Digestibility of Selected Resistant Starches in Humans

Dietary fiber consists mainly of carbohydrates which are not digested in the human small intestine. A high intake of dietary fiber is associated with many health benefits including, but not limited to, improved bowel function and reduced risk of weight gain, cardiovascular disease and diabetes. Therefore, there is great interest in the food industry to produce products enriched with dietary fiber to promote health. One way to achieve this is to use more whole grains (wheat, rice, oats, barley) as ingredients in products such as breakfast cereals, breads and food bars. However, the use of whole grains results in products which have a different taste and/or texture than the usual products made from refined grains. Thus, other types of high-fiber ingredients have been developed which can be incorporated into food products with less effect on their taste and/or texture. Examples of such ingredients are inulin (an oligosaccharide containing fructose) and resistant starch. Resistant starch, defined as starch which is not digested in the human small intestine, is present in small amounts (2-5% of total starch) in many normal foods.

Starch is the most abundant energy containing nutrient in the human diet; it consists of 2 types of polysaccharides: amylose, is a linear polymer consisting of long chains of glucose molecules joined by 1-4 linkages; and amylopectin, a highly branched polymer consisting of long chains of glucose molecules joined by 1-4 linkages with numerous 1-6 linkage branch points. Most (70-80%) of the starch in normal starchy foods (eg. cereals and potatoes) is amylopectin. Amylopectin is highly digestible because its branched structure makes it readily able to gelatinize, the process whereby adjacent starch molecules swell and separate from each other under the influence of moist heat (ie. cooking). By contrast amylose is less digestible because its linear structure allows adjacent molecules to associate by hydrogen bonding which reduces their ability to gelatinize. Some types of commercially available resistant starch come from strains of corn which produce starch containing 70 to 100% amylose.

The digestibility of starch is usually determined in-vitro using methods involving digestion of the starch with α-amylase under pH and temperature conditions thought to mimick digestion in the human small intestine. However, there is evidence that such methods may overestimate the amount of resistant starch by as much as 100%. Methods used to estimate starch digestibility in-vivo include the breath hydrogen method and the measurement of the amount of carbohydrate in the ileal effluent of subjects with an ileostomy. The latter is considered to be the best in-vivo method which involves preparation of subjects with a polysaccharide-free diet the day before then consumption of the test carbohydrate with breakfast. Subjects collect ileal effluent during the day during which time they consume a polysaccharide-free diet. There is evidence that resistant starch consumed at breakfast is completely recovered in ileal effluent 8-10hr after consumption. The objective of this study is to determine the amount of carbohydrates in 3 commercially available starches (Hi-Maize® 260, Hylon® VII and Amioca corn starch) which escape digestion in the human small intestine.