Abstract

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. Since then, epidemiological studies and clinical trials have highlighted the potential role for dietary fiber in the prevention of cardiovascular and gastrointestinal disease. Today, nutrition societies consider dietary fiber to be an essential part of a balanced diet for disease prevention. Health authorities have now approved health claims on the benefits of moderate to high fiber intakes, based on the increasingly available scientific evidence. 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.

Methods

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 and suggest potential lipid-lowering mechanisms including satiety effects and moderate to low bile acid-binding capacities. In humans, there is little evidence showing a link between insoluble fiber and coronary heart disease risk factors as well as mortality. 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. 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. 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. 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. 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. 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. 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.