Crohn's Disease and environmental factors in the New Zealand context : a thesis presented in partial fulfilment of the requirements for the degree of Doctoral of Philosophy in Nutritional Science at Massey University, Manawatū, New Zealand

Abstract

Background: Inflammatory bowel disease (IBD), consisting of Crohn’s disease (CD) and ulcerative colitis (UC), are lesser-known chronic diseases of the gastrointestinal tract. The causes of IBD are unknown, although research indicates an interplay of genetic, immunological, and environmental factors. The incidence and prevalence of CD in New Zealand (NZ) are among the highest worldwide, and unlike many other Western countries, evidence suggests the incidence rate is still increasing.

Objective: The objective was to investigate the involvement of environmental factors in the aetiology, pathogenesis, and symptomatology of CD in NZ. Specifically, pathogenic bacterium Mycobacterium avium subspecies paratuberculosis (MAP), vitamin D, diet, and urbanisation.

Methods: Patients with IBD and controls from around NZ completed a questionnaire on environmental factor exposure. Foods implicated in symptom triggering or exacerbation, the possible mechanism(s) involved, and whether vitamin D can confer protection, were investigated using an in vitro digestion method and in vitro model of the intestinal barrier. Serum vitamin D concentrations were measured and compared in patients and controls in order to explore a possible association between vitamin D and IBD. Lastly, the incidence and prevalence of IBD in the Manawatū region was determined, and the urban and rural incidence were compared.

Results: Questionnaire derived data showed significant associations between CD and exposure to rural sources of microorganisms, and a major urban birthplace (≥100,000 residents), while rainwater for drinking and cooking during childhood was protective. No associations were observed between CD and MAP exposure. Over 50% of patients implicated dietary elements in symptom onset and/or exacerbation. The in vitro investigation findings suggest this may result from tight junction damage. Vitamin D concentrations did not differ between patients and controls, however, were significantly lower in CD patients that reported recent disease activity. In the Manawatū region, the mean annual incidence and 2013-point prevalence of CD were 17.7 and 250.4 per 100,000, respectively, and urban residence at diagnosis was associated with a six-fold greater IBD incidence compared to rural residence.

Conclusions: The findings demonstrate that vitamin D, diet, and urbanisation are involved in CD. A greater understanding of environmental factors, especially modifiable factors, could provide opportunities for reducing CD risk, managing symptoms, or slowing disease progression.