Basically, three distinct characteristics of dietary fiber shape their role in digestive health: bulking, fermentation and viscosity. Rhonda Witwer, senior business development manager, nutrition, Corn Products International/National Starch Food Innovation, explained the activity involved and summarized recent scientific studies. She provided a grid prepared by Dan Gallaher at University of Minnesota that charts the different mechanisms of dietary fiber.
Fibers such as wheat bran, cellulose and psyllium hold a lot of water and promote regularity by helping move intestinal contents through the digestive tract but are only minimally fermented, she observed. Thus, bulking fibers promote the largest regularity effects but the fewest fermentation benefits. “A recent clinical study (Wolever et al., 2001,American Journal of Clinical Nutrition75: 1023) found that people eating cereal high in psyllium required six months of daily consumption before the fermentation biomarkers in their blood significantly changed,” she noted.
But this slow pace has its advantages, too. Peggy Steele, Danisco global business director, Danisco BioActives, Madison, WI, cited the example of polydextrose. Danisco offers Litesse polydextrose. Because of its complex structure, she explained, it acts throughout its transit of the colon, thus optimizing pH to suppress detrimental bacteria, reducing carcinogenic compounds, improving bowel function and minimizing gas production.
Craig et al. (1998, “Polydextrose as soluble fiber: physiological and analytical aspects,”Cereal Foods World43 : 370) reviewed data on polydextrose as fiber. The most comprehensive clinical study confirming the fiber properties of polydextrose was undertaken by Zhong et al. (2000, “Studies on the effects of polydextrose intake on physiological functions in Chinese People,”American Journal of Clinical Nutrition72: 1503). This study, which involved 120 healthy male subjects, showed improvement in all the aspects of bowel function studied with a dose related response from as low as 4 g per day. This was achieved without undesirable side-effects. Attenuation of glucose uptake was also demonstrated, with the glycemic response of glucose reduced to 88 in the presence of 12 g of polydextrose. This work has recently been repeated in Japan with similar results.
Several studies have investigated the impact of polydextrose consumption on serum cholesterol. Most recently a positive result in gerbils was followed by a pilot study with hyperlipaemic humans. In this investigation a polydextrose intake of 30 g/day over 4 weeks mediated a 6% reduction in LDL-cholesterol in a subgroup of responders, including Pronczuk and Hayes (2005, “Hypocholesterolemic effect of dietary polydextrose in gerbils and humans,”British Journal of Nutrition[in press]).
The overall conclusion from both animal and human data is that polydextrose has a moderate beneficial effect on serum and liver cholesterol metabolism, not unlike that of other soluble, fermentable fibers.
Such thick fibers form a shield within the intestinal tract, according to research cited by Jit Ang, executive vice-president, technical services, International Fiber Corp. (IFC), North Tonawanda, NY. “High fiber intakes are associated with decreased risk of diverticulosis,” he said. “The protective effect against the disease was strongest for non-viscous dietary fiber, particularly cellulose.”
Studies that he quoted as establishing these effects were done by Marlett et al. (2002, “Position of the American Dietetic Association: health implications of dietary fiber,”Journal of the American Dietetics Association102 : 993); a report from the Institute of Medicine (2002, “Dietary, functional and total fiber,” Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids); Farrell et al. (2001, “Diverticular disease in the elderly,”Gastroenterol Clinician North America30: 475); and Aldoori et al. (1998, “A prospective study of dietary fiber types and symptomatic diverticular disease in men,”Journal of Nutrition128 : 714).
“Research overwhelmingly demonstrates that fermentation of indigestible carbohydrates triggers significant health benefits, including promoting the health and functioning of the digestive tract, supporting immune health and aiding mineral absorption,” Ms. Witwer said. Such fibers are preferentially fermented by beneficial bacteria, contributing short-chain fatty acids (SCFAs) responsible for the positive effects.
Resistant wheat starch, for example, can change the gut microbiota in humans. A clinical study done at the University of Nebraska cited by Ody Maningat, PhD, vice-president, applications technology and technical services, MGP Ingredients, Inc., Atchison, KS, found phylum changes increasing the population of bothBifidobacterium adolenscensisandParabacteroidetes distasoniswhile reducing the level of Firmicutes species. Total cell counts increased threefold.
“The phylum-level changes induced have been shown to be associated with lean human subjects,” Dr. Maningat summarized. “And bifidobacteria were strongly correlated with immunological and metabolic improvements in animal models of type 2 diabetes and hypercholesterolemia.” The resistant wheat starch further reduced the amount of Erysipelotrichaceae, a bacterial family that includes members that have been associated with an increase in liver fat in women.
A 2011 study involved soluble corn fiber, another type of resistant starch, fed at the rate of 21 g per day and reported improvements in several fecal chemistry markers related to gut health, including an increase in beneficial Bifidobacteria.
“Many soluble fibers have prebiotic effects, but sometimes these come at the expense of digestive tolerance,” said Neelesh Varde, PhD, senior project manager, fiber platform, Roquette America, Keokuk, IA. He described clinical trials with feeding rates as high as 45 g per day with soluble corn fiber that found excellent tolerance, and more recent clinical studies showed statistically significant effects in improving satiety at 8 g per day.
High-viscosity fibers such as beta-glucan slow absorption of cholesterol and glucose because their thickening effect reduces migration of these compounds to the mucosal barrier. “However, viscous fibers present formulation difficulties,” Ms. Witwer said, “and can interfere with taste, which limits their usage.”
Looking at low-viscosity fibers such as GOS and scFOS, scientists found much to like, related to their fermentation by beneficial Bifidobacteria species. “Significant increases in Bifidobacteria can be seen with as little as 1 g scFOS per day,” Ms. Witwer said, summarizing recent scientific trials. Other research by Bowling et al. (1993, “Reversal by short-chain fatty acids of colonic fluid secretion induced by enteral feeding,”The Lancet342: 1266) showed that scFOS enhances water and electrolyte absorption in the colon, both critical factors for reducing constipation and diarrhea.
GOS is the third-largest solid component of breast milk, and its fermentation in the large intestine lowers pH. “It prevents adhesion of pathogenic bacteria to the colonic cell wall,” Ms. Witwer noted. She described two studies that found GOS acting as a prebiotic to stimulate Bifidobacteria and alleviate symptoms of irritable bowl syndrome as well as ulcerative colitis. These were done by Silk et al. (2009, “Clinical trial: the effects of a trans-galactooligosaccharide prebiotic on faecal microbiota and symptoms in irritable bowel syndrome,”Alimentary Pharmacology & Therapeutics29 : 508) and Ishikawa et al. (2011, “Beneficial effects of probiotic Bifidobacterium and galacto-oligosaccharide in patients with ulcerative colitis: a randomized controlled study,”Digestion84: 128).
Healthy humans also benefit. According to a newly published study by Hughes et al. (2001, “Galactooligosaccharide supplementation reduces stress-induced gastrointestinal dysfunction and days of cold or flu: a randomized, double-blind, controlled trial in healthy university students,”American Journal of Clinical Nutrition93 : 1305), students with healthy weights were fed GOS while undergoing final exams. Even in such stressful times, the students experienced 40% fewer colds and flus compared with students consuming sucrose as a control. “This study confirms that immune health is intimately related to gastrointestinal health and that dietary consumption of GOS improves both,” Ms. Witwer said.
Whether a fiber is soluble or insoluble — or a blend of both — determines much about its activity in the body and in foods. Mr. Ang described insoluble fiber as “the most commonly known digestion helper,” yet more recent studies indicate that secondary benefits such as reducing transit time of toxins through the colon and balancing colonic pH may help to prevent colon cancer. “Insoluble fibers such as cellulose help cleanse the digestive tract,” he said, describing the fibers as a sponge that holds toxins and sweeps the walls of the colon during elimination. Mr. Ang also noted that sugar beet fiber, which is two-thirds insoluble and one-third soluble fiber, has been proven to lower blood glucose levels.
Some dietary fiber studies have even changed how scientists look at digestion. For example, researchers discovered that obese individuals carry different types of bacteria in their gut, producing different fermentation byproducts, than lean individuals. “Traditionally, the digestive system has been viewed as simply an extractor of nutrients,” Ms. Witwer said. “But the health effects of fermentation are demonstrating that the large intestine actively contributes to health, or the lack of it, via fermentation.”