Osteomalacia and cardiometabolic disorders are favored in morbidly obese patients due to an inadequate vitamin D (VD) status. Former trials supplementing orally VD (20-50 μg/day) in crystalline form after sleeve gastrectomy (SG) could not stabilize serum 25-hydroxycholecalciferol levels at predefined concentrations (≥50 nmol/l). We hypothesized that VD in an oily suspension would increase its bioavailability resulting in normal serum VD levels minimizing markers of cardiometabolic risk.Morbidly obese patients (n = 94, BMI 51.8 ± 11.5 kg/m(2)) received orally 80 μg/day VD3 dissolved in oil or placebo (pure oil) in a randomized, double-blind, parallel-group study for 12 weeks after SG. 25-hydroxycholecalciferol, parathyroid hormone, albumin, alkaline phosphatase, phosphate, magnesium, calcium, creatinine, C-reactive protein, lipids, glucose, and glycated hemoglobin were determined in serum/plasma before surgery and after 4 and 12 weeks of supplementation. Intake of energy, fat, and VD were monitored using a 3-day food record.Seventy-nine patients were included in statistical analysis. Preoperatively, 77.2 and 40.5 % presented 25-hydroxycholecalciferol levels <75 and <50 nmol/l, respectively. After 12 weeks of supplementation, significantly more patients in the VD group exhibited levels >50 nmol/l (92 %) and >75 nmol/l (68 %) compared to the placebo group (54 and 22 %, respectively). Parameters of mineral metabolism and cardiometabolic risk were not modulated by intervention.Supplementation of 80 μg/day VD3 by oil is an effective and safe measure to prevent VD deficiency and to treat a preexisting undersupply in patients after SG. Cardiometabolic risk factors were, however, not affected; probably, higher VD doses might be necessary.This trial was registered retrospectively on November 14, 2014, at the German Clinical Trials Register as DRKS00007143.