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Whole grains or whole grain foods, based on the Whole Grains Council (2004), contain “all the essential parts and naturally‐occurring nutrients of the entire grain seed in their original proportions” and its carbohydrates include endosperm starch and non‐starch polysaccharides (dietary fibre), and minor amounts of inulin (Van Loo et al. 1995, in wheat, rye, barley) and simple sugars. Whole grains comprise cereals and pseudocereals, but not legumes or oilseeds. Starch, as a semi‐crystalline entity, is presented in a granular form in the endosperm of different shapes and sizes. In cereals, there are pores leading to channels within the starch granules that facilitate more rapid enzyme digestion of raw starch than is the case in granules without channels such as in tubers. Molecularly, starch is composed of essentially linear amylose and highly branched amylopectin that are both homopolymers providing energy to the body in the form of glucose. The nutritional character of starch is represented by the rate and extent of its digestion in the small intestine and, based on an in vivo‐supported in vitro digestion method (Englyst et al. 1992), it is described by its content of rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS). The latter is considered as a type of dietary fibre with a prebiotic property (Bird et al. 2010). Whole grain dietary fibre is composed of a heterogeneous class of mostly polysaccharides with high complexity in their sugar unit and linkage compositions, molecular structures, and 3‐dimensional conformations, which lead to different physiochemical properties as well as human body functions, including modulation of starch digestion and glucose absorption, and, through changes of the gut microbiota, impacting of the immune system and intestinal epithelial barrier function (Anderson et al. 2009). The physiological quality of available or glycemic carbohydrates is captured in a number of ways with the most common being glycemic index (GI) (Jenkins et al. 1981), which is based on postprandial glycemic response. Many low GI foods have a high amount of dietary fiber (Björck and Elmstathl 2003), suggesting that dietary fibres may lower glycemic response. The combination of glycemic and non‐glycemic carbohydrates, as indispensable parts of whole grains, is related to the concept of whole grain foods having good “carbohydrate quality.”

A good understanding of the mechanism and role carbohydrates play in the health benefits associated with whole grain foods is a necessary basis to develop strategies to optimize their impact in unprocessed and processed foods. This is of particular importance for the starch component, as whole grains that are further processed to commercial products often significantly increase glycemic response. Many whole grain foods, as approved in regulatory systems, are also composed of recombined flour streams. The outcome of these processes on the health‐related property of final whole grain food products is not well understood and conceivably could be neutral, negative or positive in regards to glycemic response, as well as on the action of the dietary fiber component in the gastrointestinal tract, including its effect on the gut microbiota. The study of Mozaffarian et al. (Mozaffarian et al. 2013) on whole grain products showed that a ratio of total carbohydrate (i.e., mainly glycemic carbohydrate) to dietary fibre of ≤ 10:1 was characteristic of healthy whole grain food products. In addition to the composition of dietary carbohydrates, the spatial arrangement of starch and dietary fibre in the food matrix is also important to the digestion property of starch and probably to the health benefit of whole grain foods (Heaton et al. 1988; Björck et al. 1994).