Background: Vitamin D is often described as a fat-soluble vitamin, but it also exhibits hormonal properties in its active form through its action via vitamin D receptors found in most tissues in the human body. Vitamin D can directly enhance insulin synthesis and its release from pancreatic bita-cells as well as increase the expression of the insulin receptor in peripheral tissues.

Aim of the study: To assess vitamin D level in children with type one diabetes and to evaluate the effect of vitamin D supplement therapy on the level of HbA1c.

Patient and method: This study included (125) patients with type one diabetes, aged (2-15 years); selected between November 2021 and September 2022. Five patients had celiac disease and were excluded, 61 patients were newly diagnosed, 59 patients had established type one diabetes and 33 controls. Vitamin D and HbA1c were measured for all patients and healthy matched control, vitamin D deficient patients received supplement for three months, and vitamin D level and HbA1c were measured after treatment.

Results: Study included 61 patients with newly diagnosed diabetes; 26 male and 35 female, and 33 healthy matched control, 15 male and 18 female, and 59 patients with established diabetes, 28 male and 31 female, serum vitamin D was low in all groups, vitamin D status was not associated with age, sex, duration of the disease, body mass index, HbA1c, seasonal distribution, family income and education in newly and established diabetic patients, after three months treatment; HbA1c significantly reduced with increased serum vitamin D (p value < 0.05).

Conclusion: Study revealed vitamin D level is low in newly, established diabetes and in control subjects, and it is not associated with patient’s clinical, demographic and laberatory characteristics. Treatment of vitamin D improved glycemic control of patients as observed by the reduction of HbA1c level.