Giles Coren meets the most committed and extreme food zealots from across the globe, people who live by their health regimes like a religion.

Immersing himself in their worlds and living by their rules, he investigates three diets – Calorie Restriction, the Paleo and Fruitarianism – seeing for himself the possible advantages and drawbacks of each one by meeting people who have been living in this way for many years.

In New York, he learns that by restricting his calorie intake he could expect to live to 120 years old, and how an unusual transplant could help revolutionise his digestive system.

Heading west to California, Giles joins would-be cavemen and cavewomen on a primal retreat and discovers the Paleo diet. Inspired by our stone-age ancestors, it cuts out all food introduced since the Neolithic era.

In LA, he meets body builders, ultra runners, families and truck drivers who all adhere to Fruitarianism – a diet that consist entirely of fruit. Could all that time spent peeling and chopping be worth it?

But Giles is not there just to observe. Far from it. Challenging those who advocate each way of life, he looks at both the positive and negative sides of each regime, sharing his brutally frank opinions on the people he meets, food he eats, and the activities he participates in.

He also personally tests each of the diets, putting himself through some difficult, unpleasant and extraordinary experiences.