A resistant starch is any starch that resists enzymes during digestion. It’s also resistant to “cooking out,” and tends to form crystal structures that are resistant to digestion. Resistant starch in the diet, in specific amounts, may assist humans in resisting the accumulation of fat.
Starch molecules occur in two forms: amylose, a linear polymer composed of at least 200 (and up to several thousand) glucose molecules linked end-to-end; and amylopectin, a much larger, highly branched polymer containing up to hundreds of thousands of molecules of glucose linked in ways that form numerous short- and long-chain branches.
The ratio of amylose to amylopectin, as well as the molecular size of the molecules, varies widely in different crops, such as corn, wheat, rice and beans. Granules of common cornstarch are composed of approximately 75 percent amylopectin and 25 per cent amylose.
Amylose molecules have a natural tendency to form double-helix configurations, which aggregate into tightly packed, highly stable crystallites (by a process known as retrogradation). The terminal branches of amylopectin can also form short helices, but the resulting crystallites are much less stable and are easily disrupted. From resistant starch to fiber is a matter of degree, but it is now apparent these compounds have important roles in digestion.
Resistant starch and digestion
Since research in the mid-1980s, the definition of resistant starch has grown and changed. Studies in 1988 by Kay Behall of USDA on the effect of high-amylase starch on healthy subjects showed definite changes in the way the starch was digested, and the data suggested high amylose starch reduced the accumulation of triglycerides in blood. Results also suggested differences in insulin action when subjects consumed muffins, cornflakes and other foods that included a large proportion of high-amylose cornstarch.
National Starch introduced Hi-maize resistant starches in 2002 and flour and meal from Hi-maize high-amylose corn earlier this year.
As the crystalline structure of starches became better understood, the characteristic of certain fractions of high-amylose starch was redefined as “resistant.” Such starch could come from high-amylose grain or other starches that had been either chemically treated or cooked and allowed to retrograde (crystallize). Since then, four types of resistant starch have been defined:
- RS1 (resistant starch that is physically encased by whole grains)
- RS2 (a granular resistant starch)
- RS3 (nongranular, retrograded or crystalline resistant starch)
- RS4 (a manufactured resistant starch)
This makes resistant starch somewhat of a moving target. How foods are prepared, how starches are processed and how they relate to fiber is important. For instance, a cold, boiled potato contains about 13.5 percent resistant starch of the RS 3 type, while the same boiled potato served hot contains about 6.7 percent resistant RS3 starch. Whole-meal bread may contain 1.6 percent RS3, and lentils (cooked and cooled) contain 16.4 percent resistant starch in combined RS1 and RS3 types.
In late 2004, the journal <I>Nutrition and Metabolism</I> reported that consuming 5.4 percent of total carbohydrate as resistant starch resulted in increased metabolism of fat (http://www.nutritionandmetabolism.com/content/1/1/8). The report was by Janine Higgins of the University of Colorado Health Sciences Center, Center for Human Nutrition (Denver) and a team including researchers from the University of Vermont (Burlington), University of Wollongong (Australia) and the Dept. of Preventive and Social Medicine at the University of Otago (New Zealand).
“Our research has shown that, just by eating a meal containing resistant starch, you can burn 20 to 25 percent more fat, and this increase is sustained throughout the day – even if only one meal contains resistant starch,” Higgins commented. “Also, (the) effect is sustained if you keep eating resistant starch on a daily basis.”
Such discoveries about resistant starch fit with consumer interest in avoiding obesity, diabetes, heart disease and other maladies. According to Higgins, the reason resistant starch works is due to an anomaly in the way digestion proceeds. “The resistant starch actually changes the order in which the body burns food,” she explains. “Usually carbohydrates are used first, but resistant starch seems to move fat to the top of the list to be burned for energy before it has a chance to be stored.”
Note to Marketing
|Exactly how marketers can claim the effects of resistant starch on a label is a tough question. In the U.S., resistant starch, unless it tests as fiber, is labeled as corn starch or modified food starch. If a firm decides to make a reduced-calorie label claim, it’s wise to discuss it either with the company’s legal staff or FDA.|
A major problem with the development of fiber and resistant starch as ingredients that can be labeled on foods is the difficulty of determining how much resistant starch is present in a food. In 2002, the American Organization of Analytical Chemists (AOAC) published a method for determining the amount of resistant starch. Different tests are used to quantify fiber. Unless the carbohydrate tests for fiber, it is labeled as starch, which is less descriptive of its actual role in digestion. While there is still some controversy about the method, it is being used.