More research is needed to explain how lifestyle choices, dishwasher use and children's exposure to microbes influence risk of developing allergies and related diseases.
The study results - published in the journal Pediatrics - support the currently popular "hygiene hypothesis."
According to proponents of the hygiene hypothesis, children in developed countries grow up in an over-sanitized world, where detergents and relatively little exposure to animals prevent our immune systems from becoming acquainted with and building up an immunity against common bacteria. Consequently, the theory suggests, the immune system misfires when it encounters these microorganisms, leading to allergies, eczema and asthma.
Previously, the allergists behind the new study, from the Department of Pediatrics at the University of Gothenburg in Sweden, found that if parents suck their baby's pacifier it may reduce the child's risk of developing allergies. The researchers thought this reduced risk was caused by a change to babies' microbiomes following exposure to their parents' saliva.
For the new study, the Gothenburg team surveyed the parents of 1,029 children aged 7-8 years who lived in two regions of Sweden. The parents answered questions on their children's allergies and whether they had asthma or eczema, and provided information on how the family washes their dishes and how often they eat fermented or farm-fresh foods.
The data from the questionnaires show that children from families that hand-washed their dishes had lower rates of allergies than children from families who used a dishwasher. The kids from families who did not use a dishwasher also had significantly lower rates of eczema and slightly lower rates of asthma and hay fever than their dishwasher-owning peers.
Children who ate more fermented or farm-fresh foods also exhibited lower rates of allergies. The children with the lowest rates of allergies in the study were those whose families both hand-washed their dishes and ate a lot of food that came directly from farms.
Previous studies have shown that dishwashers are much more effective at removing bacteria than washing by hand, which means that families are exposed to a greater range of bacteria when eating from hand-washed crockery and cutlery.
Results are 'interesting,' but more research is needed
A linked commentary from physicians at the University of California-San Francisco (UCSF) says that while the results are interesting, it is too soon to put them into practical use. The physicians state that more research is needed to explain how lifestyle choices, dishwasher use and children's exposure to microbes influence risk of developing allergies and related diseases.
The UCSF researchers say that according to the hygiene hypothesis, the greatest protective effect on the immune system occurs before infants reach 6 months of age. However, this creates problems for the findings of the study, because babies this young would have very limited exposure to hand-washed dishes and utensils - especially if they were breastfed.
Other potential confounding factors that may have affected the study's results include housing, socioeconomic status and immigration status, all of which are linked with allergy risk.
In contrast to the suggestions made by the new study that dishwashers over-sanitize our living environments, a 2011 study published in the British Mycological Society journal Fungal Biology found that 62% of tested dishwashers contained potentially pathogenic fungi on their door seals.
Last week, Medical News Today looked at a study published in the British Journal of Dermatology, which found that incidence of work-related dermatitis among health workers in the UK had increased four-fold following a national campaign against superbugs that required hospital workers to wash their hands more frequently with soap.