Feb. 7, 2001 Champaign, IL — Legumes often fall far below popular grains and moisture-laden fruits and vegetables on the list of foods Americans eat to try to meet the American Dietetic Association-recommended 25 to 35 grams of dietary fiber per day. University of Illinois researchers, however, say many legumes (beans, lentils and peas) should be on more plates.
In the February issue of the Journal of Nutrition, UI animal scientists fill a knowledge gap in the ADA’s 1997 position paper on dietary fiber. Legumes, they report, contain substantially higher percentages of resistant starch than do cereal grains, flours and grain-based food products. Resistant starch does not digest easily. It goes past the stomach and small intestine before settling in the colon. There, bacteria attack it just as they do a dietary fiber, producing butyrate – a short-chained fatty acid desirable for its cancer-preventing qualities.
The study provides the first database of the percentages of starch and fiber in common food and feed ingredients. The researchers also determined how and where in vitro digestion occurred by studying digestion in the lower part of the small intestine in a dog model representative of the human digestive tract.
“The nice thing about legumes is that they have a great deal of dietary fiber plus the resistant starch,” said George C. Fahey Jr., who led the study. “You always think of legumes for their protein, as you should. With their protein, fiber and resistant starch, these foodstuffs offer good nutrition. Until now, we never knew legumes had so much of their starch in the form of resistant starch.” Of the 29 food and feed ingredients studied, the legumes (seven varieties) contained substantially higher percentages of both dietary fiber and resistant starch. Black beans, for instance, contain the highest amount of total dietary fiber (43 percent), and 63 percent of their total starch content is resistant starch that makes it to the colon.
Cereal grains, especially barley and corn, followed legumes in their percentages of resistant starch that reach the colon, but like all non-legumes tested they dropped significantly in fiber content. Heavily processed flours and grain-based products dropped off most dramatically in their resistant starch content with a range of just under 2 percent in rice to 15 percent in rolled oats reaching the colon.
“Flours don’t have much resistant starch, because they are processed so much,” Fahey said. “A lot of grain-based foods also don’t have very much resistant starch. If we eat grain-based materials that are not heavily processed and legumes, which we usually eat after minimal cooking, we get a lot of resistant starch and a lot of fiber as colonic foods.”
The Illinois Council on Food and Agricultural Research funded the work. Co-authors of the paper with Fahey are Neal R. Merchen, a professor of animal sciences; Christine M. Grieshop and Avinash R. Patil, both postdoctoral researchers; and graduate students Geoff E. Bednar and Sean M. Murray.
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