"In general, athletes' perceptions are definitely off. They usually think they need far more than they do," Shutt says. And they usually think it all needs to come in the form of carbs: pasta, rice, and pizza.
According to Shutt, if you're training about 20 hours per week (standard for most athletes preparing for an Ironman), then you're burning an average of just 2,500-4,500 calories per day, depending on your size, metabolism, gender, and type of training. That's just an average, though.
Sure, if you're a big guy, then on your long training day of a six-hour ride and an hour-long run, you might literally need 10,000 calories, because you might have actually burned 6,000-7,000 just in training. But, there's probably also a day where you're burning only 2,500-3,000 calories, including both your baseline and training. If you're a small woman, then those numbers could be lower.