Linda F. Benedict, Martin, Roy J, Keenan, Michael J.
Michael J. Keenan and Roy J. Martin
Americans do not consume enough fiber in their diets. Fiber is a diverse substance that comes from or can be synthesized from foods. One of the major differences among fibers is the property of fermentability. Some fiber components are fermentable, and others are not. For a food substance to be a fiber, it must not be digestible, which means the substance reaches the large intestine. Bacteria that populate the large intestine feed on the fiber if it is fermentable. Fermentable fibers are often called prebiotics. Bacteria that colonize the large intestine are often referred to as the microbiota. People possess more bacterial cells in their large intestine than they have cells in their bodies. When people buy capsules or tablets containing what are considered beneficial bacteria, these bacteria are called probiotics.
Resistant starch is a term that did not exist until the 1980s, when starch was discovered in the large intestine of individuals during autopsies. There are two types of starch compounds. One, amylose, is a single chain of glucose compounds with two ends that amylase enzymes can act on in digestion of the starch in the small intestine. The other, amylopectin, has many branches for the amylase enzyme in the small intestine to digest.
Resistant starches of type 1 (RS1) and type 2 (RS2) usually are high in amylose (Table1). RS1 is found in whole-grain products that have not been overly processed, and RS2 can be found in a high-amylose cornstarch. Cornstarch from the grocery store is about 80 percent amylopectin and 20 percent amylose, while high-amylose cornstarch is about 60 percent amylose and 40 percent amylopectin. The latter is the result of a reduction in the amount of the branching enzyme that promotes the production of amylopectin in a naturally occurring corn variety. The resistant starch in this corn variety is an RS2. However, the major property that promotes resistance to digestion by the amylase enzymes in our small intestine is the granular structure.
High-amylose or high-amylopectin starches can be resistant to digestion when the granular structure prevents access to the starch by amylase enzymes. Resistant starch 3 (RS3) occurs in raw potatoes, but the granular structure of the starches is destroyed when the potato is cooked, and then a granular structure reforms when the potato is cooled as in potato salad. Adding various chemical components such as fatty acids to starch compounds or chemically linking the starch compounds together creates resistant starch, or type 4 (RS4).
Feeding an RS2 in pre-clinical studies in rodents has allowed LSU AgCenter researchers to make many observations and investigate the mechanism for beneficial health effects. When anyone is fed resistant starch, it has a lower energy value than starch that is completely digested because the bacteria in the large intestine use a certain amount of the energy contained in the resistant starch when they ferment it. By measuring the energy value of the RS2 used in the studies, AgCenter researchers have been able to ensure the control diets and diets with resistant starch were equal in energy so they could determine if a reduction in body fat was the result of more than a reduced dietary energy. They observed reduced abdominal fat when diets were of equal energy. The research team also observed improvements in functioning of the pancreas and glucose handling when a rat model for type 2 diabetes was fed resistant starch; and in elderly mice, eating resistant starch improved their food intake response to a fast, their stability in agility tasks and their performance in mental tasks.
The mechanism for these beneficial health effects is likely the fermentation of resistant starch into products such as short-chain fatty acids that appear to lead to the production of hormones that can increase energy expenditure and lead to beneficial signaling in the brain. AgCenter researchers have observed an increased use of fat for energy in mice. Additionally, the very presence of an increased amount of the bacteria that ferment resistant starch may likely promote beneficial health effects.
Several studies by other investigators have demonstrated that bacteria can interact with the host organism and alter metabolism. For example, using a metal tube inserted into the stomach of the mice through the mouth, researchers have transplanted the microbiota from genetically obese mice into normal, healthy, bacteria-free mice, and those mice became obese. In several studies, the AgCenter researchers have demonstrated increases in the bacteria known to ferment resistant starch and global bacterial changes in the whole microbiota.
Recently, AgCenter researchers demonstrated that a highfat (42 percent of energy) diet partially reduces the fermenta tion and effects of resistant starch compared to a low-fat diet (18 percent of energy), but a moderate-fat diet (28 percent of energy) is no different in fermentation and effects than the low-fat diet. The latter is encouraging because average fat consumption of Americans is in the moderate range; therefore, most Americans should benefit from consumption of resistant starch.
Future studies need to discriminate between the effects of fermentation of resistant starch and the effects of the presence of the bacteria in the large intestine of organisms that are fed resistant starch. Many of these types of studies have involved transferring bacteria into mice that have been raised without the presence of any bacteria (called germ-free). However, several scientists have begun investigating how to transfer bacteria to rodents that already have bacteria in their large intestine. These studies use strong antibiotic drugs to reduce the number of bacteria so that the bacteria transferred from individuals fed resistant starch will take hold in the large intestine of the target individuals.
Other studies need to investigate lower effective dietary levels of resistant starch other than the higher levels used in mechanistic studies. Recently, another lab used lower dietary levels of resistant starch and demonstrated that less body fat was observed when rats were fed resistant starch at levels as low as eight percent of the weight of the diet. Around 10 percent of the weight of the diet in rodents matches the human requirement for fiber. However, their control diets were higher in energy than the diets with resistant starch.
Additionally, AgCenter researchers have begun to investigate effects of resistant starch using whole-grain resistant starch (from whole-grain flour rather than isolated starch from corn flour). They have found in preliminary studies that the whole-grain flour is fermented to a greater extent than the isolated starch.
Michael J. Keenan is an associate professor, and Roy J. Martin is a professor emeritus in the School of Nutrition and Food Sciences.
(This article was published in the fall 2013 issue of Louisiana Agriculture magazine.)