Dietary Fiber and Risk of Inflammatory Bowel Disease: Fact or Hype?

Abstract

In our modern society, it has become a dogma that a diet poor in fruits and vegetables and high in fat and carbohydrates is the culprit behind many diseases, including inflammatory bowel disease, despite the relatively weak evidence.1Hou J.K. et al.Am J Gastroenterol. 2011; 106: 563-573Crossref PubMed Scopus (564) Google Scholar Ananthakrishnan et al in their study also enshrine this dogma in a prospective study on the long-term intake of dietary fiber in a large group of highly educated white females followed over 26 years.2Ananthakrishnan A.N. et al.Gastroenterology. 2013; 145: 970-977Abstract Full Text Full Text PDF PubMed Scopus (336) Google Scholar The study raises a number of questions that have been partly addressed in the accompanying editorial. Beside recall bias, the food frequency questionnaire has only a fair to moderate correlation with fiber intake (0.6) and no data was available for fruits or vegetables in the quoted paper.3Willet W.C. et al.Am J Epidemiol. 1985; 122: 51-65Crossref PubMed Scopus (3453) Google Scholar There is also a tendency for over-reporting of fruits and vegetables and under-reporting of meats and dairy products among women.4Feskanich D. et al.J Am Diet Assoc. 1993; 93: 790-796Abstract Full Text Full Text PDF PubMed Scopus (860) Google Scholar Potential for misclassifying women into the lowest and the highest quintiles could have occurred in 5% of cases.5Jain M.G. et al.Am J Clin Nutr. 1982; 36: 931-935Crossref PubMed Scopus (116) Google Scholar It is also important for authors to consider the time period effects as possible confounders.6Kovach C.R. et al.J Gerontol Nurs. 1989; 15: 11-15Crossref PubMed Scopus (7) Google Scholar The response rates to questionnaire from Nurses' Health Study (NHS) I and NHS II cohorts were not clearly reported but if based on study inclusion rates, the response was 63% from NHS I and higher at 80% in the NHS II cohort. The relatively small numbers of patients diagnosed with Crohn's disease and ulcerative colitis could affect the calculation of risk. The fact that more than half of living respondents denied the diagnosis further supports that over-reporting is relatively common among women. Fewer than half of the original respondents were scrutinized on their medical records and, even then, the accuracy of information in the records had to be questioned. It is unknown on what is the optimal intake of daily fiber that patients should be advised on, even though guidelines have suggested an intake between 25 and 35 g/d of soluble fiber. Even less is known about the optimal intake of fruits and vegetables. It may be difficult to provide solid advice to patients at the moment based on the available data and we hope to see more conclusive evidence in the future. Is this dogma on dietary fiber a fact or hype is food for thought for everyone. A Prospective Study of Long-term Intake of Dietary Fiber and Risk of Crohn's Disease and Ulcerative ColitisGastroenterologyVol. 145Issue 5PreviewIncreased intake of dietary fiber has been proposed to reduce the risk of inflammatory bowel disease (Crohn's disease [CD] and ulcerative colitis [UC]). However, few prospective studies have examined associations between long-term intake of dietary fiber and risk of incident CD or UC. Full-Text PDF ReplyGastroenterologyVol. 146Issue 4PreviewWe thank the authors for their insightful comments on our study.1 Stein et al suggest the possibility that reverse causation may have explained our findings by hypothesizing that women with subclinical Crohn’s disease may have reduced their fiber intake in response to symptoms before formal diagnosis. This is unlikely for several reasons. First, a key strength of our cohorts is that our dietary information was not obtained from a single time point, but rather based on cumulatively averaged intake from food frequency questionnaires administered serially over several decades. Full-Text PDF