European Journal of Clinical Nutrition (2007) 61 (Suppl 1), S5–S18 & 2007 Nature Publishing Group All rights reserved 0954-3007/07 $30.00 www.nature.com/ejcn REVIEW Carbohydrate terminology and classification JH Cummings1 and AM Stephen2 1Gut group, Division of Pathology and Neuroscience, Ninewells Hospital and Medical School, Dundee, UK and 2Population Nutrition Research, MRC Human Nutrition Research, Elsie Widdowson Laboratory, Cambridge, UK Dietary carbohydrates are a group of chemically defined substances with a range of physical and physiological properties and health benefits. As with other macronutrients, the primary classification of dietary carbohydrate is based on chemistry, that is character of individual monomers, degree of polymerization (DP) and type of linkage (a or b), as agreed at the Food and Agriculture Organization/World Health Organization Expert Consultation in 1997. This divides carbohydrates into three main groups, sugars (DP 1–2), oligosaccharides (short-chain carbohydrates) (DP 3–9) and polysaccharides (DPX10). Within this classification, a number of terms are used such as mono- and disaccharides, polyols, oligosaccharides, starch, modified starch, non-starch polysaccharides, total carbohydrate, sugars, etc. While effects of carbohydrates are ultimately related to their primary chemistry, they are modified by their physical properties. These include water solubility, hydration, gel formation, crystalline state, association with other molecules such as protein, lipid and divalent cations and aggregation into complex structures in cell walls and other specialized plant tissues. A classification based on chemistry is essential for a system of measurement, predication of properties and estimation of intakes, but does not allow a simple translation into nutritional effects since each class of carbohydrate has overlapping physiological properties and effects on health. This dichotomy has led to the use of a number of terms to describe carbohydrate in foods, for example intrinsic and extrinsic sugars, prebiotic, resistant starch, dietary fibre, available and unavailable carbohydrate, complex carbohydrate, glycaemic and whole grain. This paper reviews these terms and suggests that some are more useful than others. A clearer understanding of what is meant by any particular word used to describe carbohydrate is essential to progress in translating the growing knowledge of the physiological properties of carbohydrate into public health messages. European Journal of Clinical Nutrition (2007) 61 (Suppl 1), S5–S18. doi:10.1038/sj.ejcn.1602936 Keywords: carbohydrate; sugars; oligosaccharides; starch; dietary fibre; classification Introduction density, large bowel cancer, constipation and resistance to gut infection. The dietary carbohydrates are a diverse group of substances with a range of chemical, physical and physiological proper- ties. While carbohydrates are principally substrates for energy metabolism, they can affect satiety, blood glucose Classification and insulin, lipid metabolism and, through fermentation, exert a major control on colonic function, including bowel As for other macronutrients, the primary classification of habit, transit, the metabolism and balance of the commensal dietary carbohydrates, as proposed at the Joint Food and flora and large bowel epithelial cell health. They may also be Agriculture Organization (FAO)/World Health Organization immunomodulatory and influence calcium absorption. (WHO) Expert Consultation on Carbohydrates in human These properties have implications for our overall health; nutrition convened in Rome in 1997 (FAO, 1998), is by contributing particularly to the control of body weight, molecular size, as determined by degree of polymerization a a diabetes and ageing, cardiovascular disease, bone mineral (DP), the type of linkage ( or non- ) and character of individual monomers (Table 1). This classification is analo- gous to that used for dietary fat, which is based on carbon chain length, number and position of double bonds and their configuration as cis or trans. A chemical approach is Correspondence: Professor JH Cummings, Division of Pathology and Neuroscience, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK. necessary for a coherent and enforceable approach to E-mail: [email protected] measurement and labelling forms the basis for terminology Carbohydrate terminology and classification JH Cummings and AM Stephen S6 Table 1 The major dietary carbohydrates Two principal approaches to total carbohydrate are used, first, that derived ‘by difference’ and second, the direct Class (DPa) Subgroup Principal components measurement of the individual components that are then Sugars (1–2) Monosaccharides Glucose, fructose, galactose combined to give a total. Calculating carbohydrate ‘by Disaccharides Sucrose, lactose, maltose, difference’ has been used since the early 20th century and trehalose is still widely used around the world (Atwater and Woods, Polyols (sugar Sorbitol, mannitol, lactitol, 1986; United States Department of Agriculture, 2007). The alcohols) xylitol, erythritol, isomalt, maltitol moisture, protein, fat, ash and alcohol content of a food are Oligosaccharides Malto- Maltodextrins determined, subtracted from the total weight of the food and (3–9) (short-chain oligosaccharides the remainder, or ‘difference’, is considered to be carbohy- carbohydrates) (a-glucans) drate. There are, however, a number of problems with this Non-a-glucan Raffinose, stachyose, fructo and oligosaccharides galacto oligosaccharides, approach in that the ‘by difference’ figure includes non- polydextrose, inulin carbohydrate components such as lignin, organic acids, Polysaccharides Starch Amylose, amylopectin, modified tannins, waxes and some Maillard products. In addition to X ( 10) (a-glucans) starches this error, it combines all the analytical errors from the other Non-starch Cellulose, hemicellulose, pectin, polysaccharides arabinoxylans, b-glucan, analyses. Also, a single global figure for carbohydrates in (NSPs) glucomannans, plant gums and food is uninformative because it fails to identify the many mucilages, hydrocolloids types of carbohydrates and thus to allow some under- standing of the potential health benefits of those foods. aDegree of polymerization or number of monomeric (single sugar) units. Based on Food and Agriculture Organization/World Health Organization Direct analysis of carbohydrate components and summa- ‘Carbohydrates in Human Nutrition’ report (1998), and Cummings et al. tion to obtain a total carbohydrate value has been the basis (1997). of carbohydrate analysis in the UK since 1929, when the first values were published by McCance and Lawrence (1929). and an understanding of the physiological and health effects Those countries that use McCance and Widdowson’s, The of these macronutrients. Composition of Foods (Food Standards Agency/Institute A chemical approach divides carbohydrates into three of Food Research, 2002) also express carbohydrate using main groups, sugars (DP1–2), oligosaccharides (short-chain this approach. The total figure obtained is for what McCance carbohydrates) (DP3–9) and polysaccharides (DPX10). and Lawrence called ‘available carbohydrate’ and therefore Sugars comprise (i) monosaccharides, (ii) disaccharides and differs from carbohydrate by difference in that it does not (iii) polyols (sugar alcohols). Oligosaccharides are either (a) contain the plant cell wall polysaccharides (fibre). In malto-oligosaccharides (a-glucans), principally occurring addition, it is not complicated by analytical difficulties with from the hydrolysis of starch and (b) non-a-glucan such as other food components. Dietary intake of total carbohydrate raffinose and stachyose (a galactosides), fructo- and galacto- and its components using direct analysis enables examina- oligosaccharides and other oligosaccharides. Polysaccharides tion of geographic variations and changes in intake over may be divided into starch (a-1:4 and 1:6 glucans) and non- time of individual carbohydrate types and their relationship starch polysaccharides (NSPs), of which the major compo- with health outcomes. Total carbohydrate by direct measure- nents are the polysaccharides of the plant cell wall such as ment is preferable and simplified methods to do this should cellulose, hemicellulose and pectin but also includes plant be developed. gums, mucilages and hydrocolloids. Some carbohydrates, Figures obtained for carbohydrate by difference and like inulin, do not fit neatly into this scheme because they carbohydrate analysed directly are not always the same, exist in nature in multiple molecular forms. Inulin, GFN, particularly for complex mixtures, and foods containing fibre from plants may have from 2 to 200 fructose units and so or certain types of starch, like pasta (Stephen, 2006). This crosses the boundary between oligosaccharides and poly- results in apparently different carbohydrate intakes for the saccharides (Roberfroid, 2005). same list of foods consumed, as shown in Table 2. Fifty-two A variety of methodologies are available for the measure- dietary records from a study conducted in Canada, where ment of the carbohydrate content of food and the compo- carbohydrate by difference is used (Health Canada, 2005) nents are listed in Table 1 (Englyst et al., 2007). were subsequently analysed in the UK using values based on McCance and Widdowson’s The Composition of Foods (Hol- land et al., 1991b, 1992). In this study, energy intake was 12% higher and carbohydrate