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Can Fiber Worsen Constipation?

iStock-1250173635“A diet poor in fiber should not be assumed to be the cause of chronic constipation. Some patients may be helped by a fiber-rich diet but many patients with more severe constipation get worse symptoms when increasing dietary fiber intake.” (Source)

It’s nearly impossible to find an article about chronic constipation in which increasing dietary fiber is not listed among the recommendations for ameliorating this issue. But a surprising body of research indicates that this conventional advice is not evidence-based and may even be counterproductive for some patients. It’s true that increasing intake of indigestible material will increase stool bulk, but this increased stool mass does not always result in a more rapid intestinal transit time, nor to more frequent or more pleasant bowel movements. Let’s take a closer look at the possibility that the long-honored recommendation to increase dietary fiber may actually exacerbate the problem it is purported to improve.    

First, it’s important to get clear on terminology. A low frequency of bowel movements (BMs) is not what defines constipation. There is no physiological necessity or requirement for the human body to pass stool on a daily basis. The Rome III criteria for constipation include six factors, only one of which is related to BM frequency. 

According to the American Society of Colon & Rectal Surgeons, “The belief that one must have a bowel movement every day simply is not accurate and can lead to unnecessary concern and even abuse of laxatives. In fact, if one’s daily bowel movement is hard, requires great effort to expel, or does not satisfactorily empty, the individual would still be considered to have constipation in spite of having a ‘normal frequency.’  On the other hand, if one has a movement every third day but it is not hard, does not require straining and completely evacuates, then one may very well consider this normal bowel movement, in spite of the fact it is not a daily event.” This explanation of constipation shows that there may be patients who believe they are constipated when they are not, and there may be others who do have constipation but who don’t realize it. This may result in people pursuing unnecessary and potentially harmful “treatment,” and others who could benefit from treatment not seeking it out. 

A paper from the World Journal of Gastroenterology did not mince words in emphasizing the potential adverse effects of increasing dietary fiber for patients with chronic constipation: “…although stool frequency may be increased by the mass effect of fiber packing in the colon in normal individuals, this is not so in individuals who are chronically constipated.” The authors point out that many people with chronic constipation have altered colonic morphology and therefore don’t have the “expected” response to increased fiber: “An increase in the fiber intake in these patients will not result in a decrease in stool transit time. There is, in fact, a deleterious effect of increasing faecal load without effectively increasing evacuation!” 

Assuming a “normal” response to increased dietary fiber, it makes sense to recommend this for patients with constipation. However, in many people, constipation results from impaired or reduced intestinal motility. In these individuals, more fiber would be likely to worsen constipation, not improve it. If stool is already not progressing through the colon in a timely manner, then increased dietary fiber would logically result in even larger and bulkier stools that continue to stagnate in the bowel, leading to worsened bloating and discomfort. Researchers have observed precisely this outcome: “For many patients, fiber exacerbates bloating and distension, leading to poor compliance.” It’s completely logical that a patient might fail to “comply” with advice that appears to worsen their condition.

Why would a patient want to experience these uncomfortable effects of a higher fiber intake, when the increased fiber may not even help the underlying problem anyway? 

“The formation of large amounts of faecal material can actually have a detrimental effect on the patient. Faeces that is bulky and hard is more difficult to evacuate in a patient with a pre-existing evacuatory problem. Increasing faecal loading by increasing the fiber intake to increase stool frequency cannot be logical if one is trying to decrease colonic load as a motive.” (Source)

In noting that insoluble fiber passes through the gut largely unchanged, researchers have written, “It is thus obvious that the more fiber is ingested, the more stools will have to be passed. Pure insoluble fiber is the ultimate junk food. It is neither digestible nor absorbable and therefore devoid of nutrition. People who ingest fiber are ingesting them to make faeces only.” Calling fiber “the ultimate junk food” is pretty risqué for a paper in a peer-reviewed publication, but the authors did acknowledge that dietary fiber—soluble fiber, in particular—does have benefits for delaying gastric emptying and reducing the glycemic impact of meals in people with diabetes. (And the fermentation of various fibers in the colon results in production of butyric acid and other short-chain fatty acids, which may have beneficial effects for immune function and possibly also psychiatric and neurological issues.) 

Questioning the validity of recommending increased fiber intake for people with constipation may seem heretical, but “conventional wisdom” in several other health issues has been shown to hold up poorly under scrutiny. For starters, saturated fat doesn’t cause cardiovascular disease, amyloid plaques aren’t the main causal agents in Alzheimer’s, and elevated LDL-cholesterol is only a small contributor to risk for heart disease. A high fiber intake may indeed be beneficial for some individuals, but we should not assume it will be helpful for all, and it is necessary to acknowledge the possibility that this frequently-given advice may have the opposite effect intended in some patients:

“Despite the widespread use of fiber supplementation, this approach is effective in only a subset of patients, and clinical trial evidence supporting the use of increased fiber intake is limited.” (Source)

The clinical trial evidence is indeed limited, and in fact there is evidence directly to the contrary. A fascinating study in a gastroenterology journal showed that reducing or completely eliminating dietary fiber resulted in improvements in idiopathic chronic constipation. Subjects were instructed to follow a no-fiber diet for two weeks, and then to reduce habitual fiber intake “to a level that they found acceptable.” Constipation symptoms, abdominal bloating or pain, anal bleeding, and difficulty in evacuation of stools were recorded at 1 and 6 months. 

At the 6-month mark, out of 63 total subjects, 41 remained on a no-fiber diet and 16 continued with reduced fiber intake. (The remaining 6 had returned to a higher fiber intake due to personal or religious reasons.) All subjects in the no-fiber group reported complete resolution of all symptoms. The reduced fiber group reported some degree of improvement, and the subjects who had resumed their usual (higher) fiber intake experienced no change. The conclusion was clear: “Idiopathic constipation and its associated symptoms can be effectively reduced by stopping or even lowering the intake of dietary fiber.” 

As always, treatment should be tailored to the individual situation. Some patients do respond to increased dietary fiber with improved bowel function but increasing fiber intake may have the opposite effect in others. It may not be prudent for this traditional recommendation to be given indiscriminately.

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