I just read an interesting question & answer at www.cyclingnews.com - "Eating on the bike". I don't know that I've seen anything definitive about caloric absorption rates (during exercise or otherwise) so the response from Scott Saifer is pretty interesting:

"... typical [caloric] absorption rates quoted are in the vicinity of 150 calories per hour for small, un-fit people, up to 325 or so for large, fit, genetically endowed people on good days ..."

The original question was posed because energy food manufacturers have suggested/recommended serving information that would have you consume 900+ calories per hour, which makes no sense whatsoever, except for the company that stands to make more cash by selling you more energy food.