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Whole Grains and Health


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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).

      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.

      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).

      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.

      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.

      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