Inflammatory bowel diseases: interrelationships between dietary vitamin D, exposure to UV radiation and the fecal microbiome

ABSTRACT

Introduction: Environmental factors and an altered fecal microbiome are believed to be central to the pathogenesis of inflammatory bowel diseases (IBD). Vitamin D and ultraviolet radiation (UVR) are environmental factors that are associated by several pathways, including changes to the gastrointestinal microbiome, with the development and course of IBD.

Area covered: This review explores the interaction of vitamin D, and UVR, with the intestinal innate and adaptive immune systems, and how they may influence the gut microbiome and the subsequent development, and progression, of IBD.

Expert opinion: Vitamin D and UVR both regulate innate and adaptive immunity through a combination of common and independent mechanisms, with the overall effect being the promotion of immune tolerance. Vitamin D, and to a lesser extent UVR, can modify the gastrointestinal microbiome either directly, or through immune-mediated mechanisms and this may explain the effect on intestinal inflammation in animal models of IBD and some clinical studies. Thus, both vitamin D and UVR exposure can be considered potential ‘master regulators’ of gastrointestinal immunity, fine-tuning the complex interaction between genetics, host immunity and the gut microbiome. Further research and increased understanding of environment-host interactions is essential to achieving the ultimate goal of preventing and curing IBD.

KEYWORDS:

• Vitamin D
• ultraviolet radiation
• Crohn’s disease
• ulcerative colitis
• inflammatory bowel disease
• microbiome

Article Highlights

• CD and UC are increasing in both incidence and prevalence.

• The cause of these diseases is not completely understood but environmental triggers, particularly those that interact with the gastrointestinal microbiome and the immune system, are thought to be of great relevance.

• Epidemiological studies have identified associations of IBD onset and disease activity with vitamin D levels and ultraviolet radiation exposure.

• Vitamin D and ultraviolet radiation can both influence the gut microbiome and intestinal immune responses.

• Vitamin D supplementation reduces intestinal inflammation in many animal models of IBD, though the optimal dose is not known, with high doses leading to worse inflammation in some studies.

• Clinical studies of vitamin D supplementation have been small and heterogenous in design with mixed results at treating or preventing relapse in IBD. Overall meta-analyses suggest a small but significant effect.

• UVR has not been adequately studied as a therapeutic strategy, animal models have shown mixed results. Further preclinical work is needed before considering clinical trials.

• Ongoing research into environmental factors, their interaction with host immunity and genetics are needed to better understand the pathophysiology of IBD, develop new therapeutic strategies and ultimately try to prevent disease onset.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper is not funded.