1 Ann. Neurol. 52: 276, 2002. 2 Gastroenterology 123: 1823, 2002. 3 JAMA 281: 2106, 1999. 4 Ann. Intern. Med. 144: 785, 2006. 5 Arch. Intern. Med. 156: 521, 1996. 6 J. Appl. Physiol. 85: 883, 1998.