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?

To what extent can we change the state of our gut microbiome? In an episode centred on the beginning and end of life, Giles hears of some innovative and controversial studies. How is the bacteria in your gut established by the way in which you’re born?

Maria Gloria Dominguez Bello from Rutgers University describes her experiments of introducing babies born by C-section to bacteria from their mother’s birth canal. Dr Claire Steves from Kings College London who researches the microbiome and dementia, reveals how the gut may affect how we age.

Giles and Tim investigate the effect of a vegetarian and vegan diet on the intestines with Dr David Katz, director of Yale University Prevention Center and an internationally renowned authority on nutrition. The historian Dr Elsa Richardson from the University of Strathclyde describes how the roots of the vegetarian diet go back further than we may think. How did the Victorians try to improve their gut health?

In the last episode in the series, Tim and Giles find out how they can both physically change how they eat in order to improve their gut health and overall wellbeing. Giles meets British food writer and historian Bee Wilson, the author of First Bite: How We Learn To Eat. How can repeated exposure to certain foods change our tastes?

We also hear from BJ Fogg who runs the behavioural change lab at Stanford University on how to establish new eating habits. How has Tim transformed his gut over the course of the series?