The Hygiene Hypothesis


When my now-toddler was an infant, we followed the “hygiene hypothesis.” This is the theory that childhood exposure to pathogens affects the risk of disease later in life. We passed our son around to anyone who wanted a cuddle, washed his bottles and my breast pump parts with soap instead of sterilizing them, and let him get dirty when it came to playtime. Our dog, Bowie, was also often in the house and loved to play gently with our son once he was up and crawling.

A series of studies soon to be presented at the American Academy of Allergy, Asthma and Immunology’s Annual Meeting claim that an infant’s immune system development and susceptibility to asthma and allergies may be influenced by a number of factors that shape what bacteria is in their gut.  

The research team, lead by Dr. Christine Cole Johnson, says the findings further support the “hygiene hypothesis.”  

“For years now, we’ve always thought that a sterile environment was not good for babies. Our research shows why. Exposure to these micro-organisms, or bacteria, in the first few months after birth actually help stimulate the immune system,” says Dr. Johnson. “The immune system is designed to be exposed to bacteria on a grand scale,” she adds. “If you minimize those exposures, the immune system won’t develop optimally.”

The researchers analyzed stool samples obtained from babies at one month and six months old. The results of their study found that a mother’s race, the baby’s gestational age at birth, exposure to tobacco in the womb or after birth, the presence of pets in the home and whether or not the baby was born via Caesarean section all influenced an infant’s gut microbiome composition. 

Whether or not the baby was breastfed also had a significant impact. Babies who were breastfed at one and six months have specific microbiome compositions that were different to formula-fed babies. Babies who were breastfed at one month had a lower risk for pet-related allergies. This backs up the results of a 2014 Danish study that found that longer breastfeeding duration was associated with higher levels of good gut bacteria in infants. 

Other studies back up Johnson’s work. One 2014 study found that exposing babies to bacteria and allergens before the age of 12 months may reduce their lifetime risk of allergies, wheezing and asthma. 

Dr Johnson says the research proves that exposure to a higher and more diverse amount of environmental bacteria and specific types of gut bacteria appear to boost the immune system, protecting against allergies and asthma. 

Our son is now three years old, and (touch wood!) he’s never been sick. Even attending a local daycare with a half-dozen other children who frequently have coughs and colds, he’s healthy and happy. I’m happy we followed the “hygiene hypothesis” for another reason, too – who has time to sterilise all those bottles and pump parts? 

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