A number of explanations have been suggested to explain the differing rates of inflammatory bowel disease (IBD) between the Western world countries (North America, Europe, Australia and New Zealand) and newly industrialized countries, such as those in Asia. While rates of IBD in Asia are low compared to that of Western countries, there is a significant increase in diagnosed cases of IBD in Asia.
Environmental risk factors are thought to be a driving factor in the rise of IBD throughout the world. But, do environmental risk factors influence the development of IBD similarly across populations? Recent studies have shed light on this question – suggesting that some environmental risk factors are shared across populations, whereas others are quite different. This article reviews the ACCESS study cohort led by Dr. Siew Ng. The ACCESS study compared environmental risk factors in patients diagnosed with IBD from eight Asian countries to patients with IBD in Australia. This article looks at some of the environmental risk factors for IBD that are believed to either protect against or contribute to the development of IBD, and include:
- Cigarette smoking
- Physical activity
- Hygiene and antibiotics
Breastfeeding has been shown to reduce the risk of developing IBD in both Western countries and in Asia when infants were breastfed for more than 12 months. In Asia, infants who were breastfed were seven to 10 times less likely to develop either Crohn’s disease or ulcerative colitis.
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While smoking is known as a risk factor for developing Crohn’s disease in the Western world, this association may not be consistent throughout the world. For example, one study demonstrated that smoking was not a risk factor for Crohn’s disease for individuals living in countries in Asia, despite high rates of smoking, but quadrupled the risk of developing Crohn’s disease in Australia. In contrast, quitting smoking increased the risk of developing ulcerative colitis in both the Western world and in Asia.
Consistent physical activity may help prevent the development of Crohn’s disease. A study conducted in Asian countries showed that those with sedentary lifestyles were twice as likely to develop Crohn’s disease as more active individuals.
Hygiene and antibiotics
The hygiene hypothesis, postulates that improved sanitation increases the risk of developing IBD. This view states that modern, sanitized conditions expose children to fewer microbes and therefore their immune systems may not fully develop. Consequently, these children become more susceptible to IBD later in life. The hygiene hypothesis is a widely accepted theory in the Western world. It is also substantiated by several studies from North America that have shown that exposure to antibiotics in childhood increase the risk of developing IBD later in life. In contrast, studies from Asia show that children exposed to antibiotics and improved sanitary conditions (flush toilets) had a protective factor against ulcerative colitis. So, these mixed results suggest that different environmental factors may influence the development of IBD in Asia.
Though several environmental risk factors are undeniably involved in the development and spread of IBD globally, it is unclear which risk factors play the greatest part in the ongoing development of IBD. The contrasting findings of particular risk factors between Western and Asian countries are not surprising given the genetic susceptibility of individuals in different cultures and exposure to very different environmental interactions that affect the composition of intestinal microbes.
What seems more certain is that IBD is emerging as a global disease and appears to be on the rise. Future studies are needed to better understand the common and the contrasting risk factors between Western and Asian countries.
Note: This article compares environmental risk factors that are shared and dissimilar between Asia and the Western world. These comments are discussed in a commentary by Dr. Gil Kaplan published in Nature Reviews Gastroenterology and Hepatology. For a more comprehensive overview of all the environmental risk factors studied in IBD, please see this open access article published in the Canadian Journal of Gastroenterology and Hepatology.