Gastrointestinal symptoms in long-distance runners, cyclists, and triathletes: prevalence, medication, and etiology - Peters, H P; Bos, M; Seebregts, L; Akkermans, L M; van Berge Henegouwen, G P; Bol, E; Mosterd, W L; de Vries, W R (Am J Gastroenterol, 1999)