Procedures
IASP
EVENTS
Participants in the saxagliptin group took 5-mg of saxagliptin once daily. Participants in the saxagliptin plus vitamin D group received saxagliptin in the same manner concomitantly with 2000U of vitamin D3 orally once daily. The dose of metformin was individualized by investigators to achieve optimal glycemic control. In case of inadequate glycemic control, any insulin regimen was approved at the investigator’s discretion. No additional antidiabetic treatment was allowed during the trial. Participants were required to record insulin dose, concomitant use of drug, self-monitoring blood glucose levels and hypoglycaemic events in participant diaries at home. Unused investigational drug was returned at each visit to assess adherence.Follow-up clinic visits after randomization were scheduled for month 1, 2, 3, 6, 10, 12, 18, 24 with telephone visits at every-4-week interval between the clinic visits. MMTTs contains 1658kJ (72% from carbohydrate, 9.5% from protein, 18.5% from fat) and were scheduled at baseline and month 6, 12, 18 and 24 to evaluate β-cell function. Saxagliptin was held for 72 h before the MMTT, and insulin injection was withheld on the day of the MMTT. Fasting C-peptide was taken after overnight fasting for at least 8 h. (Supplementary Methods). C-peptide and serum 25(OH)D levels were assessed with a chemiluminescence method using the Adiva Centaur System kit (Siemens, Munich, Germany). GADA was assessed using radioligand assay, with sensitivity and specificity being 82% and 97.8% confirmed by Islet Autoantibody Standardization Program (IASP). C-peptide, 25(OH)D and islet autoantibodies were measured by the core laboratory of the Second Xiangya Hospital, Central South University (Changsha, China) (Supplementary Methods).