Giles, a geneticist at the University of Cambridge, uses a 21ft knitted scarf to give Tim a guided tour of his gut. Tim is an American chef who runs a busy Japanese restaurant in London and he is hoping to transform his energy levels, wellbeing and mood by focusing on his intestines. They learn about the importance of the gut microbiome, the vast ecosystem of bacteria, yeasts, fungi and viruses in our digestive pipes, which weighs more than the brain.

At Kings College London, Tim meets Professor Tim Spector (author of The Diet Myth) who analyses his microbiome through a stool sample. What will Professor Spector find in Tim’s gut?

Now that Tim has received the results of his microbiome profile, he wants to find out what’s good to eat for a healthy gut. Why should we be filling it with bacteria?

Tim and Giles sample kombucha, a sweet fermented tea and they hear from Sandor Katz who discovered fermentation after swapping a busy life in New York for off-grid commune dwelling in rural Tennessee. What are the health benefits of eating fermented foods?

At the APC Microbiome Institute at University College Cork, Tim meets Dr John Cryan and Dr Ted Dinan, who are world leaders in research on the connection between our gut and our mood. They’ll explain how what we eat not only affects our physical health, but also our mental health.

In the near future, could mood disorders be treated by changing our diet to consume better bacteria?