The human body has limited glycogen reserves (about 90 minutes of intense effort). For a long or extra-long triathlon, you need to regularly provide "fuel" from outside sources. The golden rule is to test your nutrition during training (the famous "Gut Training") to get your stomach used to digesting during exercise.
It's all about timing and digestion:
Used by the world’s top marathon runners, Hydrogel technology allows for very high concentrations of carbohydrates to be encapsulated without causing nausea or stomach cramps. The gel turns into hydrogel upon contact with stomach acid, transporting energy directly to the intestine for record absorption.