Childhood immunity and infections

In the multi-variate model, several childhood factors were found to be protective for CD and UC. Having being breast fed for 12 months or longer (aOR 0.10; 95% CI 0.04 to 0.30), use of antibiotics before the age of 15 years (aOR 0.19; 95% CI 0.07 to 0.52) and having dogs during childhood (aOR 0.54; 95% CI 0.35 to 0.83) were associated with a reduced risk of developing CD in Asians (table 2). The prevalence of antibiotic use during childhood was 12.5% in Asians and 8.0% in Australians. The reduced risks associated with breast feeding, antibiotic use, having dogs and development of CD were also observed when analysis was performed in the combined Asian and Australian cohorts (table 2). Vaccinations history did not influence the risk of CD when analysis was confined to Asian cohorts. In the combined Asian and Australian cohort, BCG vaccination increased the risk for CD (aOR 2.14; 95% CI 1.03 to 4.42) (table 2).

Having being breast fed for 12 months or longer (aOR 0.16; 95% CI 0.08 to 0.31), antibiotic use in childhood (aOR 0.48; 95% CI 0.27 to 0.87) and having aquarium fish (aOR 0.46, 95% CI 0.29 to 0.73) were protective for the development of UC in Asians. In addition, subjects who had inhouse water tap (aOR 0.67; 95% CI 0.48 to 0.93), hot water tap (aOR 0.65; 95% CI 0.46 to 0.91) or flush toilet (aOR 0.71; 95% CI 0.51 to 0.98) during childhood were less likely to develop UC. When analysis was performed in all incident cases (Asian and Caucasian inclusive), the protective effect of breast feeding, antibiotic use, having fish, and having a hot water tap or flush toilet for development of UC remained statistically significant. Concerning childhood immunisations, vaccination against pertussis (aOR 0.61; 95% CI 0.38 to 0.99) was associated with decreased risk of UC. There were no associations between any of the childhood infections and risk of developing CD or UC (tables 2 and 3).

After adjustment for multiple testing, breast feeding for longer than 12 months remained significant as a protective factor for both CD and UC and having dogs during childhood protected against CD.

Smoking

In Asia, prevalence of smoking was similar between CD cases and controls (11% vs. 11%) whereas in Australia, prevalence of smoking was higher in CD subjects than matched controls (40% vs. 19%). Smoking was not a significant risk factor for Asians with CD. However, smoking was associated with a more than fourfold increased risk of CD when analysis was confined to the Australian Caucasian cohort (aOR 4.30; 95% CI 1.22 to 15.16). In Asians with UC, the prevalence of smoking in cases and controls was similar (11% vs. 11%). More UC patients than controls were ex-smokers in Asia (14% vs. 7%) and Australia (43% vs. 15%). Ex-smoking was associated with an increased risk of UC in Asians (aOR 2.02; 95% CI 1.22 to 3.35) and Australian Caucasians (aOR 3.73; 95% CI 1.14 to 12.16) (tables 4 and 5).

Dietary factors before diagnosis

In univariate analysis, frequent juice intake was associated with increased risk of CD (aOR 1.95; 95% CI 1.02 to 3.72) whereas daily cornflakes intake was associated with an increased risk of UC. Following adjustment, these factors reached borderline significance as risk factors. In multi-variate analysis, daily tea consumption was associated with a reduced risk of CD (aOR 0.62; 95% CI 0.43 to 0.91), whereas both daily tea (aOR 0.63; 95% CI 0.46 to 0.86) and coffee consumption (aOR 0.51; 95% CI 0.36 to 0.72) were associated with a reduced risk of UC in Asians. The protective effect of tea and coffee consumption for IBD development was also observed when analysis was performed in both combined Asian and Australia cohort. Overall, no significant associations were seen between CD or UC and the consumption of fruits, vegetables, eggs, bread, cereal, juice or soft drinks (tables 4 and 5).

Other risk factors

Daily physical activity when compared with less frequent physical activity was found to be protective for CD (aOR 0.58; 95% CI 0.35 to 0.96). There were no significant associations between the use of oral contraceptive pill or cholecystectomy, Westernised diet, or major stressful event before diagnosis and the risk of CD or UC (tables 4 and 5).

Multi-variate model

After including all significant findings from univariate analysis into a multi-variate model, we found that breast feeding ≥12 months, antibiotic use in childhood, early contact with pets, daily tea or coffee consumption and ex-smoking remained significantly associated with IBD development. The only factor that was no longer significant was the association of sanitary condition including inhouse water tap, hot water tap or flush toilet and the risk of UC (see online supplementary table S2).

Discussion

Our study addressed for the first time, in a large population-based cohort from Asia, the hypothesis that development of IBD is associated with early childhood immunological and hygienic events. We showed that breast feeding was protective for the development of CD and UC. There was a duration-response effect for both diseases whereby protective effect was only significant when duration of breast feeding was greater than 12 months. Childhood contact with pets and consumption of tea and coffee were novel protective factors for disease development in this study. These findings conducted in a large population-based prospectively collected cohort support two emerging hypothesis on IBD aetiology: first, that environmental exposures in early life are implicated in disease aetiology and, second, that markers of altered intestinal microbiota including immunological, hygiene and dietary factors may modulate risk of IBD later in life. Consistent with the literature, ex-smoking was associated with an elevated risk of UC.

The geographical variation of IBD provides an opportunity to investigate possible environmental aetiological factors. The strongest environmental associations identified in the West are cigarette smoking and appendectomy, although neither alone could explain disease variation in Asia. Incidence and prevalence may have stabilised in high-incidence areas such as North America and Europe but they continue to rise in previously low-incidence areas such as Eastern Europe, Asia and much of the developing world.1 ,15 This epidemiological shift likely relates to ‘Westernisation’ of lifestyle, changes in feeding patterns and improved hygiene as part of socioeconomic development in these countries. Environmental influence may occur at different rates in different geographic areas and populations. For instance, southern parts of mainland China including Hong Kong and Singapore are more developed and urbanised than northern regions. There has been conflicting evidence as to whether breast feeding increases the risk or provides a protective role for development of IBD.9 ,16 ,17 Two meta-analyses showed that breast feeding protects against development of CD and UC in adults and early-onset IBD.7 ,8 The most striking finding from this study is the marked protective effect of breast feeding (>90%) on the risk of CD and UC; the beneficial effect was most prominent when breast feeding was continued for 12 months or longer. These data are consistent with recent case-control studies from New Zealand and Denmark demonstrating a duration-dependent effect for breast feeding, with a negative association seen after at least 3 and 6 months, respectively.14 ,18 In some parts of China, breastfeeding rates at 4 months are 22% compared with the national target of 80%. The potential mechanism of action of breast feeding impacting on development of IBD most likely relates to changes in gut microbiota. Higher concentrations of bifidobacteria and less anaerobic bacteria have been found in stool of breast fed compared with bottle-fed infants.19 As the gut microbiota continues to change up until 2 years of age, there is a potential need for a longer duration of breast feeding to impact a child's risk of IBD development.

Diet-induced changes to gut-associated microbial communities are suspected to contribute to the growing epidemic of IBD; particularly, increased consumption of refined sugar, fast food, fatty acids, cereals and bread and reduced intake of fruits, vegetables and fibre are associated with disease development.18 ,20–22 In the EpiCOM study, Eastern European patients exhibited higher occurrences of ‘Westernised’ dietary habits than Western European patients.23 We found that more CD and UC subjects have reported a higher juice and cornflakes intake, respectively when compared with controls. Westernised diet has been shown to induce dysbiosis,24 ,25 alter host homeostasis and promote adherent invasive Escherichia coli gut colonisation in genetically susceptible mice.26 In contrast, the consumption of tea and coffee which may contain antioxidants and caffeine was associated with a protective effect. Oral caffeine administration has been shown to ameliorate acute colitis in intestinal epithelial cells.27 In addition, green tea polyphenols can improve antioxidants levels and attenuated severity of colitis analogous to sulfasalazine.28

It has been proposed that a lack of exposure to enteric pathogens in early childhood increases the risk of IBD later in life.29 ,30 Although we found no association between childhood infections and disease development, having dogs at a young age markedly reduced risk of CD. Others have shown that exposure to cats before the age of 5 years was protective for later development of CD.31 It could be that exposure to shed microorganisms from pets at an earlier age modulates the immune system and protects against CD development. In contrast to data from others,29 ,30 we found that UC was less common in subjects who had access to a hot water tap and flush toilet during childhood. These data need to be interpreted with caution as many of these factors of modern living may have become ubiquitous since they are now considered normal even in the majority of developing countries.29

A striking finding in our cohort, in contrast to the literature, is the protective effect of antibiotics for CD and UC. Several observational studies have shown an association between antibiotic use and subsequent diagnosis of IBD although causality or biological mechanisms cannot be inferred.32–34 The paradoxical antibiotic effect could be a surrogate marker of exposure to GI infections and antibiotic use may be markers of frequent childhood infections that lead to induction of tolerance. Our data represent the first to report an inverse association between antibiotic use and development of IBD suggesting that antibiotics may not necessarily be a contributing factor to the rising incidence in Asia. While genetic makeup may protect from IBD, certain environmental factors may also be protective including those related to diet and hygiene such as antibiotic use. Alternatively, an imbalance in normal gut microbiota, due to antibiotic use, might have a sustained effect on GI immune tolerance and sensitivity to pathogens, possibly favouring or protecting the onset of IBD depending on the consequence of changes to the composition of the resident microbiota. This effect could be dependent on the type, duration and time point at which antibiotics were given. It is currently unclear whether the effect of antibiotics on the microbiota is long-lasting, and the importance of transient changes on the microbiota remains questionable. Regarding childhood vaccinations, the association between BCG vaccination and increased risk of CD and UC was particularly convincing, as has been shown by others.17 BCG is associated with a Th1 immune response.

To date, the most convincing factor shown to be associated with IBD is smoking, with opposite effects on UC and CD.4 When data were limited to Asian IBD subjects, smoking was not a risk factor for CD. This may be in part because smoking rates in CD was lower in Asia than in Australia. Alternative explanations include differences in genetic makeup and/or environmental exposure (e.g., type of tobacco, way of smoking). It is likely that smoking does not cause CD but modulates the disease once present.35 Furthermore, smoking in CD may not play the same role in different ethnic groups as it does in Western populations due to differences in genetic heterogeneity.

Our data were consistent with findings from case-control and population-based studies reporting an inverse association between regular physical activity and risk of IBD.36 In the Nurses’ Health study, active women had a 44% reduction in risk of developing CD compared with sedentary women.36 Although the exact mechanism for this association is not clear, physical activity may induce autophagy and regulate innate immunity to reduce chronic inflammation.37

A key strength of this study is that it is one of the first to assess environmental risk factors in a non-Western cohort and a population of emerging disease incidence. Our cohort was unselected and population-based with sizeable samples. All cases were prospectively included and followed-up, hence eliminating the potential bias associated with subjects from referral centres. Both cases and controls were drawn from the same population and geography and were likely to be representative of the general population. The main challenge in investigating environmental risk factors for IBD is that the exposures of interest may occur in early childhood, distant in time from when a patient is diagnosed with IBD. Furthermore, the timing of exposure may be an important predictor of risk. Herein, the collection of data immediately after diagnosis and the short median time from symptom onset to diagnosis of IBD can potentially minimise risk of recall bias and timing of exposures can be more accurately defined. Socioeconomic factors may be responsible for variation in the occurrence of IBD reported worldwide. Given that higher social class has been associated with an increased risk of IBD, we carefully adjusted for socioeconomic status based on GNI for each country. Although our data do not prove causality, they provide evidence suggesting that environmental risk factors for IBD are likely to vary between different populations. Last, we have separated the estimates of CD and UC as the two conditions may have different or opposite risk factors.

Our study results should also be interpreted in the light of several limitations. First and possibly the most important relates to recruitment of controls. An ideal population control would include subjects randomly selected from the electoral roll. However, this was not possible in most centres in Asia. We randomly invited consecutive consenting subjects from the streets within the same residential/postal areas of cases. The use of hospital employees as controls may have potential for bias as such personnel may be more likely to be from a higher socioeconomic stratum. Nonetheless, these individuals comprise the minority of controls.38 Second, missing data are inevitable in questionnaire studies. Although certain factors including immunisations and infections have had lower response rates, major factors studied which showed significance have received >90% responses. Third, the findings especially questions regarding early lifetime factors are likely to be subjected to recall bias. Compared with hospital-based or retrospective cohort, the inception nature of this study decreases recall bias although elimination of any recall bias is unavoidable in questionnaire-based study. We believe that some factors may be less affected by recall bias than others, for example breast feeding. Wherever possible, data were recorded categorically with ‘yes,’ ‘no’ and ‘unsure/don't know’ to reduce information bias due to participants being forced to provide an answer for questions that they were unsure of. Bias may also be introduced since healthcare professionals interviewed the patients for the questionnaire. Fourth, false positive results may occur due to chance arising from the evaluation of 87 questions. Last, to our knowledge, formal validation of the IOIBD questionnaire has not been conducted.11 Nonetheless, analyses of test–retest reliability indicated moderate to high reliability across all questions used in the questionnaire. Because questionnaires designed for IBD patients outside of Asia may not be representative of Asian countries, there is a need for a modified questionnaire related to Asian subjects, particularly regarding dietary factors, and for it to be prospectively validated.

In conclusion, we reported for the first time in a population-based cohort in Asia-Pacific the role of dietary and immunological alterations early in life and development of IBD. CD and UC share overlapping environmental factors. We found a duration-response protective association between breast feeding and disease development, and contact with childhood pets is a novel protective factor. These observed associations indicate that early childhood factors and markers of altered intestinal microbiota including antibiotic use may modulate the risk of IBD later in life and that this period requires further evaluation. The strong protective association between breast feeding could potentially motivate female patients with IBD to breast feed, especially since their children have an increased risk of the disease. Attention should be directed from solely investigating the organisms that cause the disease toward factors associated with protection against its occurrence.