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Historically, dietary fibre was a term first used by Hipsley (1953) to describe nondigestible food constituents referring to plant cell wall materials, including cellulose, hemicelluloses and lignin. With the advancement of scientific investigation, as well as new methodologies for dietary fibre measurement, dietary fibre is essentially now considered another food macronutrient. Organizations and regulatory agencies have their own definitions, such as the American Association of Cereal Chemists (AACC 2001), CODEX Alimentarius Commission (CODEX 2009), European Food Safety Authority (EFSA 2010), US Food and Drug Administration (FDA 2016) and the Institute of Medicine (IOM 2001). Among these definitions, CODEX Alimentarius is broadly accepted and represents a relatively comprehensive and flexible definition reflecting the current knowledge of dietary fibre. Dietary fibre as defined by CODEX Alimentarius is as follows:

Table 4.1 Classification of whole grain carbohydrates.

Carbohydrate	2nd Level Classification		Examples	Digestibility
Simple	Monosaccharide		Glucose, fructose, galactose	Direct Absorption
Carbohydrate	Disaccharide		Sucrose	Rapid
Complex	Starch	RDS	Cooked waxy starch	Rapid
Carbohydrate	(glycemic carbohydrate)	SDS	Raw corn starch	Slow
	Non‐starch	RS	Retrograded amylose	Fermented
	polysaccharides	Soluble	β‐glucan, arabinoxylan, pectin	Partially
	(dietary fibre)	Insoluble	Cellulose, hemicelluloses	fermented
Oligosaccharide			Fructans	Fermented

Table 4.2 The composition of whole grain carbohydrates (% dry weight) (Choct 1997; Collins et al. 2010).

Cereal	Starch	Arabinoxylan	β‐Glucan	Cellulose	Glucomannan	Galactan
Wheat	68	8.1	0.8	2.0	–*	0.3	Barley
60	7.9	4.3	3.9	0.2	0.2	Rye	69
8.9	2.0	1.5	0.3	0.3	Triticale	53	10.8
1.7	2.5	0.6	0.5	Sorghum	77	2.1	0.2
2.2	0.1	0.15	Corn	78	5.2	–	2.0
0.2	0.6	Oats	50	3.1	4.3	2.3	–
–	Brown rice	66	2.6	0.13	2.6	–	–
Pearl millet	63	2.5	–	1.5	–	–

“Dietary fibre means carbohydrate polymers with ten or more monomeric units,* which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories:

1 Edible carbohydrate polymers naturally occurring in the food as consumed,

2 Carbohydrate polymers, which have been obtained from food raw material by physical, enzymatic or chemical means and which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities,

3 Synthetic carbohydrate polymers which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities.

*Decision on whether to include carbohydrates from three to nine monomeric units should be left to national authorities.

By this definition, dietary fibres that have been isolated or synthesized and used as ingredients must also be shown to have a physiological benefit, but importantly whole grain fibres do not have this requirement. This fibre requirement of physiological benefit is also incorporated into the new FDA dietary fibre definition of 2016 (US FDA 2016). Cellulose, hemicellulose, lignin, gums, modified celluloses, mucilages, oligosaccharides, pectins and associated minor substances such as waxes, cutin, and suberin all fall within the scope of dietary fibre. By the late 1990s, the methodologies for dietary fibre analysis had been refined by AOAC and AACC (DeVries et al. 1999). However, novel digestion‐resistant materials developed by the food industry were not always within their scope, and a new method, (AOAC Official Method 2009.01, 2009), was developed to capture analytically all fibre materials meeting the definitions. Based on Footnote 2 found in the AOAC method, resistant oligosaccharides, resistant starch and resistant maltodextrin all satisfy the definition of dietary fibre (Jones 2014).

Dietary fibre is, in a general way, often separated into water‐soluble and water‐insoluble types. Some water‐soluble dietary fibres, such a β‐glucans and pectins, have a high viscosity property that can slow the transit of food in the gastrointestinal tract, reduce the postprandial glucose spike, and aid in the excretion of bile acids and cholesterol. They are also readily fermented in the colon and produce short chain fatty acids (acetate, propionate and butyrate) that have certain health benefits and modulate the microbiota. Some water‐insoluble dietary fibres have water retention capacity to promote bowel movement and generally have a lesser degree of fermentation. In whole grain foods, β‐glucans and some arabinoxylans are the main water‐soluble dietary fibres in cereals (Carpita and Gibeaut 1993) and xyloglucans and pectins in pseudocereals like amaranth and quinoa (Lamothe et al. 2015), while the major water‐insoluble dietary fibres are cellulose, the insoluble cross‐linked arabinoxylans, and lignin (Table 4.2). Depending on the botanical source of the dietary fibre as well as individual genotypes and growth conditions, there may be variability in degree of heterogeneity in structure within the same type of fibre polysaccharide type. Such a wide range of fibre structures in whole grains could differentially influence the gut microbiome and resulting metabolic and physiological responses (Hamaker and Tuncil 2014). Additionally, as illustrated from the molecular architecture of the plant cell wall (Figure 4.1), dietary fibre polysaccharides in whole grain usually exist in a cell wall form consisting of a fibrillar phase of cellulose microfibrils in a matrix composed of non‐cellulosic polysaccharides with a variety of different structures along with structural proteins, glycoproteins and phenolic components. At a more macro‐level, whole grain cell wall structures may contain starch (Figure 4.2), and make it more difficult to access, and thus slowing its digestion or making it resistant. The principal whole grain non‐starch polysaccharides and their molecular structures (shown in Figure 4.3) are briefly discussed in the following, along with their potential association with starch in the context of the whole grain food matrix as related to carbohydrate quality in whole grain foods.

Figure 4.1 Depiction of plant cell wall structure consisting of cellulose microfibrils (blue), non‐cellulosic polysaccharides, termed hemicelluloses (green), and pectins (yellow/brown).

Source: Johnson et al. 2018.

Figure 4.2 The endosperm cell wall in oats and barley as it contains starch (dark spheres) and other constituents.

Source: Tosh 2013.