The bulk of the research on dietary fiber started in the 1970s when what had been considered as a fad, lacking scientific evidence, gained the interest of the scientific community as improved designs and analytical methods were used to examine the health benefits of dietary fiber.(1) Since then, epidemiological studies and clinical trials have highlighted the potential role for dietary fiber in the prevention of cardiovascular and gastrointestinal disease.(2-8) Today, nutrition societies consider dietary fiber to be an essential part of a balanced diet for disease prevention.(9) Health authorities have now approved health claims on the benefits of moderate to high fiber intakes, based on the increasingly available scientific evidence.(10) The purpose of this review is to identify those claims, analyze the supporting evidence behind the role of soluble and insoluble fiber as health promoters, and examine the potential for further health claims, based on recent studies in fiber research.
The literature was searched using mainly PubMed (accessed in 2006 and 2007). References cited in this review comprise peer-reviewed original research articles, reviews, metaanalyses as well as government regulation reports. The bulk of the evidence includes work published after the year 2000; although, findings published between 1960 and 1990 were also used to illustrate the early work in the field of fiber research.
Fiber Efficacy for Lowering Total and LDL Cholesterol, and Decreasing Risk of Coronary Heart Disease
Recent animal studies show a significant effect of fiber on blood lipids(11) and suggest potential lipid-lowering mechanisms including satiety effects and moderate to low bile acid-binding capacities.(12) In humans, there is little evidence showing a link between insoluble fiber and coronary heart disease risk factors as well as mortality.(13-16) A common limitation to human studies examining the lipid-lowering effects of insoluble fiber is a lack of control on the diet and lifestyles of subjects or populations assessed. Thus, there is a contributing confounder by way of the effect of a healthy lifestyle on blood lipid levels. A review by Truswell (2002) showed that 27 of 34 human studies testing efficacy of insoluble fiber saw no reductions in plasma total cholesterol.(17) In addition, experiments that did show significant reductions had inherent weaknesses such as short durations and one-way designs (control-test) where subjects ate ad libitum.(17) Thus, the controversy behind the efficacy of insoluble fiber in decreasing risk factors of cardiovascular disease remains unresolved.
In contrast, there is an abundance of evidence supporting beneficial effects of soluble fiber on plasma lipid levels. At least forty human trials have indicated high efficacy of oatmeal or oat bran in reducing plasma lipids.(17) Doses of soluble fiber ranging from 3 g to 8 g have induced significant reductions ranging from 2.0% to up to 24% in total and low-density-lipoprotein (LDL) cholesterol in both hypercholesterolemic and non-hypercholesterolemic individuals.(18-21) Unlike human studies involving insoluble fiber, the clinical trials mentioned here are characterized by their strong designs, including large sample sizes and controlled study diets, reinforcing the validity of their findings. A pooled analysis of cohort studies conducted in the United States and Europe showed that incremental intakes of 10 g/day of cereal and fruit fiber intakes were associated with 25% and 30% reduction in coronary death, respectively.(22) In addition, studies have demonstrated that higher intakes of soluble fiber may reduce the incidence of metabolic syndrome characterized by elevated LDL cholesterol levels, decreasing the risk of cardiovascular disease.(23-24) Cholesterol lowering action of soluble dietary fiber was explained by Yoshida et al (2005) as a result of an increased fecal sterol excretion and/ or production of short-chain fatty acids previously shown to play a role in the suppression of cholesterol biosynthesis.(25)
A dose response effect of soluble fiber appears to exist, where an increase in the amount consumed is associated with a greater reduction in blood lipids, mainly total cholesterol and LDL cholesterol, however, no optimal dose is evident in the literature.(17-26) One study examined the lipid-lowering effects of 0.4 g, 3 g, or 6 g of beta-glucan in conjunction with a Step 1 diet which consists of 50% of energy from carbohydrates, 20% from protein and 30% fat, with less than 10% contributed by saturated fat and less than 300 mg/day of cholesterol in the diet. Results of the trial showed that LDL cholesterol levels were reduced by around 4%, 10%, and 14%, respectively,20 compared to a Step 1 diet alone. Considering that 3 g of soluble fiber can be found in approximately one cup of oat cereals, it is clear that such improvements in cholesterol levels can be attained with fairly low intakes of soluble dietary fiber.
Average daily intakes of total dietary fiber in Europe and the USA lie between 15 and 20 g, while the dietary review papers recommended intakes for fiber are reported to be 38 g/day and 25 g/day for men and women, respectively, with no upper tolerable intake set to this date.(9) Adequate intakes can be attained by consuming the recommended servings of whole grains, fruits and vegetables, making fiber supplementation unnecessary. However, it is clear that an adequate consumption of soluble fiber plays a role in reducing the risk of coronary heart disease by decreasing total and LDL cholesterol levels.
The American Heart Association’s year 2000 dietary guidelines state that there is a moderate cholesterol-lowering effect of soluble fiber over and above the effect of a diet low in saturated fat and cholesterol. As well, the US Food and Drug Administration has authorized a health claim stating that soluble fiber in certain foods such as whole oats reduces risk of coronary heart disease.(10)
Fiber Prevents Constipation
Insoluble dietary fiber plays an important role in promoting normal bowel movement by acting like a sponge in the distal colon, therefore increasing stool bulk. Through such action stool movement through the colon is promoted, thus reducing transit time.(27) In addition to the type of fiber, the effect of fiber on stool output is dependent on the amount of fiber present. Regular consumption of dietary fiber will help increase water content and plasticity of stools, thus promoting regularity. Since the stool holds more water as a result of fiber intake, it is important to increase fluid consumption to a recommended 2 L per day in order to further the action of fiber on stool movement.(27)
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.(28-29) 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.(30) Similar results were observed in a crossover trial using 100 mg/kg body weight of the soluble fiber glucomannan.(31) 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.(32) 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.(33-34) 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.(35) 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.(36)
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(37-38) called “a disease of Western civilization” for its high prevalence in developed countries.(39) 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(37) which, in turn, results in excessive segmentation. However, to date no human studies exist which confirm this mechanism.(37) Nonetheless, low fiber diets have repeatedly been correlated with the incidence of diverticulosis.(37-38) 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.(40)
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.(41) 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.(42) 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.(43) 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.
Fiber Decreases Risk of Colorectal Cancer
In general, many large epidemiological and case-control studies have resulted in data describing conflicting outcomes examining the relationship between colorectal cancer and fiber intake.(44-45) A combined analysis of 13 case-control studies found relative risks of colon cancer to be 0.53 with high versus low fiber intakes.(46) Similarly, a case-control study of 2000 cases found intakes high in fiber correlated with a low relative risk.(47) In contrast, large epidemiological studies such as the Nurses’ Health Study(48) and the Health Professionals Study(49) did not find any such relationships. Lupton (2000) suggests that a higher intake in fermentable fiber is more protective because it results in lower colonic pH and production of the short-chain fatty acid butyrate, a primary energy source for colon cells.(50) The recent review from Obrador examined the evidence supporting the role of dietary fiber in the prevention of colorectal cancer. While most epidemiological and intervention studies analyzed in this review show little strength in terms of cancer prevention, meta-analyses on case control studies report reductions of up to 50% in the risk of colorectal cancer associated with higher intakes of dietary fiber.(45) However, the evidence remains weak since case-control designs are often associated with various biases, namely recall bias and the influence of the subject’s health status on the validity of self-reporting.
Among factors contributing to the inconsistency in the data available are the source and form of fiber. While some studies have shown both soluble and insoluble sources to be protective,(44), (51) others state that insoluble fiber is the only form that could play a role in cancer prevention.(52-53) The diet accompanying fiber intake could play an important role as well, as it is hypothesized that the type of fat consumed affects gut susceptibility to cancer.(50) Namely, oils that induce apoptosis, such as fish oil, may be more protective.(50) Confounding the effect of dietary fiber on colorectal cancer is the natural presence of phytochemicals in such food groups as fruits, vegetables and whole grains. In fact, polyphenols and others have shown great antioxidant, antiproliferative and apoptotic activities(54-56) and could be responsible for the anti-carcinogenic effect of high-fiber foods.
Although there seems to be a significant body of evidence in favor of dietary fiber as a prevention of colorectal cancer,(45-47) data from well-designed studies characterized by large sample sizes show no effect of dietary fiber on cancer development or progression.(48-49) Confounding factors such as composition of meals accompanying fiber intake and antioxidant properties of fiber-rich foods have made the relationship between dietary fiber and colorectal cancer inconclusive, creating the need for further research to isolate the effect of fiber on cancer development. Moreover, it should be noted that cancer itself is a complex disease that can arise secondary to other health conditions, diet and lifestyle. Health claims pertaining to colorectal cancer prevention cannot be justified based on the current literature, however given the natural benefits of fiber containing foods, a potential health claim which states that regular consumption of fiber may decrease the risk of colon cancer could be appropriate in orienting the consumer towards healthier food choices. In conclusion, evidence shows that soluble fiber reduces the risk of cardiovascular disease, and that insoluble fiber protects against disturbances of the digestive tract. Although more extensive research is needed to confirm the beneficial role of dietary fiber in the prevention of colorectal cancer and diverticulosis, the fact remains that fiber is an important and safe component of a healthy diet and has been positively linked to lower incidences of these diseases.
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