Objective The objective of this study is to investigate the effect of prolonged medically supervised fasting (PMSF) for 6 months on vitamin D levels and diabetes-related clinical outcomes in type 2 diabetes mellitus (T2DM) patients. Methods This is an open-label, parallel arm, randomized control trial; 170 T2DM patients from a complementary medicine setting, aged between 20 and 70 years, with a body mass index >25 kg/m 2 and glycated hemoglobin (HbA1C) levels >6% and <12%, treated with lifestyle advice and/or oral hypoglycemic drugs, will be randomized to receive PMSF or a diabetic diet (DD), along with their usual care. The patients will be initially trained in PMSF and DD for a duration of 10 days in the study setting, followed by 5 months of practice at their respective residences. The PMSF will fast for five days a month for three consecutive months after the initial exposure, whereas the DD group will follow the same diet for 6 months. Other than fasting days, the PMSF group will follow the same DD as the control group. The primary outcome is an improvement in vitamin D levels, and the secondary outcomes include serum calcium, insulin levels, insulin resistance, blood cell counts, anthropometrics, quality of life, medication score, compliance rate, and disease perception. Discussion In this study, we hypothesize that PMSF would gradually increase vitamin D levels, which would increase calcium levels, which would promote insulin secretion and upregulate its function. This may help reduce the need for diabetic medication and result in diabetic remission.