Ultra-endurance athletes try to extend their limits in performance. In ultra-endurance races, athletes face limits in nutrition regarding both energy intake and fluid metabolism. The purpose of this review is to focus on the decrease in body mass, aspects of energy and fluid balance, and exercise-associated hyponatremia in ultra-endurance performance. An ultra-endurance performance lasting 24 hours or longer may lead to an energy deficit of approximately 7,000 kcal per day. This energy deficit may result in a decrease of body mass, including a decrease in both fat mass and skeletal muscle mass. The energy deficit cannot be completely compensated by increasing energy intake. Adequate fluid intake is required during an ultra-endurance performance to prevent dehydration. In case of fluid overload, both exercise-associated hyponatremia and swelling of limbs may occur. Limited fluid intake of approximately 300-400 ml per hour may prevent both exercise-associated hyponatremia and swelling of limbs. In summary, in ultra-endurance performances, an energy deficit seems to be unavoidable, and athletes are at risk to develop both exercise-associated hyponatremia and limb swelling in case of fluid overload.