Whole Grains and Health. Группа авторов
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 grain carbohydrates consist principally of dietary fibres and starch. They are considered to be the main contributors of the health benefits ascribed to whole grain foods. While starch is quantitatively the major component of whole grains, relatively less is known relating starch quality to the health attributes of whole grains. Whole grain foods are broadly considered low GI foods, though their processing can change this attribute. Thus, the quality of starch is an important factor requiring further investigation, particularly in the area of glucose release kinetics and location of digestion in the gut related to potential gut‐brain axis and ileal brake responses. The role of whole grain dietary fibres on the gut microbiome is large and a growing area of research, and likely an expanded research effort is needed for a more precise and fundamental understanding of the action of different fibres and their physical forms on the microbiota and their consequent effect on health. Last, the complex interactions in whole grain foods among starch, dietary fibre and other food components require more studies to develop novel food processing technologies for processed foods to maximize the health benefits.
4.10 References
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