Primer on Resistant Starches

Oct. 30, 2005
Contributing Editor Mark Anthony, Ph.D., provides a primer on resistant starches, discussing their forms, functions and applications.
Resistant starch boosts processed foods' fiber content without changing their taste, texture or color. Photo courtesy of MGP Ingredients.

As an energy store of plant foods - fruits, vegetables, grains, beans, tubers and potatoes - polysaccharides are built by assembling glucose molecules into chains, and then packing the chains away as granules. The two basic types of starch chains, amylose (a relatively linear molecule) and amylopectin (more complexly branched) exist in plants in a characteristic mixture of chain lengths and types, consisting of hundreds to thousands of glucose units. To understand resistant starches, it's important to understand how starches in general are digested. We cook grains, beans and potatoes to cause starch granules to swell and burst, releasing the starch molecules via gelatinization. From this point, starch-digesting enzymes in the intestine liberate the glucose. The freed glucose is absorbed into the blood and carried to the liver, which uses it for energy or stores it as glycogen or fat. What is resistant starch resisting?Resistant starch steadfastly "resists" its digestive fate in the small intestine, remaining untouched until it journeys to the colon where bacteria take it apart by fermentation. The products of this disassembly are short-chain fatty acids, (acetate, butyrate and propionate) instead of glucose. These fatty acids help to maintain healthy bowel function and enhance uptake of fluids and electrolytes. Butyrate and propionate feed both the cells lining the colon and the friendly resident bacteria. In this circumstance, resistant starches fit the definition of fiber along with non-starch polysaccharides; they reach the colon undigested, enhance fecal bulk and provide useful material for colon-friendly bacteria.There are different types of resistant starch - that is, different ways starch can escape digestion in the small intestines:
  • RS1 –
      a starch that is indigestible by virtue of the presence of whole-grain fibers.
  • RS2 –
      a starch that is indigestible because some of the granules didn't swell and burst, such as from an undercooked source or a starch with a high amylose content.
  • RS3 –
      a starch cooled and crystallized post-gelatinization (also called "retrograde" starch)
  • RS4 –
    a starch modified to resist digestion; it does not occur naturally in foods.
Fiber vs. resistant starchIt's been observed for decades that dietary fiber is negatively associated with colon cancer, but the mechanism is still a matter of debate. It was once believed that the more quickly food passed through the digestive system, the less time potential carcinogens had to alter DNA and produce damage. But this model has been called into question, and resistant starch may provide some of the answers.The RS2 type of resistance starch (i.e. from high-amylose corn) has been shown to reduce DNA damage and lessen destruction to the lining of the colon in animal studies of high-protein/low-fiber diets. It's believed to work in conjunction with probiotic bacteria to remove cancer cells from the distal portion of the colon.
Resistant starch boosts crunchiness in items like the croutons in this soup, and improves al dente texture of pasta. Photo courtesy of National Starch Food Innovation.

The protective mechanism appears to be production of short-chain fatty acids (particularly butyrate). Butyrate not only provides energy to healthy colon cells, it may halt the progression of tumor cells. Hence, resistant starch, working either alone, complemented by other fermentable fibers (so-called non-starch polysaccharides), or in conjunction with friendly colon bacteria may provide natural protection against colon cancer. The RS2 and RS3 types appear to hold much promise in this regard and are the subjects of ongoing studies.Foods with higher amounts of amylose tend to have more resistant starch, due to the slow release of glucose from the incomplete gelatinization. This may, in turn, effect a lowered insulin response. Since blood-triglyceride levels reflect carbohydrate intake, high-amylose foods potentially lower triglycerides.Replacing a portion of dietary carbohydrates in a meal with RS2 high-amylose resistant maize has been shown to increase fat oxidation and decrease fat storage independent of the immediate glucose or insulin response. Even a reduction in plasma cholesterol may be within the healthful realm of such high-amylose starches, as has suggested studies with high amylose barley.Americans are encouraged to increase their fiber intake by increasing consumption of fruits, vegetables, whole grains and legumes. The difficulty is that we've become accustomed to staple foods without much fiber, or the texture and feel that accompanies high-fiber foods. Resistant starch is one of the products being exploited by food manufacturers to bridge both worlds.For example, it's often included as a flour ingredient to reformulate breads, muffins, and other baked goods traditionally low in fiber. Steve Hamm, director of marketing, specialty ingredients, for MGP Ingredients (, Atcheson, Kan., calls resistant starch "invisible fiber," since it doesn't change the taste, texture or color of processed foods."Resistant starch can help consumers fill the gap between typical and recommended fiber intake without giving up the foods they enjoy," says Rhonda Witwer, Ph.D., development manager-nutrition for National Starch Food Innovation (, Bridgewater, N.J. How the body handles resistant starch is a good example of nature displaying efficiency, taking food that escaped digestion by the normal route, and using colon bacteria to extract energy or create useful and potentially protective products. There is more complexity to carbohydrates than first meets the eye (or palate). Starches whose natural properties are preserved are protective, not damaging; strengthening, not fattening. A challenge for processors will be using and marketing resistant starch products without adding to the confusion that starch - and carbohydrates in general - have experienced by the public in the past. If successful, however, the results could be positive for all involved.

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