[Randomized controlled trials] evaluating nutrients are often confounded when ‘low’ nutrient status is not established, since nutrients reach a threshold effect in which greater amounts do not provide enhanced physiologic effects. As such, providing vitamin D to volunteers who are vitamin D replete should not be expected to demonstrate beneficial effects,” they explain in their position statement on the use of vitamins and minerals in skeletal health.
The current study’s cutoff of 30 ng/mL for vitamin D deficiency is the same as that recommended by AACE and ACE.
For the analysis, 680 women ages 35-74 completed interviews and physical exams and gave blood samples. Their mean blood glucose was 105 mg/dL, while the average 25(OH)D level was 26.4 ng/mL. Among the cohort, 3.5% reported taking vitamin D supplementation, which — as expected — was negatively associated with having low vitamin D (OR 0.44, 95% CI 0.19-0.99, P=0.049) levels. Women who reported having habitual sun exposure also saw lower odds of having a serum vitamin D level under 30 ng/mL (OR 0.68, 95% CI 0.48-0.96, P=0.028), but no significant risk of excessive blood glucose (OR 1.03, 95% CI 0.85-1.25).
A glucose level of 100 mg/dL or higher wasn’t only associated with low vitamin D levels, but was also significantly tied to obesity, hypertension, and older age among women:
- BMI ≥30: OR 1.42 (95% CI 1.19-1.69, P<0.001)
- SBP ≥130 or DBP ≥85 mmHg: OR 1.27 (95% CI 1.03-1.56, P=0.025)
- Age 60+: OR 1.33 (95% CI 1.11-1.61, P=0.003)
Among the study’s limitations were that data on sun exposure, vitamin D supplementation, medication use, and other potential confounders were self-reported. Also, the cohort was exclusive to women living in a tropical region of Brazil where sunlight is relatively strong and plentiful.
The study was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo.
Valladares and co-authors reported no disclosures.