The Hygiene Hypothesis -- Redefine, Rename, or Just Clean It Up?

Linda Brookes, MSc; Laurence E. Cheng, MD, PhD


April 06, 2015

In This Article

What Do Recent Studies Add to Our Knowledge?

Two recently published studies reported by Medscape[19,20] appeared to lend further support to the hygiene hypothesis, although, as Dr Cheng pointed out, "They did not really answer the same question." One of the studies, carried out in more than 1000 Swedish children aged 7-8 years found that children living in households where dishes were washed by hand had a 43% lower risk of developing allergic disease, primarily eczema, than children from households with a machine dishwasher.[21] The investigators concluded that the less-efficient hand-dishwashing method probably induced tolerance by increased microbial exposure. In the accompanying editorial, however, Dr Cheng and his coauthor cautioned that the study had "definite limitations."[18] "It is interesting; it is an association, but in many ways, as our commentary laid out, there are some holes in their argument," Dr Cheng told Medscape. "The study looked at 8-year-olds as opposed to very young children—less than 6 months old—in whom we believe this sort of protective effect happens," he explained. He also questioned how only hand dishwashing showed an effect despite "much clearer and heavy microbial exposures" seen with ingestion from fermented foods or [unpasteurized dairy] foods from a farm that appeared to have no effect. "These associations, although they can drive the formation of testable hypotheses, remain just associations," he stressed.

Despite adjustment for a number of potential confounding variables such as pet keeping and parental history of allergy, Dr Cheng believes that hand dishwashing in the study was a proxy for the real reason behind the effect. "Hand dishwashing vs machine dishwashing is probably passed down culturally; maybe you do it, along with a number of other learned behaviors, because it's what your parents did," he suggested. "People who want to use this study to support their arguments will use it, whereas others will say this is yet another study that doesn't bring us closer to a genuine understanding," he cautioned. "I am somewhere in the middle; I think it is interesting, but I would not use this paper as a reason to stop using my machine dishwasher."


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