The Tour de France riders burn more calories per day than running a marathon

Professional Tour de France cyclists burn between 6,000 and 8,000 calories per day during the race's three weeks — compared to approximately 2,600 for running a marathon. The overall calorie deficit over the three-week race is so severe that riders typically lose 5–8% of their body mass even while eating constantly. Teams employ nutritionists to deliver specialised food in musettes (feed bags) at pre-designated points and in team cars. Studies of Tour riders found that their gut microbiome — optimised for extracting energy from carbohydrates — is measurably different from that of sedentary people.

Source: Jeukendrup et al., International Journal of Sport Nutrition (2000)