We invite Dr Sam Impey of Hexis back to the show to discuss the concept of maximal sustainable energy expenditure and its potential to unlock endurance performance. We explore the International Atomic Energies WLO Walter database, which measures energy expenditure in professional athletes, and dive into the gold standard of measuring energy expenditure: Double labeled water. We look at the limitations of these calculations and the difficulty of capturing exercise energy expenditure accurately. Plus, we explore the suggested ‘upper limit’ of two and a half times resting metabolic rate for exercise energy expenditure. We discuss fueling strategies in elite cycling and how training the gut can help athletes reach the upper limits of what they can digest, both in the particular day of racing and in the meals that follow. Plus we examine the importance of sustaining energy levels throughout the day. LINKS: Dr Sam Impey on Twitter at https://twitter.com/SamImpey_    Hexis at https://www.hexis.live/   Hexis Performance Lab at https://www.hexis.live/performance-lab