Patients with fat malabsorption due to Crohn's disease, ulcerative colitis, or other causes including cystic fibrosis, among others, or who have undergone gastric bypass have increased incidence of vitamin D deficiency relative to the general population. Given that vitamin D is fat soluble and absorbed in the proximal small intestine, it has been documented that vitamin D deficiency in people with a fat malabsorption syndrome is due to decreased absorption of vitamin D.