Resistant Starch, Panacea, but Why?

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I am definitely not a bandwagon kind of guy (except maybe in the case of fecal transplants.)  So embracing evil, high-glycemic starch as a medicinal godsend was tough.  As a biochemist raised on structural analysis of carbohydrates, the idea of starch as a mixture of linear amylose and branched amylopectin, was straightforward.  Amylase in saliva and pancreatic solutions digests the linear alpha,1-4,glucan stretches and pullulanase (bacterial) degrades the alpha,1-6, glucan branch points.  But this suggests that starch should be digested before it gets to the colon and most of the gut flora.  This ignores the spirals of amylose that resist amylase digestion, i.e. resistant starch (RS), and that lead to the puzzling role of RS as a special food for gut flora and a health panacea.

Why is resistant starch, RS, difficult to digest and terrific for gut flora?  

There are several conceptual difficulties to understanding why RS is Real Special:

• Polysaccharides are not all simple linear chains of sugars, e.g. they branch (amylopectin) or form helices (RS, amylose).
• Sugars and polysaccharides have hydrophobic patches.  They are amphipathic like soap.
• Amylose (RS) forms a spiral with a hydrophobic surface of each sugar facing inward to make a hydrophobic core and hydrophobic patches on the outer surface that structures water to hold the spirals together.  The same principle holds together the double helices of DNA around a central hydrophobic core of stacked base pairs.
• Polysaccharides (soluble fiber) are made of many different sugars, e.g. glucans, mannans, xylans, galacturonans, etc.
• Starch is enzymatically hydrolyzed to glucose, which most gut flora can ferment.  Other sugars from other polysaccharides (e.g. pectin, polygalacturonan) must first be converted to glucose for fermentation.
• Soluble fiber polysaccharides made from multiple sugars, e.g. arabinogalactans or xyloglucans, require multiple bacterial enzymes for digestion by gut flora.
• Several hundred bacterial enzymes of the gut flora are required to digest the complex soluble fibers of food plants in a typical diet.  Resistant starch requires two.
• Food intolerances (also mistakenly called food allergies) result from missing bacteria and their enzymes to fully digest soluble fibers.
• Novel soluble fibers or sugars are used as laxatives, and they lose their loosening impact as your gut flora adapts to digest the new fiber.  Normal, softened stools are half bacteria.
• Amylose spirals are used as a storage form of glucose in seeds, potatoes, roots, etc., because enzymes can’t attack their glycosidic bonds to hydrolyze the starch into amylodextrins and glucose.
• Bacteria digest amylose by attaching the spirals to their cell walls and using wall-bound enzymes to tear the amylose apart.  It's like the different requirements of a wood chipper (pancreatic amylase) versus a man with a chain saw (bacterial amylase).
• The spirals of RS melt during cooking and become susceptible to gut amylase.  Melted amylose can sometimes slowly reform enzyme-resistant spirals, RS, when chilled.  Al dente or chilled pasta has more RS and raises blood sugar less than soft pasta.

Resistant Starch, a Unique Soluble Fiber  

Humans only produce enzymes (amylases) to degrade one of the hundreds of plant polysaccharides, linear starch.  RS is not degraded by human amylases and, as with other soluble fibers, it is degraded by bacterial enzymes in the colon and is fermented to short chain fatty acids.  The difference is that the glucose released from hydrolysis of RS is used directly by common gut bacteria, whereas the other sugars released from other soluble fibers require enzymes produced by particular species of bacteria to be converted first to glucose.  RS is a unique form of starch and a unique soluble fiber.

Short Chain Fatty Acids and Immune Cells Required for Health

Gut flora eat soluble fiber and produce short chain fatty acids, e.g. acetic, butyric, propionic acids, that feed the cells of the intestines and lower inflammation.  The development of both the aggressive and the suppressive (Tregs) halves of the immune system requires healthy gut bacteria.  The bacteria that digest RS, for example, are Clostridia (see EM right, note bacterium dissolving its way into the grain of RS), the type of gut flora that also stimulates Tregs and prevents autoimmunity.  Thus, the beneficial impact of dietary RS results from feeding gut flora.  Most people already support gut flora that can utilize RS, so most people benefit from RS in their diet.  Some people have severely damaged gut flora, dysbiosis and constipation, and they may need to eat live, fermented foods (not just dairy probiotics) to recruit enough new bacteria to benefit from RS.  Other healthy people may already have healthy gut flora that can exploit all of the soluble fiber in a compatible healthy diet, and need no further enhancement of their health by RS.  Health always requires gut flora complementary to diet and each change in diet requires accommodation by corresponding changes in gut flora.  Some changes in diet may require new species of gut bacteria.

Does Altered RS in Modern Bread Explain Gluten Intolerance?

The RS remaining in today’s superfine flour that is rapidly cooked into bread and other foods, may be very different from previous generations.  Traditional hydration and exposure to fermenting microorganisms may have produced breads with higher levels of RS that contributed to healthier gut flora.  Healthier gut flora would in turn produce less intestinal inflammation and a reduced response to gluten.