Vitamin D supplements seem to strengthen the gut wall, so reducing the incidence of leaky gut, one of the characteristics of the disease.
Researchers from St James's Hospital in Dublin gave 27 Crohn's disease patients either vitamin D supplements, delivering 2000 IUs a day, or a placebo for three months. At the end of the trial, those who had taken the supplements had less intestinal permeability, and also were showing signs of reduced inflammation. They also reported experiencing a better quality of life while taking the supplements.
(Source: United European Gastroenterology Journal, 2015; 3(3): 294)