We measured, during a marathon swimming event 2006 in the lake of Zurich, load intensity, energy intake, energy expenditure and impact on both muscle and fat mass in a trained long-distance swimmer. The swimmer had a load intensity of 125 bpm corresponding to 51% VO2(max). Despite adequate energy intake during the load, an energy deficit of 500 kcal per hour occurred, which resulted in a loss of 1.1 kg body mass within 9 hours of swimming.