Living with a dog is good for you — we already knew that: it helps combat loneliness and depression, reduces the risk of heart attacks and halves the likelihood of developing allergies. According to new research under way in the United States, the intestinal bacteria of our dogs could have benefits for our bodies, contributing to our overall wellbeing.

The study, conducted by researchers at the University of Arizona, is trying to establish whether it is the microorganisms that dogs share with us that have a positive effect on our physical and mental health. "Humans and dogs have evolved side by side over millennia, yet we still haven't managed to understand the secret behind the special bond between the two," said Kim Kelly, lead author of the study and anthropologist at the University of Arizona. "The answer may be hiding just beneath the surface."

The hypothesis is based on previous research suggesting that dogs and their owners, after living together in the same home for a while, share the same types of intestinal bacteria. A healthy population of microorganisms is essential for physical and mental health, particularly in old age; the researchers' aim is precisely to understand whether dogs are unwittingly helping their owners strengthen their immune systems.

To test this theory, Kelly's team is recruiting volunteers over the age of 50 to analyse, over a three-month period, how living with a dog may encourage the growth of beneficial microorganisms in the human gut.

The study will examine several factors, including intestinal bacteria, diet, levels of physical activity and immune function. Participants must not have taken antibiotics in the six months prior to the study, nor have previously lived with a dog.

"We think that dogs could act as probiotics to improve the health of the bacteria living in our gut," said researcher Charles Raison. "The collective community of these bacteria, known as the microbiota, plays an important role in our mental and physical health, particularly as we age."