Objective:To investigate the effect of vitamin D supplementation on weight loss efficacy and insulin resistance (IR) in obese patients.Methods:It was a randomized controlled trial. A total of 190 obese patients with vitamin D deficiency were selected from the obesity clinic of Tianjin Union Medical Center from March to December in 2023. The patients were divided into control group (95 cases) and vitamin D group (95 cases) according to random number table. The control group was given energy-limited high-protein diet combined with moderate intensity exercise, and the vitamin D group was supplemented with vitamin D on the basis of the control group, 14 000 U/week for 24 weeks. A total of 25 dropped out of the study from the two groups for various reasons. Finally, 79 cases in the control group and 86 cases in the vitamin D group were included in the analysis. Independent sample t test and rank sum test were used to compare serum 25 hydroxyvitamin D [25 (OH) D] level, body weight, body mass index, fat mass, visceral fat area, fasting blood glucose, fasting insulin and glycated hemoglobin (HbA 1c) between the two groups at baseline and after intervention. Homeostasis model insulin resistance index (HOMA-IR) was used to evaluate the degree of insulin resistance (IR), and the effects of vitamin D supplementation on weight loss efficiency and IR in those patients were analyzed. Results:There was no significant differences in serum 25(OH)D level between the two groups before intervention ( P>0.05); the serum 25(OH)D level in the vitamin D group was significantly higher than that in the control group after intervention [(30.90±7.55) vs (16.00±4.34) μg/L] ( t=-15.35, P<0.001). The body weight, body mass index, fat mass, visceral fat area, fasting blood glucose, fasting insulin, HbA 1c, and HOMA-IR were all significantly lower after the intervention than those before the intervention in both groups [control group: (93.32±13.47) vs (98.95±14.31) kg, (33.74±5.09) vs (35.80±5.52) kg/m 2, (39.77±11.87) vs (44.12±12.79) kg, (183.76±40.95) vs (204.01±32.18) m 2, 5.00 (4.55, 5.67) vs 5.24 (4.68, 6.42) mmol/L, 16.78 (13.94, 24.30) vs 22.56 (15.95, 31.2) mU/L, 5.55%±0.53% vs 6.05%±0.99%, 4.11 (3.14, 5.57) vs 5.51 (3.61, 8.49); vitamin D group: (88.14±17.66) vs (104.43±22.02) kg, (31.02±5.10) vs (36.66±5.98) kg/m 2, (35.51±12.87) vs (46.67±13.33) kg, (166.50±49.50) vs (213.64±40.14) m 2, 4.70 (4.35, 5.07) vs 5.17 (4.77, 6.30) mmol/L, 13.18 (9.87, 18.84) vs 21.67 (15.78. 32.74) mU/L, 5.43%±0.48% vs 6.21%±1.22%, 2.88 (1.99, 4.21) vs 5.19 (3.82, 9.27)], and the body weight, body mass index, adiposity, visceral fat area, fasting blood glucose, fasting insulin, and HOMA-IR were all significantly lower in vitamin D group than those in the control group [(88.14±17.66) vs (93.32±13.47) kg, (31.02±5.10) vs (33.74±5.09) kg/m 2, (35.51±12.87) vs (39.77±11.87) kg, (166.50±49.50) vs (183.76±40.95) m 2, 4.70 (4.35, 5.07) vs 5.00 (4.55, 5.67) mmol/L, 13.18 (9.87, 18.84) vs 16.78 (13.94, 24.30) mU/L, and 2.88 (1.99, 4.21) vs 4.11 (3.14, 5.57), respectivley] (all P<0.05). The IR remission rate was significantly higher in the vitamin D group than that in the control group after the intervention (37.3% vs 15.3%) ( χ2=8.071, P=0.002). Conclusion:Supplementation of vitamin D on the basis of energy-limited high-protein diet combined with moderate intensity exercise can significantly improve the efficacy of weight loss and IR in obese patients with vitamin D deficiency.