In ultra-endurance races, athletes face limits in nutrition regarding energy and fluid metabolism. An ultra-endurance performance lasting for 24 hours or longer leads to a mean daily energy deficit of ~7,000 kcal. This energy deficit leads to a decrease in body mass, covered by a decrease in both fat mass and skeletal muscle mass. The energy deficit cannot be prevented by adequate energy intake. To avoid dehydration during an ultra-endurance performance, adequate fluid intake is required. In case of fluid overload, both exercise-associated hyponatremia and swelling of limbs may occur. Adequate ad libitum fluid intake of ~300-400 ml per hour may prevent both exercise-associated hyponatremia and swelling of limbs. To summarize, in ultra-endurance races, an energy deficit seems to be unavoidable. Potential strategies might be to increase pre-race body mass by a diet to increase fat mass and/or strength training to augment skeletal muscle mass. Another possibility could be increasing energy intake during racing by consuming a fat-rich diet. However, future studies are required to investigate these aspects.