Practitioners are quickly becoming familiar with the use of prebiotics to enhance the effects of probiotics and optimize the microbiome. In addition to supplemental prebiotics, dietary resistant starches – which are another form of prebiotic, fermentable fiber – confer similar benefits as other prebiotics but may augment the benefit since they can be incorporated into daily meals.

Resistant starches are unable to absorb liquid or do not fit into the enzymatic binding site of amylase, so they resist the action of amylase in the small intestine. (The term “resistant starch” is derived from this resistance to enzymatic breakdown.) The undigested starch progresses to the large intestine where it is fermented by resident microorganisms, producing metabolites such as short-chain fatty acids (SCFAs), that provide numerous health benefits.

There are five different types of resistant starch:

Type 1 is classified as physically inaccessible starch. Synthesized in the endosperm of cereal grains, legumes, and seeds, this type of starch is housed within a protein-based shell that liquid cannot penetrate, protecting the encapsulated starch from enzymatic hydrolysis. Examples include whole and coarsely ground grain kernels and seeds.

Type 2 is a granular starch with B- or C-type polymorph which are highly resistant to enzymatic hydrolysis. Uncooked potato starch, green banana starch, gingko starch, and high-amylose maize starch are examples of type 2 resistant starch. Cooking, however, disassembles the B- or C-type polymorph, rendering the starch highly digestible.

Type 3 is retrograded amylose and starch that has high gelatinization temperatures. Most of its structure is not dissociated by normal cooking. Some starch molecules may be digestible when warm, but when cooled at refrigeration temperature, the amylose forms double helices, loses its water-binding capacity, and no longer fits into the amylase enzymatic binding site thereby becoming a resistant starch. Potatoes and rice that have been cooked and then cooled fall into this category.

Type 4 includes starch that has been chemically modified by either adding chemical derivatives such as octenyl succinic or acyl groups or cross-linking so the starch is unable to absorb water and is usually resistant to bacterial fermentation as well.

Type 5 is an amylose-lipid complex. These starches interact with lipids to form single-helical complexes with fatty acids and fatty alcohols, preventing cleavage by amylase.

Two of the most sought-after health benefits of resistant starch are optimization of commensal microbes and improvement of glycemic control. These benefits provide secondary health effects for patients, such as weight management, improved metabolic markers, modulation of inflammation and oxidative stress. Bacterial fermentation of resistant starch produces gases (methane, hydrogen, carbon dioxide), SCFAs (acetate, propionate, butyrate, and valerate), and lesser amounts of organic acids (lactate, succinate, and formate), branched SCFAs (isobutyrate and isovalerate), and alcohols (methanol and ethanol). Many of these byproducts lower the pH of the large intestine, which positively influences the classes of bacteria that survive and grow, as well as gene expression of both bacteria and colonic cells.

SCFAs are also important for the prevention of colon cancer, as they reduce cell proliferation, induce cell apoptosis of tumor cell lines, and support the expression of tumor suppressor genes. In fact, one study showed increases in >2000 genes and specifically in genes associated with cell growth, proliferation, differentiation, and apoptosis, all likely tied in a complex manner to improved gut health. Finally, the byproducts of resistant starch bacterial fermentation are strongly associated with improved insulin sensitivity, with the potential for decreasing risk for type 2 diabetes and related metabolic dysfunction.

In the presence of resistant starches, microbiome diversity shifts as the phylum Firmicutes is reduced and Bacteroidetes is increased. This shift has been associated with a more favorable body mass index and improved immunity. Type 4 resistant starch, specifically, has been proven to improve immune-metabolic functions in those with metabolic syndrome. In one study, cholesterol, fasting glucose, glycosylated hemoglobin, and proinflammatory markers in the blood as well as waist circumference and % body fat were lower in type 4 resistant starch intervention group.

Prebiotic intervention in the form of resistant starch ensures a more effective optimization of gastrointestinal health and the microbiome than probiotics alone. No one would expect new grass seed to take root and grow without adequate water. Likewise, resistant starches help facilitate the growth and survival of health-promoting bacteria.