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THE TRUTH BEHIND DIETARY FIBER

Fiber used to have a negative connotation in the field of nutrition. In the 1970’s, a paradigm shift occurred in the thinking of medical and nutrition researchers, no longer was fiber non-nutritive and unimportant; now fiber played a significant physiological role for a well-rounded healthy diet. Research on dietary fiber (DF) spilled over into many other areas, including a significant rethinking of the dietary regimen for diabetics to help reduce the symptoms of that disease. The research effort continues with indications that not only low fat and high fiber are good for well-being, but a complex of carbohydrates, including those with slow digestibility, are important.

 

Definition of Dietary Fiber

For decades there was crude fiber (CF), “the residue of plant (based) food left after extraction with solvent, dilute acid, and dilute alkali” (Williams and Olmstead, 1935). The term “dietary fiber” was first applied by Hipsley (1953) as a shorthand term for the constituents of the plant cell wall. This new term “dietary fiber” clearly was an attempt to distinguish something above and beyond CF. In 1976, the DF definition was broadened (Trowell et al. 1976) to a physiological-chemical definition including all undigestible dietary polysaccharides in addition to the other fiber components. These non-digestible polysaccharides were included in the definition because they were found to have the physiological actions that we attribute to DF, but could not necessarily be chemically identified as having their origins in the cell wall. The 1976 definition gained wide acceptance and became the basis definition for the current AOAC International Official analytical methods. The current working definition of DF is “Dietary Fiber consists of the remnants of plant cells, polysaccharides, lignin and associated substances resistant to hydrolysis (digestion) by the alimentary enzymes of humans.”

 

Benefits of Dietary Fiber

Insoluble Dietary Fiber

Table 1. Relative risk heart attack versus TDF consumption


(Total Dietary Fiber. J.W. De Vries, 2009)

 

The most obvious benefit of increased intake of IDF (particularly wheat bran) is the feeling of well-being resulting from increased laxation. IDF typically has a high water binding capacity which results in the formation of softer stools that pass through the system faster. The softer stools reduce the pressure necessary for elimination, thus, less constipation. Rimmet. al. (1996) reported increased consumption of IDF correlated with reduced coronary heart disease, (an effect usually seen for SDF) after tracking the health habits of some 44,000 US male medical professionals for six years. Self-reported food intakes were audited by plasma carotenoid and tocopherol testing. Effectiveness of DF was measured by reduction in heart attacks. Table 1 summarizes the relative risk of heart attack vs TDF consumed per day. Relative risks are adjusted for saturated fat level in the diet, vitamin E intake, age, body mass index and physical activity level of the subjects, smoking, alcohol consumption, hypertension, hypercholesterolemia, family history, and profession. The quintile that consumed the highest level of TDF had 2/3 the risk of those consuming the lowest amount of DF. Looking at the heart health response to the three major sources of fiber, i.e. fruits, vegetables, and cereals, each have an effect, with cereal fiber showing the greatest impact.

 

Soluble Dietary Fiber

While not as effective as IDF in promoting laxation, SDF does exert a positive effect through a different mechanism. As SDF is fermented in the colon, a substantial quantity of bacterial mass accumulates which is soft, bulky, and water retaining, the resulting fecal properties being similar to those from IDF. SDF fermentation also generates significant quantities of gases which exercise the colon during transit. (This may be healthy and beneficial, but is not necessarily considered a social benefit of SDF.) Decreased risk of coronary heart disease is correlated with increases in consumption of DF, typically SDF. Increased risk of coronary heart disease is also correlated with a significant number of other risk factors which are reduced by DF, such as diabetes, high serum cholesterol, high levels of low density lipoprotein (LDL) associated cholesterol, and low levels of high density lipoprotein (HDL) associated cholesterol, obesity, and possibly hyperinsulinemia.

Increased intake of SDFs such as guar gum, locust bean gum, oat gum and pectin can significantly decrease total and LDL serum cholesterol while maintaining or increasing HDL cholesterol levels. SDFs increase fermentation in the large intestine, increasing the production of short chain fatty acids, helping to remove bile salts from the system and depressing cholesterol production.

 

REFERENCES:

Hipsley, E.H. 1985. Dietary “fibre” and pregnancy toxaemia.British Medical Journal 2:420-422.

 Williams, R.D., and Olmstead, W.D. 1935. A biochemical method for determining indigestible residue (crude fiber) in feces: Lignin, cellulose, and non-water soluble hemicellulose. J. Biol. Chem.108:653-66

 Rimm, E.B., Ascherio, A., Giovannucci, E., Spielgelman, D., Stampfer, M.J., and Willett, W.C., 1996. Vegetable, Fruit, and Cereal Fiber Intake and Risk of Coronary Heart Disease Among Men. JAMA275.6:447-451.