Center for Functional Gastrointestinal and Motility Disorders, Division of Gastroenterology and Motility (Medicine), and Division of Urogynecology (OBGYN), University of North Carolina at Chapel Hill, Chapel Hill, North Carolina

Correspondence: William E Whitehead, PhD, Division of Gastroenterology and Hepatology, Bioinformatics Building Room 4112, CB#7080, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7080.

Received 13 June 2007; Accepted 21 June 2007.

Abstract

Twin studies have traditionally been used to assess the heritability of diseases such as irritable bowel syndrome (IBS) by comparing concordance rates in monozygotic twins (identical genetic endowment) to dizygotic twins (half of genes shared). Wojczynski et al. used twins in a novel way—they studied monozygotic twins who were discordant for IBS (but who shared identical genes) to show that the comorbidity of IBS with major depressive disorder could NOT be due to genetic influences. This paradigm provides the most rigorous method for separating genetic from environmental influences and should be adopted by other researchers. However, the authors' conclusion that major depressive disorder and IBS are part of the same pathophysiological process is questioned on the basis of (a) incomplete co-occurrence of IBS and major depressive disorder (13–45% co-occurrence) and (b) lack of specificity—the authors show that chronic widespread pain (related to fibromyalgia) and chronic fatigue are also strongly associated with IBS. This study provides precise, generalizable estimates from a large population-based study for the comorbidity of IBS with major depressive disorder, chronic widespread pain, and chronic fatigue.