Human studies have concluded that total dietary fiber given at doses ranging from 20-26 g helps to normalize and improve bowel function by decreasing the incidence of constipation in elderly populations. In these studies, ingestion of fiber-rich foods or supplements resulted in significant improvements of clinical symptoms of constipation and discontinuation of laxatives. Fiber supplementation in children with chronic constipation demonstrated that the recommended ‘age in years plus 5 g/day’ dosage of total fiber resulted in a reduction in the frequency of clinical symptoms. Similar results were observed in a crossover trial using 100 mg/kg body weight of the soluble fiber glucomannan. Constipation is also commonly faced by patients administered enteral feeding. To this end, Silk et al (2001) studied the effect of fiber supplementation of enteral formulas on bowel function. Results show a reduction in gut transit time and an increased stool wet weight as compared to fiber-free formula. Although two metaanalyses examining randomized controlled trials have shown no benefits to dietary fiber in alleviating the symptoms of constipation, the choice of a study population composed of irritable bowel syndrome patients decreases the applicability of the authors’ findings to the general population. In fact, an earlier systematic review, looking at a more heterogeneous population, argued that fiber supplementation did lead to a modest improvement in bowel movement frequency and a decrease in abdominal pain. The review outlines data from 1815 patients with chronic constipation having participated in 36 controlled trials. Although special populations, namely patients with irritable bowel syndrome and diverticulosis, were included in three studies, these populations only accounted for 45 patients of the total sample size. The beneficial effect of dietary fiber on constipation is supported by the evidence in the literature linking the intake of fiber to reduced intensity and frequency of symptoms in the general population. Therefore, health claims on the benefits of dietary fiber for constipation are well founded, especially in populations at risk where more fiber is associated with the need for less laxative medication and accompanying side effects.
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Fiber Effect on Diverticulosis Prevention
In the 1960s and 1970s, Painter and colleagues first proposed a low fiber diet as a cause of diverticular disease called “a disease of Western civilization” for its high prevalence in developed countries. A suggested mechanism for the development of diverticulosis is that low fiber intake results in greater water absorption due to slower gut transit. Thus, smaller, firmer stools are produced, leading to high intraluminal pressure which, in turn, results in excessive segmentation. However, to date no human studies exist which confirm this mechanism. Nonetheless, low fiber diets have repeatedly been correlated with the incidence of diverticulosis. In fact, epidemiological studies predicted that 60% of individuals over 60 years of age, from Westernized countries, will be diagnosed with diverticular disease, partly due to a low intake of dietary fiber.

Fiber supplementation has been examined in animal models in order to explain a possible mechanism through which fiber may act to prevent the disease. It was suggested that fiber may influence the nature of collagen cross-linking in the bowel through decreased production of short chain fatty acids by gut microflora. Although animal models have shown a positive response to high-fiber diets, to date very little evidence exists on fiber supplementation and its link to diverticulosis. Results from a prospective study involving a cohort of healthy males show a more significant inverse relationship between diverticulosis and insoluble fiber than it does for soluble fiber. While no recent randomized placebo-controlled trials have been conducted to evaluate the effect of fiber on diverticular disease, earlier human trials do not hold enough evidence qualifying dietary fiber as a treatment for diverticulosis symptoms. Although patients diagnosed with the disease are commonly prescribed a high fiber diet, there is a need for further investigation into the role of fiber in the development and/or progression of diverticulosis through placebo-controlled clinical trials. At present, consumption of a diet low in fiber is considered a main cause of diverticulosis, therefore there should be a preventative health claim on the risks of diverticulosis linked to a low-fiber intake.