MedWire News: Israeli study results provide further evidence to suggest that high levels of hygiene in childhood may be associated with an increased risk of inflammatory bowel disease.

The 'hygiene hypothesis', which was first proposed in the late 1980s to explain the rise in allergic conditions, argues that exposure to bacteria primes a child's immune system to combat the development of certain diseases and conditions.

Therefore, high levels of hygiene during childhood may not allow this priming to occur, resulting in the development of conditions characterised by an over-reactive immune system, such as inflammatory bowel disease.

To investigate further, Dr Eran Israeli, from Hadassah Medical Center in Jerusalem, and team studied the incidence of inflammatory bowel disease and surrogate markers of hygiene, such as number of siblings, birth order and living environment, among 399,251 Israeli adolescents aged between 16 and 19 years.

In total, 768 participants had been diagnosed with inflammatory bowel disease.

The researchers found that adolescents with just one sibling were more than two-and-a-half times more likely to have inflammatory bowel disease than those with five or more siblings.

Birth order also appeared to be associated with the risk of inflammatory bowel disease, with fifth-born participants having a 2.4-fold greater risk of the disorder than first-born participants.

Furthermore, living in an urban environment was associated with a 1.4-fold increased risk of inflammatory bowel disease compared with living in a rural environment.

Writing in the American Journal of Gastroenterology, Dr Israeli and team conclude: "Surrogate markers of childhood hygiene are associated with the risk for inflammatory bowel disease, thus reinforcing the 'hygiene hypothesis'."

They add: "The effects of poor hygiene are diverse. In some situations, poor hygiene can lead to increased pathogenic infections, but in others, it results in a higher exposure to harmless micro-organisms and priming of the regulatory immune system, thus lowering the risk for development of inflammatory bowel disease."