At the Evergreen Air & Space museum (home of the Spruce Goose), they have an old SR-71 on display. Part of the exhibit has a TV playing a video loop with pilots and crew talking about their experiences. One of the pilots recalls how they'd have a "high-protein, low-residue" breakfast like steak & eggs before a long mission. The idea was to provide long lasting energy without having to go "number two" in the suit. He says that some of the crew who would help them out of the flight suit at the end of a mission were female, so it'd be especially embarrassing if you'd pooped yourself. I got the impression that, while rare, it did happen sometimes. :-\