Elite marathon runners can lose up to 10 pounds (4.5 kg) during a single race, mostly through fluid loss and energy depletion.

The main driver of weight loss during a marathon is thermoregulation through sweating. Research published in the Journal of Applied Physiology shows that elite runners can lose between 0.5 to 2.5 liters of sweat per hour, depending on temperature and intensity. For a runner crossing the finish line in just over two hours, this totals more than 4 liters of water weight.Beyond fluid loss, the body also depletes glycogen—energy stored in muscles and the liver. A typical adult stores about 500 grams of glycogen, which is chemically bound to roughly 1.5 kilograms of water. As the body burns through these reserves over the 42.2-kilometer distance, that associated water is released and excreted, adding to the weight loss on the scale.The International Marathon Medical Directors Association (IMMDA) notes that losing 2% to 3% of body mass is common and generally safe for trained athletes. Elite competitors sometimes push further, losing up to 8% of body weight under extreme conditions. This is temporary: the body rapidly restores fluid balance and glycogen levels within 24 to 48 hours after the race.