Seasonal variation of vitamin D and HbA_1c levels in patients with type 1 diabetes mellitus in the Middle East

Khaled El Baba, Mira S Zantout, Randa Akel, Sami T Azar
Department of Internal Medicine, Division of Endocrinology and Metabolism, American University of Beirut Medical Center, Beirut, Lebanon

Introduction: The discovery that many tissues express the vitamin D receptor and are able to transform the 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (active metabolite) has led to the hypothesis that vitamin D could have a role in the pathogenesis and prevention of diabetes mellitus. Accumulating evidence has linked vitamin D deficiency with immunological disturbance in type 1 diabetes mellitus. Several epidemiological studies have shown lower serum 25-hydroxyvitamin D concentrations in diabetic individuals compared to nondiabetic individuals.
Patients and methods: A total of 105 Lebanese patients with type 1 diabetes mellitus were recruited. Levels of HbA_1c and vitamin D were recorded during two phases with phase 1 being the period from June–September 2009 and phase 2 from January–April 2009.
Results: Vitamin D levels at the end of sunny season (phase 2), reflecting the previous sunny months, were found to be higher than in the early summer season (phase 1) reflecting the previous cloudy months season, with a mean change of 0.70 ± 6.87 ng/mL (P = 0.029). HbA_1c levels were higher in phase 2 than in phase 1 with an increase of 0.175% ± 1.46% (P = 0.113).
Conclusion: Vitamin D levels were higher at the end of the sunny season than at the end of cloudy season. HbA_1c was also lower (but not significantly so) in the early summer season (cloudy months). There was a weak correlation between seasonal changes of vitamin D levels and the control of blood glucose as monitored by HbA_1c in a cohort of Lebanese patients with type 1 diabetes mellitus.

Keywords: type 1 diabetes, vitamin D, HbA_1c, seasonal variation