Here, we show that patients who were vitamin D-deficient before surgery exhibited a ~14 % higher RYGB surgery-induced weight loss when they carried two copies of the A-allele in the fat mass and obesity (FTO) gene rs9939609, compared to vitamin D-deficient patients who were homozygous for the FTO T-allele. The gene discussed is FTO; the disease is obesity due to melanocortin 4 receptor deficiency.