Poo.

It’s the one conversation topic that most young children seemingly never tire of.

And if you’re the parent of a young child, you might think you’ve had enough poo talk to last a lifetime.

But a Victorian couple want you to use that poo chat to help both you and your child live a little longer.

Felice Jacka and Rob Craw have created a new book, There’s a Zoo in My Poo, which teaches adults and children how eating fruit, vegetables and wholegrains helps their mental and physical health.

It is a book that Felice, who is professor of nutritional and epidemiological psychiatry at Deakin University, is uniquely qualified to write.

Not only has she led important research into gut health and its effects on the brain, she is also well versed in poo jokes.

“We have so many poo jokes in our lab because we collect so many stool samples for the research that we do,” Felice tells ABC RN’s Life Matters.

“Poo is often in our conversations.”

The “zoo” in the book’s title refers to the microbes that live in the intestines and come out in faeces.

There are approximately 40 trillion bacteria living on and in the human body — about the same as the number of cells.

As the book notes, that means “you are as much bug as you are human”.

Rob says his illustrations of the bacteria were “resurrected deep from my subconscious imagination”.

“It was a process of finding characters that would engage those young readers with the information,” he says.

But the text of the book is grounded in scientific fact.

Junk food ‘shrinks your brain’

In general, eating healthily encourages the bacteria that help keep you healthy, while eating fatty, sugary or salty foods hurts those good bacteria while encouraging bacteria that make you sick.

But our diet not only affects our physical health — it also influences us mentally.

In 2015, Felice led research that generated international headlines proclaiming “junk food shrinks your brain”.

“We did the first study that linked the quality of people’s diets to the size of their hippocampus,” Felice says.

“This is a key part of the brain that’s essential for learning and memory as well as for mental health.”

Parents who improve their own diet are also helping any future children they may have, according to a 2013 study Felice led, which looked at more than 23,000 mothers and their babies.

The study examined the quality of the mothers’ diets during pregnancy and tracked the children’s emotional health over their first five years of life.

“Mums who had more junk and processed foods in their diet, even when we took into account a whole host of other really important factors that might explain this, their children had higher levels of behaviours that we know are linked to mental health problems later on,” Felice says.

Felice also points to research showing obese men are more likely to father children diagnosed to be on the autism spectrum.

“That might shock some people,” she says.

‘The king of poos’

Each section of the book starts with Dr Seuss-like rhyming couplets and Rob’s illustrations of the various gut microbes appear to be influenced by the US author’s fanciful characters.

The cartoonish microbes appear throughout the book eating various foods, attacking each other and even playing with a dog.

Towards the middle of the book, two microbes hold a hand-illustrated Bristol Stool Chart.

The chart, which was developed in 1997 as a diagnostic tool by the Bristol Royal Infirmary, classifies human faeces into seven categories, providing a visual representation of each.

The categories range from type 1, which is the hardest, to type 7, which is “entirely liquid”.

“The king of the poos is the type 4, which is like a sausage and smooth and soft,” Felice says.

“But a type 3 is OK as well — that’s like a sausage with cracks in the surface.”

She says examining your poo can give you an insight into the health of your body, letting you know if, for example, you need to eat more fibre or drink more water.

Rob says he hadn’t heard of the Bristol Stool Chart before Felice told him about it while they were devising the book.

“I thought this is fantastic, because this will give me a good opportunity to use the science and marry it with some interesting images of a good bug and a bad bug.”

Getting kids onboard

Felice says unhealthy diet is now the leading cause of illness and early death worldwide.

“[Being] overweight now kills more people than underweight,” she says.

Packaged junk foods are now available everywhere from petrol stations to swimming pools, Felice says, and are designed to interact with our brains’ reward systems.

She says simply telling people to avoid junk foods because they’re unhealthy “just doesn’t cut through”.

“We wanted to create something that was very accessible for people, something very easy to understand — but also concrete.”

By detailing how the food they eat affects their bodies and brains, Felice and Rob hope to inspire children to make better food choices for themselves — which will in turn influence what foods their parents buy.

And Felice says the early feedback from parents is promising.

“They’re starting to come to us saying: my three-year-old this morning ate a banana and said, ‘man, the bugs in my bum are going to love this’. So I think it’s a concept that most kids will get on board with.”