Large Bowel Effects

Constipation/Diarrhea/Irritable Bowel Syndrome (IBS)

Constipation can be defined as infrequent elimination (e.g., <3 bowel movements per week) of small/ hard difficult to pass stools that require straining [5]. It is a misconception that consuming the

Weight loss in a 6-month study comparing a restricted diet alone (Diet

Fig. 23.1 Weight loss in a 6-month study comparing a restricted diet alone (Diet: AHA step 2 diet) to the same diet with added fiber supplements: psyllium (PSY) 3.5 g twice a day versus guar gum (GG) 3.5 g twice a day in patients with metabolic syndrome. Note that only psyllium showed weight loss across all 6-months of treatment

recommended levels of fiber will reduce the risk of constipation. As discussed previously, the adequate intake guidelines for dietary fiber were based on reducing the risk of CVD, not reducing the risk of constipation. The American Gastroenterological Association concluded that “Constipation was associated with low-dietary fiber intake in some, but not other studies. However, these associations do not necessarily indicate causation. Although it is reasonable to try and modify these risk factors, doing so may not improve bowel function” [16]. One reason for this inconsistency in population-based studies is that “dietary fiber” does not differentiate between specific fiber characteristics, and not all fiber types provide a laxative effect (some can actually be constipating). For fiber to soften stool and significantly increase stool volume, it must resist fermentation to remain intact and present throughout the large bowel (must be present in stool), and it must increase stool water content, which is the primary mechanism for both a stool-softening effect and increased stool bulk [17]. There are two mechanisms by which a fiber can significantly increase stool water content and stool bulk: (1) poorly fermented coarse insoluble fiber particles, such as wheat bran, mechanically irritate the gut mucosa, stimulating water/mucous secretion (fine/smooth particles only add to the dry weight of stool and can be constipating); and (2) non-fermented viscous/gel-forming fiber, such as psyllium, retains its water-holding capacity throughout the large bowel to resist dehydration [17]. Both mechanisms increase stool water content (softer stool) and stool bulk.

The texture of stool is correlated with percentage of water content, and small changes in stool water content can lead to large changes in stool texture: hard stool is <72%, normal/formed stool is «74-75%, soft/formed stool is «76%, and loose stool is «80% water [5, 17]. An effective fiber (e.g., coarse wheat bran, psyllium) will increase stool water content, resulting in a significant stool-softening effect and increased stool bulk, both of which make stools easier to pass without straining. In contrast, a fiber that adds only to the dry mass of stool, with no water-holding capacity [e.g., wheat dextrin [5]], will decrease the percentage of stool water content, resulting in harder stools (constipating effect).

The observation that coarse wheat bran has a greater laxative effect than fine wheat bran suggested that the insoluble particles themselves may have a direct effect in the large bowel and led to studies comparing insoluble wheat bran to inert plastic particles (“plastic effect”) [18]. Coarse plastic particles and coarse wheat bran had a significant laxative effect, while fine/smooth plastic particles and finely ground wheat bran had no laxative effect [18]. Fine wheat bran, which does not mechanically stimulate the mucosa and has no water-holding capacity, added only to the dry weight of stool and caused a decrease in stool water content (stool-hardening/constipating effect) [18]. The laxative efficacy of insoluble fiber is highly dependent on particle size/coarseness, which is one reason why epidemiologic data assessing “dietary fiber” may be inconsistent.

In contrast to the mechanical irritating effects of coarse insoluble fiber, a non-fermented, soluble gel-forming fiber (e.g., psyllium) retains its high water-holding capacity throughout the large bowel, which significantly increases both stool bulk and stool water content (softens stool). In a clinical study of 170 patients with chronic idiopathic constipation, psyllium was shown to be superior to docusate for increasing stool water content (softer stools; p = 0.007) and the frequency of bowel movements (p = 0.02) [19]. If a soluble fiber is nonviscous, it has no water-holding capacity and can add to the dry mass of stool (similar to fine wheat bran), decreasing the percentage of stool water content (harder stools). Wheat dextrin, an artificially created “fiber” (heat/acid treated wheat starch), showed a decrease in stool water content (harder stools), consistent with reports of harder stools by healthy subjects consuming 15 g/day [20]. Even if a fiber is viscous/gel-forming but is readily fermented (e.g., guar gum, p-glucan), it loses its water-holding capacity and, like nonviscous fermentable fibers (e.g., wheat dextrin, inulin), has no laxative effect [17].

A gel-forming fiber that resists fermentation and retains its high water-holding capacity (e.g., psyllium) can actually exert a stool normalizing effect: softening hard stool in constipation and firming loose/liquid stool in diarrhea. Psyllium has been shown to be effective for softening hard stool in constipation [better than docusate [19]], firming loose/liquid stool in diarrhea [17], and reducing fecal incontinence episodes [21], making psyllium an effective fiber choice for irritable bowel syndrome [22].

 
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