property. A positive correlation between GI and RDS‐derived glucose (Englyst et al. 1999) and a negative correlation between RDS and SDS (Zhang et al. 2008) indicate that SDS is the structural basis for cereal‐based low GI foods. It should be noted that whether there remains a controversy regarding the relationship between GI and health, though a recent international consensus report supports that low GI or glycemic load foods in diets reduce certain chronic metabolic diseases such as diabetes and heart disease (Augustin et al. 2015).
Although SDS is defined by an in vitro starch digestion method (Englyst et al. 1992), studies on raw corn starch, which is a naturally pure source of SDS, showed it to produce a prolonged and sustained low postprandial glycemic and insulinemic responses similar to those of low GI foods (Seal et al. 2003; Wachters‐Hagedoorn et al. 2006). Indeed, the health benefits of SDS are also assumed to be similar to low GI foods (Lehmann and Robin 2007). Literature reports have shown metabolic and physiological effects of SDS consumption. SDS epigenetically caused a shift of the gene expression peak of SGLT1 from the upper jejunum to ileum (Shimada et al. 2009), leading to an increased glucose transporters in the ileum (Woodward et al. 2012), and consumption of SDS in the form of raw normal corn starch resulted in sustained release of incretin hormone of GLP‐1 (Wachters‐Hagedoorn et al. 2006), which is important for body weight regulation and insulin sensitivity (Larsen 2008). Our recent animal study (Hasek et al. 2018) using a slowly digestible starch‐entrapped microsphere that digests starch into the ileum showed reduced food intake behavior and decreased expression of hypothalamic orexigenic neuropeptides NPY and AgRP, which are appetite stimulators. Thus, SDS not only generates a modulated postprandial glycemic response, but also influences a variety of processes that may be important to human health.
The making of efficacious healthy SDS materials requires an understanding of the in vivo process of starch digestion related glycemic response, and also to physiological response. Our group has pursued a path of research relating location of digestion and glucose release in the ileal region of the small intestine to activation of the gut‐brain axis and ileal brake (Hasek et al. 2018; Lee et al. 2013; Romijn et al. 2008). Yet, not all SDS materials necessarily digest into the ileum, and this is also true with digestion of starch in whole grain foods. For instance, using a pig model, normal corn starch, which is a standard SDS material, was nearly all digested in the duodenum and upper jejunum with little measurable amount of starch getting to the ileum (Hasjim et al. 2010). More research needs to be done on the factors that moderate starch digestion in whole grain foods and, in particular, the role of whole grain matrices in digestion.
4.6 Physical form of whole grain foods
Food structure is a key factor affecting starch digestion, and any disruption of the physical or botanical structure of cereal grains can increase the rate of digestion and postprandial glycemic response (Björck et al. 1994). Coarse particles of whole grains were found to reduce the rate of starch digestion more than fine whole grain flour (Liljeberg et al. 1992). Particle size of grains from wheat, corn and oat were likewise found to influence in vitro starch digestion rate, with larger particles being slower digesting (Heaton et al. 1988). Thus, the physical structure in whole grain foods is an important factor contributing to the starch digestion property and is likely related physiological effect. The botanical structure of grain kernels provides a nature‐produced physical barrier to protect the nutritive contents from environment influences. Starch, as the energy provider for seed germination, is mainly located in the endosperm cellular compartment that is embedded in a matrix formed by proteins and cell wall material (Kamal‐Eldin et al. 2009). Depending on the type of endosperm, a dense packing of starch granules in the protein matrix of the vitreous endosperm significantly decreases the rate of starch digestion (Lopes et al. 2009). The influence of the protein matrix, even after cooking, still affects starch digestibility with the report of SDS content of 20% in a cooked flour compared to 0–2% in cooked isolated starch (Zhang et al. 2008). Soluble fibre in the endosperm cell wall of oats was shown to affect starch digestion, where different degrees of β‐glucan solubilization using cooking methods caused that different degrees of starch digestion (Yiu et al. 1987). Whole grain kernels also have fibre‐rich multiple‐layered bran that may decrease the accessibility of hydrolytic enzyme to some starch granules. Accordingly, the ordered botanical structure of whole grain kernels is a natural way to produce slowly digestible or perhaps even some degree of resistant starches.
Although the whole grain botanical structure provides some degree of physical barrier to starch hydrolytic enzymes, most whole grain foods are further processed before consumption. An understudied area is how to effectively process whole grain foods to retain or minimize the loss of physical barrier function important to slow starch digestion properties and moderated postprandial glycaemia. Food processing with high temperatures and shear conditions may completely disrupt grain structure and disperse gelatinize starch, leading to a high content of RDS, which from the starch perspective differs little from processed refined grain products. On the other hand, moderate processing such as with rolled oats can lead to reduced rate of starch digestion due to a minimal disruption of the physical structure of the grain (Mishra and Monro 2009). Similarly, food processing to produce a dense packing of food form may create a physical barrier property to starch digestion, such as in pasta that can contain a significant SDS.
4.7 Digestibility of dietary fibre
Dietary fibres such as arabinoxylan, pectin, cellulose, β‐glucan and resistant starch are often mentioned regarding the health benefit of whole grain foods (Lattimer and Haub 2010; Cho et al. 2013). Related to starch digestion and glucose absorption, viscous‐forming fibres (e.g., β‐glucans, water‐soluble arabinoxylans) in some whole grain foods have been shown to moderate diffusion kinetics of α‐amylase and its digested products to the small intestine epithelial cells for glucose production by the mucosal α‐glucosidases (Blackburn and Johnson 1981; Johnson and Gee 1981; 2013). High viscosity of β‐glucans lowered in vitro starch digestion (Kim and White 2013), and the zero‐shear viscosity of jejunal digest containing β‐glucans was found to negatively correlate with glucose absorption in a pig study (Ellis et al. 1995). Other whole grain sources do not have appreciable amounts of viscous‐forming fibres, though cellulose as an insoluble fibre has been shown to inhibit α‐amylase activity to reduce starch digestion (Dhital et al. 2015).
Short chain fatty acids generated through fibre fermentation by the colon microbiota induce the release of the gut hormone peptide YY (PYY) (Wen et al. 1998) and GLP‐1 (Tollhurst et al. 2012) to slow gastric emptying, promote insulin secretion and moderate glycemic response. This creates a “second‐meal effect” first described by Jenkins et al. (1982; Brighenti et al. 2006). Thus, dietary fibres can slow the absorption of glucose or other nutrients from both its physiochemical property such as high viscosity (Zijlstra et al. 2012) and the ileo‐colonic brake systems, which enhance carbohydrate quality of whole grain foods.
4.8 Phytochemicals
Phytochemicals mostly reside at the outer layers of cereal grains, particularly in the pericarp seed coat and aleurone layers, and can affect carbohydrate digestion and absorption in the gastrointestinal tract. Polyphenols such as phenolic acids found in whole grains have affinity to proteins like enzymes (Bandyopadhyay et al. 2012), and their inhibiting function on the activity of the α‐amylases and α‐glucosidases (maltase‐glucoamylase and sucrase‐isomaltase) have been documented (Shihabudeen et al. 2011; Forester et al. 2012; Mkandawire et al. 2013; Tu et al. 2013; et al. 2013; Simsek et al. 2015). In addition to their inhibition effect on hydrolytic enzymes, they also have been shown to interrupt the uptake of glucose by binding to the sodium‐glucose linked transporter, SGLT1 (Kobayashi et al. 2000) and glucose transporter 2, GLUT2 (Kwon et al. 2007; Stelmanska 2009; Manzano and Williamson 2010), or by reducing the expression of glucose transporters (Alzaid et al. 2013). Thus, both starch digestion and glucose absorption may be interfered by whole grain polyphenols, which could result in a reduction of postprandial glycemic response.
4.9 Future perspectives
Whole
