As js2 notes, it's not the calories that runners are worried about during a marathon. They're worried about ingesting carbs to supplement what they have stored

As js2 notes, it's not the calories that runners are worried about during a marathon. They're worried about ingesting carbs to supplement what they have stored via carb-loading over the previous day(s) before the marathon. Kipchoge has used Maurten supplements (see https://www.maurten.com/ https://www.maurten.com/ ) for a previous race. The commentators during the Ineos159 broadcast also referred to his use of Maurten supplements every 5km during the Ineos159 run. Why Maurten? One of the supposed benefits of Maurten GEL is the use of two forms of carbs (maltodextrin and fructose). Early carb supplements (Gatorade, etc) typically used one type of sugar to supply carbs to the body. The rate of carb absorption by the body was thought to be 1 gram per minute (60g/hr). Researchers discovered that fructose was ingested via a different pathway than other sugars and when combined with another sugar could increase the absorption rate of carbs up to 90g carbs/hr. see https://www.torqfitness.co.uk/news/maltodextrinfructose https://www.torqfitness.co.uk/news/maltodextrinfructose for more info. Kipchoge was taking drinks every 5km, at target pace of 21.1km/hr - 21.1/5 = ~4 times/hr Each serving of Maurten GEL has 25g. So, assuming each drink is close to one serving of GEL, he'd get ~100g carbs/hr which his body would absorb close to 90%. The support team also believed that the cause of Kipchoge's slowdown in the previous Breaking2 attempt was that Kipchoge wasn't supplementing enough and hit the wall near the end of the run. To reduce the chance of that happening, they gathered the used drink bottle after Kipchoge was finished and analyzed how much supplement was remaining. Not sure what they would do if they found he wasn't drinking enough other than verbal encouragement.