What is known about the mechanisms of dietary influences in Crohn's disease?

•Vitamin D deficiency has been implicated as a risk factor for Crohn's disease, as well as other unrelated conditions.•Animal models have reported that vitamin D increases tight junction proteins and enhances healing following an injury to the intestinal epithelium.•Very recently the effect of vitamin D supplementation and intestinal permeability has been investigated in patients with Crohn's disease, with some evidence suggesting a role for vitamin D in supporting gut barrier functioning.

Much has been written about the role of diet and risk for Crohn's disease (CD). However, the evidence is contradictory. Recent evidence has pointed to fiber playing an important role along with the possibility that dietary fat and overnutrition also have a role. Diet has a clearer place in disease modification, with some diets used in the treatment of CD. The lack of clarity stems from a poor understanding of the mechanisms underlying the relationship between diet and CD. Gut permeability is likely to play a key role in the risk for CD. Mechanisms whereby diet can affect gut permeability, including the effects of the gut microbiota, are reviewed. Modification of disease behavior is likely to be influenced by additional mechanisms, including recognition of complex food antigens. As with many other chronic diseases, a surrogate marker of CD risk would greatly aid evaluation of the dietary factors involved. Formal measures of gut permeability are too cumbersome for large-scale use, but fecal calprotectin may be a convenient measure of this. There are only preliminary data on the effect of diet and microbiota composition on fecal calprotectin and these require further investigation.