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Until approximately 15 years ago, it was believed that all starches were digested in the upper digestive tract and absorbed as sugars into the bloodstream. However, it has since been noted that some starches resist digestion in the upper digestive tract and, so, pass into the large bowel relatively intact. These starches are referred to as resistant starch (RS). In the bowel, RS is highly fermentable; not by you, because people don't have that pathway; but by many of the trillions of microorganisms that make their homes there. It has the properties of both soluble and insoluble fiber and, thus, has been studied widely for its effects on bowel health. RS has been shown to increase stool bulk and decrease transit time and prevent the proliferation of pre-cancerous bowel lesions in rats. The metabolic benefits of RS include lowering post-meal blood sugar levels after consumption and increased fat burning. How are these processes related? What do we know about the mechanisms of action of RS, if anything? Do different mechanisms underlie the bowel and systemic effects of RS consumption? How can we consume more RS?
From Science News (online 23 February 2013): "As the name suggests, you can't digest resistant starch so it ends up in the bowel in pretty much the same form it entered your mouth. As unlovely as that seems, once in the bowel this resistant starch does some important things, including decreasing bowel pH and transit time, and increasing the production of short-chain fatty acids (acetic, propionic, butyric acids). These effects promote the growth of good bugs while keeping bad bugs at bay.
A University of Colorado Cancer Center review published in this month's issue of the journal Current Opinion in Gastroenterology shows that resistant starch also helps the body resist colorectal cancer through mechanisms including killing pre-cancerous cells and reducing inflammation that can otherwise promote cancer.
"Resistant starch is found in peas, beans and other legumes, green bananas, and also in cooked and cooled starchy products like sushi rice and pasta salad. You have to consume it at room temperate or below -- as soon as you heat it, the resistant starch is gone. But consumed correctly, it appears to kill pre-cancerous cells in the bowel," says Janine Higgins, PhD, CU Cancer Center investigator and associate professor of Pediatrics at the University of Colorado School of Medicine.
Higgins describes studies showing that rats fed resistant starch show decreased numbers and sizes of lesions due to colorectal cancer, and an increased number of cells that express the protein IL-10, which acts to regulate the body's inflammatory response.
"Resistant starch may also have implications for the prevention of breast cancer," Higgins says. "For example, if you let rats get obese, get them to lose the weight, and then feed half of the rats a diet high in resistant starch -- these rats don't gain back the weight as fast as rats fed a regular, digestible starch diet. This effect on obesity may help to reduce breast cancer risk as well as having implications for the treatment of colorectal cancer."
"There are a lot of things that feed into the same model of resistant starch as a cancer-protective agent," Higgins says. "Much of this information currently comes from rodent models and small clinical trials but the evidence is encouraging." On the table now is a menu of benefits and while it's just now being studied which benefits, exactly, will pan out as mechanisms of cancer prevention, one thing is clear: resistant starch should be on the menu."
What exactly are resistant starches? For the chemically-minded, here's an excerpt from Dr. Higgins' recent review:
Resistant starch has been broadly described in five general categories. RS1 represents starch physically inaccessible to digestive enzymes due to the presence of seed coats, germ, and so on (e.g. whole grains; one of the reasons these are so widely recommended as part of the diet); RS2 are starch granules which are inaccessible to amylases (starch-digesting enzymes from mouth to small intestine) due to granule starch structure or conformation (e.g. high amylose maize starch); RS3 is the product of retrograded starch (e.g. starch cooked then cooled); RS4 encompasses starches that are chemically modified to be resistant to digestion; and RS5 are inclusion complexes formed by amylose with polar lipids.
It should be noted that it is rare for any starch or starch-derived material to be comprised of only resistant starch; usually there is also a digestible component. New forms of RS2 are, or will in the near future, be available from high amylose varieties of barley, wheat, and rice produced by transgenic or conventional breeding techniques.
A current area of debate is whether RS4 compounds have the same physiological effects as the extensively studied RS2 and RS3 starches. This is a topic of some importance as RS4 products are versatile and can be easily and inexpensively engineered to deliver resistant starch and other compounds to specific areas of the colon.
Bio
Janine Higgins, PhD, is Associate Professor of Pediatrics in the University of Colorado School of Medicine, Aurora. She received her PhD in biochemistry at the University of Sydney, with a thesis focusing on the effect of carbohydrate sub-type, in particular resistant starch, on insulin sensitivity in rats. Her research focuses on preventing weight regain following weight loss in obese rodents and the metabolic effects of resistant starch in children, adults, and rats. She is also an investigator on the NIH multi-center Treatment Options for Type 2 Diabetes in Adolescents and Youth (TODAY) study. She is currently the Colorado Clinical and Translational Sciences Institute (CCTSI) Nutrition Research Director. Her latest endeavours seek to translate the data from adult studies to children and adolescents who are the population at greatest risk for an explosion in the prevalence of the metabolic syndrome and, therefore, the population with the greatest possible benefit from resistant starch consumption.