Dogs may be man’s best friend, but what if they boost our well-being not just by being our furry companions, but by altering our microbiome? A series of experiments in mice suggests that dog owners have a unique makeup of bacterial species that encourage empathetic and social behaviours.

We know that pets improve  and play a role in shaping our gut microbiome. Research also increasingly suggests that this microbiome and even helps mould our personalities. With dogs typically topping popular pet lists, at Azabu University in Japan wanted to understand whether the animals change our microbiome in a way that prompts good well-being.

To explore this, the researchers analysed surveys where the caregivers of 343 adolescents – aged 12 to 14, who lived in Tokyo – reported on various aspects of their social behaviour, such as how often they felt lonely, were cruel to others or struggled to get on with their peers. The surveys also revealed that about a third of the adolescents had a pet dog.

The researchers found that those with dogs ranked as less socially withdrawn and behaved less aggressively than the non-dog-owners, on average. The team accounted for other factors that may influence such behaviours, such as sex and household income.

Saliva samples also revealed that several species of Streptococcus bacteria were more abundant in the adolescents with dogs, which has been .

“If you’re playing with a dog a lot, you’re going to have a lot of exposures to the microbes the dog has, from licks [and] them jumping up on you,” says at University College Cork in Ireland. These bacteria can travel down to the gastrointestinal tract, where they may , which improve mental health, he says.

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In a critical part of the study, the team transplanted oral microbes from three dog owners and three non-dog-owners into the stomachs of germ-free mice. Based on stool samples, they could tell that the microbes had reached the mice’s guts.

Over the next few weeks, the team had the animals carry out a series of behavioural tests. In one, the mice were placed in a cage with another mouse that was trapped in a tube. The researchers observed that the mice that received transplants from dog owners chewed the tube and poked their nose through holes in it significantly more often than those that received transplants from non-dog-owners.

This suggests that the former mice had more empathy and were trying to help, says Kikusui. We’ve recently learned more about care-giving among mice, with studies finding that they assist their pregnant companions with giving birth, and even give a form of first aid.

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In another test, the dog-owner transplant recipients sniffed at an unfamiliar mouse in their cage more often than the other group, which suggests they were more social, says Clarke. “These social behaviours are relevant across species, including humans,” he says. “Social networks are a positive thing for mental health – if you have low exposure to social networks, or if your social network is small, then that probably isn’t a good thing.”

Learning more about these microbial changes could one day benefit people without dogs, for instance, if we can develop probiotics that mimic them, says Clarke. But studies in other geographical locations, where microbial exposures may vary, are needed, he says.

Vegetarian and vegan dog foods just need a few tweaks to make them nutritionally complete diets.

Analyses reveal that meat-free dry kibbles meet dogs’ protein and fat requirements, lacking only sufficient iodine and B vitamins. With supplements or – better yet – improved commercial preparation, plant-based dog food could keep the animals healthy while reducing the environmental impact of the pet food industry, says at the University of Nottingham, UK.

“As long as they’re getting all of the essential nutrients from those ingredients, then the dogs are going to thrive,” she says.

Many vegans and vegetarians who own dogs struggle with the ethics of feeding meat to their omnivore pets, says , also at the University of Nottingham. Responding to that concern, pet food manufacturers have started offering plant-based foods to consumers.

Government organisations like the European Pet Food Industry Federation and the Association of American Feed Control Officials require standardised testing of commercial pet foods and additives to ensure they meet nutritional standards. Even so, most pet foods worldwide do not go through robust nutritional testing by independent research teams, says Gardner.

Brociek, Gardner and their colleagues analysed 25 commercial dry foods for healthy adult dogs – 19 containing meat and six based exclusively on plants. Among the latter, two were labelled as vegetarian and four were vegan.

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None of the foods met all the official nutritional guidelines for dogs, despite being packaged as nutritionally complete, the researchers say. Even so, all the foods had acceptable concentrations of protein, fatty acids and essential amino acids.

Most – including five of the six plant-based foods – didn’t have enough iodine, but seaweed could easily make up for that deficiency.

Vitamins in general were sufficient across the board, except for vitamin B, which came up particularly short in plant-based foods. Dogs low in vitamin B can have issues with their skin, nerves and digestive systems, so manufacturers should supplement those foods, the researchers say.

The findings point to oversights in all kinds of dog food manufacturing, not just those based on plants, says at Murdoch University in Australia. “Consumers are expecting that if products are labelled as being nutritionally sound, and that will be true, but clearly it’s not true,” he says.

“I believe a vegetarian diet can be adequate if carefully supplemented with nutrients lacking in plants – as is the case for humans who choose a vegetarian diet,” says at the University of Pisa in Italy. Owners should avoid making their own plant-based foods for their pets, she adds.

Pet dogs have evolved to be able to eat a wide variety of foods, so feeding them well-balanced, plant-based meals can satisfy their nutritional needs while avoiding the , like greenhouse gas emissions, says Gardner.

“They’re true omnivores,” he says. “As most Labrador owners – myself included – will tell you, they’ll eat anything.”

Read more: How a unique puppy kindergarten lab put the science into dog training