The study also found that women with a farm upbringing were more likely to have stronger lungs.
These findings were remarkably consistent across many countries, “suggesting that farming effects may be due to biological mechanisms rather than socio-cultural” ones,” say the researchers.
As background information in the study points out, the incidence of asthma and allergies has risen dramatically in recent decades. Many theories have been proposed for this increase, including the so-called hygiene hypothesis, the idea that a lack of exposure in childhood to various microbes and allergens (partly from living in homes that are “too clean”) can weaken the immune system and lead to illnesses.
The hygiene hypothesis was first proposed when researchers found that children raised in large families were less likely to develop hay fever, possibly because their siblings had exposed them to more germs.
Similar findings have also been found among children living on farms. But little research has been done to see if this protective “farm effect” persists into adulthood. The current study set out to fill that gap.
For the study, researchers analyzed data collected from more than 10,000 people, aged 26 to 54, who had volunteered to take part in the 14-country European Community Respiratory Health Survey II. (The survey’s title is a bit of a misnomer, as about 500 of the participants were Australians.)
The volunteers were asked where they had lived before the age of 5 — on a farm, in a country village or similar rural-like setting, in a small town or city suburb, or in an inner city.
They were also asked other environmental questions about their early life, including the number of children in their family (and if they had had to share a room with any of them), the number of pets (cats and dogs), and the size of any day-care or nursery-school classes they attended.
Each participant was tested for lung strength and sensitivity to allergens. The participants were also asked if they had a personal or family history of hay fever, asthma, or bronchial hyper-responsiveness (over-reactive airways).
Most of the study’s participants — 65 percent — had lived in a rural village, small town or city suburb before the age of 5. Another 27 percent had spent their early childhood in an inner city, and 9.2 percent had been raised on a farm.
The data showed that people who grew up on a farm were more likely to have shared a bedroom at an early age with a sibling and to have had dogs or cats as childhood pets. In addition, a family history of allergies was less prevalent in this group.
But it was where the study’s participants had spent their first five years — not the other biodiversity factors — that was most predictive of whether they developed allergies or asthma when they became adults. The former farm kids were 54 percent less likely to have asthma or hay fever and 57 percent less likely to have allergic nasal symptoms than the adults who had grown up in an inner city.
The people who were raised in a village, town or suburb were only slightly less likely to have asthma or hay fever as an adult, and no less likely to have allergic nasal symptoms, than those raised in an inner city.
The farm-raised adults were also 53 percent less likely to be sensitized to allergens than the urban-raised ones.
No difference in lung strength was observed among the groups — except among women with a farm upbringing, who had stronger lung function, on average, than people in any of the other groups. The researchers say they don’t know why this association was found only among women.
Caveats and implications
“As any parent with a small child knows, childcare centers are hotbeds of viruses and bacteria, but it turns out that’s nothing compared to a farm,” said lead author Brittany Campbell of the University of Melbourne’s Allergy & Lung Health Unit, in a released statement.
“We found that for kids in villages, towns, suburbs and cities, not even daycare or living with cats, dogs and older siblings came close to endowing the protective effects that appear to come with life on a farm,” she added.
Yet, as Campbell and her co-authors point out, this is an observational study, so it cannot definitely prove that growing up on a farm offers benefits in terms of lowering the risk of allergies and asthma. Nor is this study proof of the hygiene hypothesis.
“We still don’t know what exactly in farming is driving this association,” said senior author Shyamali Dharmage, also of the University of Melbourne, in a podcast posted on the Thorax journal’s website. “It could mean that … you are exposed to a whole lot of microbes. It could be that you are exposed to less air pollution. It could be that you are more physically active. It could be related to the lifestyle of those living on farms, such as less medication use.”
In other words, much more research will need to be done. Stay tuned.