Whole Grains and Health. Группа авторов
starch is the most abundant material in whole grains with variation of size shape, and surface properties depending on its source and processing treatment (Krok et al. 2000). Starch granules have different levels of structural features and the granule is a semi‐crystalline material. Whole grain cereal and pseudocereal (quinoa and amaranth) starches are of the A‐type showing peaks at 2θ of 9.9, 11.2, 15, 17, 18.1 and 23.3°, which differentiates them from B‐type tuber starches showing peaks at 5.6, 15, 17, 22 and 24°. A‐ and B‐type starches reflect the difference in the geometry of their unit cells, the packing density of double helices and the amount of bound water within the crystal structure (Buléon et al. 1998; Qian and Kuhn 1999). When starch is gelatinized and dispersed, the linear amylose (essentially α‐(1→4) linked) and highly branched amylopectin [containing both α‐(1→4) and α‐(1→6) linkages] can be separated by their structural difference and size. The percentage of α‐(1→6) linkage is one aspect of the fine structure of amylopectin, for example, 4.6% for normal maize starch and 5.7% for waxy maize starch (Shin et al. 2008).
4.4 Carbohydrate quality of whole grain foods
The term carbohydrate quality refers to the health‐associated aspects of carbohydrates in foods and generally is more specifically related to the quality of glycemic carbohydrates in foods. It is not usually used in reference to the dietary fibre component. For whole grain foods, the glucose‐generating starch is the glycemic carbohydrate. While the entire whole grain package is generally considered important to the beneficial health outcome of whole grains (Fardet et al. 2008), the nutritional property of its starch is an essential though understudied component.
4.5 Slow digestion property of starch
Much about the notion of carbohydrate quality of whole grains is centered on the idea that whole grain foods have a low GI compared to refined grain foods. In this case, low GI is equated with a slow digestion property of starch that moderates and extends glycemic response. Low GI foods (GI<55) are considered to be beneficial to health by preventing or therapeutically addressing obesity and associated metabolic diseases (Ludwig 2002; Livesey 2005; Fabricatore et al. 2011). High GI foods (GI >75), which have been associated with consumption of refined grain products (Ludwig et al. 1999). Thus, whole grain foods with starch of the same chemical structure can generate different nutritional outcomes based on their digestion profile or carbohydrate quality 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