Resistant starch (RS) has been the topic of numerous research publications in petfoods and nutrition over the past several years. However, it isn’t very prominent in the petfood aisle or with new product offerings. This notion of resistant starch is a starch that resists digestion in the small intestine and enters the large intestine/colon where it is fermented by the microbiome into beneficial metabolites—much like what is expected of a functional fiber. There is enough evidence in the human and animal nutrition research literature to suggest that resistant starch may have a wide array of health benefits. So, is resistant starch a “future fiber” for the next generation of petfoods?
Most consumers know “starch” as an ingredient like potato starch or corn starch used in the home for cooking. To the biochemist, “starch” is a polymer of glucose molecules connected by α (1→4) [amylose] or α (1→6) [amylopectin] glycosidic linkages. In seeds and tubers, raw starch is organized as crystals in granules and consists of varying proportions of amylose and amylopectin. When these granules swell during hydration the crystalline structure of the starch becomes disorganized and the starch is said to be gelatinized.
This process of swelling and gelatinization is most often quickened by cooking, whereby heat accelerates the change. Gelatinization increases the overall digestibility of the starch by allowing amylolytic enzymes (e.g., amylase) to access the inner molecular structure and degrade the amylose and amylopectin to glucose. However, depending upon other dietary ingredients, processing considerations and animal factors, not all starch is digested in the small intestine and some escapes into the large intestine where it can be fermented or excreted. This escaping fraction is “resistant starch.”
Resistant starch is not an ingredient. Rather, it is a fraction or property of starch that behaves like a fiber. Most starch in commercial petfoods would be readily digested into glucose for absorption. That has traditionally been a desired result of cooking—improved starch utilization in the small intestine. But in essence, resistant starch is a “starch artifact” that behaves contrary to this intention and blurs the line between starch and fiber. In theory, it could potentially be a fiber-like compound engineered or targeted to a specific purpose and functionality in the gastrointestinal tract.
However, it is far from a simple matter. Resistant starches have been classified into four categories. Type 1 (or RS1) is a starch granule that is protected or surrounded by other plant components such as a protein. Type 2 (RS2) resistant starch occurs naturally in ingredients like high amylose corn, potatoes and bananas. Type 3 (RS3) resistant starch is produced following the cooking process due to retrogradation of the gelatinized starch during the cooling process and Type 4 (RS4) resistant starch is produced by chemically modifying the starch via esterification, crosslinking or transglycosylation. The origin of the resistant starch can greatly affect its behavior during digestion.
A review by Spears and Fahey summarized the in vitro and in vivo companion animal studies conducted prior to 2002 and concluded that physiological responses to resistant starch sources were wide-ranging, from healthy fermentation in the large bowel and concurrent production of short-chain fatty acids to negligible fermentation and good laxation. More recently, Knapp and co-workers at Illinois (2008) evaluated a resistant starch from high amylose corn and reported that it contributed to a low glycemic response and consistent stools. But at this last summer’s Animal Science meetings, Beloshapka and colleagues reported in two separate abstracts that a high amylose corn decreased total tract digestibility slightly, had no impact on stool scores, lowered fecal pH and had a mixed impact on various plasma metabolome markers.
These equivocal responses may be a “dog factor” because Goudez and co-workers (2011) fed a diet supplemented with resistant starch from high amylose corn and reported improved stool consistency in giant breed dogs but not in smaller dogs. Glucose metabolism and obesity might also be influenced by resistant starch. Bauer and colleagues at Texas A&M (2006) fed a high amylose corn diet which resulted in modest increases in plasma hepatic lipase activity. They reasoned in a follow-up study that the low glycemic index of the resistant starch slowed release of glucose and may have partially explained the weight loss achieved when fed to obese female Beagle dogs (Mitsuhashi et al., 2010).
Practically speaking, we already supply a small portion of resistant starch in every pet diet that contains some level of grain, tuber or legume. Murray et al. in 2001 reported resistant starch values for various raw ingredients as follows: barley (17.0%), corn (23.6%), potato starch (66.0%), rice (26.9%), sorghum (33.8%) and wheat (13.0%). These would be classified as RS2. However, the level of resistant starch is transitory, not static. Following extrusion cooking, which is common for most petfoods and would gelatinize much of the raw starch, the RS values drop to single digits, whereas with some specialized ingredients like high amylose corn hybrids, much of the raw resistant starch value is retained following extrusion cooking (13%–15%; Gajda et al., 2005).
It almost goes without saying—more details are needed. To start, a specified dose of resistant starch has not be identified, nor has the full evaluation of how the RS type influences the desired response. Much of the work to date has been done with high amylose corn starch which can be readily purchased. Other commercial sources from potatoes, legumes and other ingredients are available but less well described in a petfood application. From a regulatory and labeling perspective, how a claim about the amount of resistant starch in a petfood could be made is unknown.
Guaranteeing starch isn’t allowed at this point, so resistant starch is probably a long way down the road. Further, if a resistant starch-rich ingredient differs from the current standards, or does not comply with “natural” designations, it remains to be seen whether consumers will accept it.
Future uses for resistant starch in petfoods will likely target specialized therapeutic diets designed for diabetes or weight management (glycemic control), gastrointestinal health (fermentation characteristics) or large breed stool composition (laxation and bulking). Further, there is some evidence to suggest that resistant starch could enhance food texture, acceptability and processing mechanics. This, along with many other aspects about resistant starch, deserves a bit more attention.
Stay up-to-date on industry ingredient issues:
Read more columns by Dr. Aldrich at www.petfoodindustry.com/ingredientissues.aspx.
