In this episode Olly wants to find out exactly how overweight he is and whether he needs to lose weight for medical reasons. Olly has a full body scan and Dr. Sadaaf Farooqi explains why Body Mass Index is not always a reliable measurement and how being overweight can affect your health.

Medical historian Louise Foxcroft takes us through how our idea of the ideal body weight has changed over time.

Giles and Olly ask why we are fat: is it because of what we eat, how much we exercise, or is it because of our genes? Italian podcast producer Jonathan Zenti tells us how he gained almost 30kg in one year and why he loves Jane Fonda workout videos.

New York Times fitness columnist Gretchen Reynolds talks about the start of the exercise boom in the United States and obesity expert Dr. Sadaaf Farooqi explains how she found one of the first genes linked with obesity.

Olly and Giles explore the world of dieting: what’s the appeal of fad diets and what are the scientific kernels of truth in them? Chef Anthony Warner helps Giles separate the real science from the pseudoscience while cooking. Medical historian Louise Foxcroft tells us about some of the more extreme historical fad diets and Olly contemplates the dangers of quick fixes.

Olly and Giles look at what constitutes a healthy diet, and how to achieve that without spending too much time or money. Nutritionist Dr. David Katz explains what carbs, proteins, and fats are, and why we shouldn’t get bogged down in calorie counting.

Dr. Pablo Monsivais lays out the socio-economics of obesity: how our income and gender might affect what we weigh. Food writer Jack Monroe gives Olly some hard-earned advice about shopping nutritiously without breaking the bank.

Olly and Giles ask why it’s so hard to lose weight. Neuroscientist Dr. Michael Graziano tells us about a study he conducted on himself, trying to understand the psychological factors behind his eating patterns. Another neuroscientist, Dr. Hannah Critchlow, explains why our brains are hard-wired to seek out high calorie foods and how to work around this.

Giles has a scan of his own brain. Dr. Clare Llewellyn and Giles talk about how our genes can also impact how much and what we like to eat.

Giles and Olly look at the most difficult part of losing weight: how to maintain the weight you’ve lost. New York Times Fitness columnist Gretchen Reynolds tells us why exercise is crucial to maintaining a steady weight. Dr. Sandra Aamodt explains why our brains hate when we start losing weight and why she practises something called “mindful eating”.

Professor of Behavioural Medicine, Dr. Paul Aveyard, gives Olly some concrete tips about how to create eating habits that are sustainable in the long term.