Why does the marathon push the human body to its absolute limits? And why do some people seem built to keep going when everyone else hits the wall? On this week’s podcast, Professor Luke O’Neill takes a biochemical deep dive into marathon running, sparked by a listener’s request. The modern marathon may trace its roots back to Ancient Greece, but what happens inside the body during those 26.2 miles is a very modern scientific story — one that turns runners into walking, sweating demonstrations of bioenergetics. 

Luke explains how the body powers long-distance running by converting energy rather than creating it, moving between carbohydrates and fats to keep muscles firing. ATP — the energy currency of life — sits at the heart of the process, with phosphocreatine, glycolysis and oxygen all playing starring roles. When carbohydrate stores finally run dry, runners hit the infamous “wall”, a moment when the body is forced to switch fuel sources, and everything suddenly feels much harder. 

The podcast also looks at how training physically reshapes marathon runners over time: denser networks of capillaries in muscles, powerful hearts with remarkably low resting heart rates, and lungs capable of shifting huge volumes of oxygen. Luke explores why elite runners can seemingly run a marathon at will. 

There’s science behind the mental side too. Endorphins and the so-called “runners high” can lift mood for days, while visualisation plays a key role in endurance. Luke even dips into the Guinness Book of World Records to uncover astonishing marathon facts, including runners in their 90s and some jaw-dropping physiological extremes. 

Along the way, Luke admits he’s never run a marathon himself — but from a biochemist’s point of view, few sports are more revealing of how the human body really works. 

You can suggest future topics by emailing Luke at [email protected].