Serious triathlon training involves energy expenditures upwards of 5,000 to 6,000 calories a day. Maintaining the level of food consumption for this kind of output means that triathletes in training are basically eating lots, and eating often. Renowned celebrity chef and Ironman triathlete Rocco DiSpirito recommends eating lots of bananas, peanut butter & jelly, lean proteins, fruits, vegetables and organic chicken stock to get the food intake levels up to that point.
Here's footage from the 2010 Hawaii Ironman triathlon world championships. The annual event in Kailua-Kona is the culmination of a series of Ironman triathlon qualification races held worldwide, and is generally regarded as an honored and prestigious event among triathlon athletes.