You need to take in large amounts of food when you exercise for more than a few hours, otherwise you will slow down and eventually have to stop. In the Race Across America, four cyclists alternated shifts as a relay team and completed the race distance of 2800 miles in 6 days, 10 hours and 51 minutes. Each rode up to 10 hours per day in approximately one hour shifts. Even though they cycled only a quarter of the time and distance, they each burned an average 6,420 calories per day, compared to the average for North American men of a little over 2000 calories per day. They ate and drank as much as they could but were able to take in only 4918 calories/day, for a deficit of 1503 calories per day (International Journal of Sports Medicine, July 2010).
Six years ago, a 33 year old bicycle racer used a continuous heart rate monitor to show that he used up more than 18,000 calories per day in the same race. He rode for 20 to 24 hours/day, sleeping no more than 4 hours/day. Yet he could eat only about half that much (9000 calories per day), and he lost 11 pounds of body fat in the nine days of competition (International Journal of Sports Medicine, July-August 2005).
More than 75 percent of North American adults weigh more than they should because they exercise too little and eat too much. These studies show that during long-term continuous intense exercise it is impossible to meet your needs for food, no matter how much you try to eat.