Not completely right. The human digestive track is able to absorb monosaccharides. Therefore they belong to the available carbohydrates.

Not completely right. In the human body digestive enzymes are present which could hydrolyse disaccharides into monosaccharide units. Therefore disaccharides belong to the available carbohydrates. Note that some people are unable to metabolise lactose (lactose intolerance). For these persons, lactose can be seen as an unavailable carbohydrate.

Not completely right. Sugar alcohols are poorly absorbed in the small intestine, but are fermented by the micro flora in the large gut. Therefore they don’t belong to the available carbohydrates.

Not completely right. Some oligosaccharides belong to the available carbohydrates, others belong to unavailable carbohydrates. Whether an oligosachharide is digested in the digestive track depends on the type of linkage between the monosaccharide units. The human body contains digestive enzymes (amylase) which are able to break down the bounds between Glucose in the alpha position but not in the beta position (see glossary: glucosidic bound). Study the information on Stachyose, FOS and/or Maltotriose again to find out which of them belong to the available carbohydrates.

Not completely right. When you study the information about starch you will learn that in starch the glucose units are linked by alpha-1,4 or alpha-1,6 linkages. The human body contains digestive enzymes (amylase) which are able to break down these alpha linkages. Although part of the starch occurs in a partially crystalline form in granules and therefore resists digestion, starch is still viewed as available carbohydrate.

Not completely right. The human body is not able to digest the plant cell wall polysaccharides Pectin and Cellulose.

Not completely right. Glycogen consist of glucose units bound by alpha linkages. The human body contains digestive enzymes (amylase) that are able to break down these alpha linkages. Therefore Glycogen is an available carbohydrate.

Not completely true. Lignin is not a polysaccharide. Above this it is not digested by the human body.

Right! Summarized, unavailable carbohydrates are mainly the non starch polysaccharides (NSPs). Together with lignin these NSPs are called fibre. Also the unavailable oligosaccharides (like FOS and Stachyose) are included in the definition of fibre. In the second part of this case more attention will be given to the definitions of fibre. Besides fibre also the sugar alcohols are unavailable for digestion.

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Exercise: Select all available carbohydrates.

So far you studied the classification of carbohydrates from a chemical point of view. However, a classification based on chemistry alone does not allow a simple translation into nutritional properties and health effects since each of the major chemical classes has a variety of gastrointestinal and metabolic properties.

A major step forward in understanding carbohydrates was made by McCance and Lawrence (1929). They divided the dietary carbohydrates into available and una vailable carbohydrates. Available carbohydrates are those digested in the small intestine. They are also called glycaemic carbohydrates. Unavailable (non-glycaemic) carbohydrates are those that resist digestion in the small intestine. However they also contribute to energy delivery in the body via fermentation processes in the colon.

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