Chris and Xand are doctors, scientists and identical twins. Well, not quite identical. Xand is 20kg heavier, clinically obese, and has a Covid induced heart condition.

Chris believes that the reason Xand is overweight is the same reason that most of us in the UK are overweight - Ultra Processed Food or UPF. It’s the main thing that we now eat and feed to our children, but most of us have never heard of it. It’s addictive, highly profitable and the main cause of the global obesity pandemic. It’s destroying our bodies, our brains and the environment.

In this series, recorded during the first coronavirus lockdown of 2020, Chris wants to help his brother quit UPF and get his health back. So, he has a plan. In an attempt to turn Xand's life around, Chris persuades his brother to eat a diet comprising 80% Ultra-processed food while learning about every aspect of it. By doing this, Chris tests two theories - that Xand is addicted to UPF, and that eating more of the stuff while learning about it, will help him quit. Chris believes that the science shows UPF is addictive and harmful to the body, not least by driving excess consumption and weight gain. By speaking with the world’s leading experts on obesity and nutrition, Xand will learn what UPF is made of, how it’s produced, whether it’s addictive, what it does to the human brain and body and how it is the number one force driving global obesity.

In episode four - Building an ultra-processed body - Xand starts the diet, eating 80% UPF while continuing to learn about it. Chris thinks the diet will work because he’s done it, subsisting on a junk food diet for four weeks while investigating the causes of childhood obesity in a BBC TV documentary. In order to understand the direct effect UPF is having on his body - appetite, hormones and weight - Xand speaks to obesity expert, Dr Janine Makaronidis from University College London. Xand also interviews Dr Kevin Hall from the National Institute of Health in the US about his ground-breaking research into the consumption of UPF. Kevin was the first scientist to prove that UPF causes weight gain.

Presented by Drs Chris and Xand Van Tulleken Produced by Hester Cant Executive Producers Philly Beaumont and Jo Rowntree A Loftus Media and van Tulleken Brothers Ltd production for BBC Radio 4