Carbohydrate chemistry is complex. This article will hopefully clarify some areas relevant to food and nutrition.
In food labelling, the sugars value given is the total of all mono- and di-saccharides in the product.
Monosaccharides (including glucose, fructose and galactose) are the simplest form of sugars, having one carbon ring in the structure
Disaccharides (including sucrose, lactose and maltose) have two rings: they break down in the digestive system into two monosaccharides (for example sucrose breaks down into glucose and fructose)
Starch consists of thousands of glucose units joined together
Oligosaccharides sit between sugars and starch. They contain at least three rings, so they do not count as sugars.
This is one reason why the starch and sugars in a food may not add up to the available carbohydrate value.
Oligosaccharides commonly occur in glucose syrups (where starch has been treated to break down mostly into glucose, but a lot of oligosaccharide is also created). They also occur naturally in some foods, particularly onions.
Polyols have a different structure to sugars, starch and oligosaccharides. They include glycerol, sorbitol, xylitol and isomalt. They contribute fewer calories than sugars.
5) available carbohydrate
Available carbohydrate is the digestible amount of carbohydrate in the food product. It, therefore does not include dietary fibre. In nutrition calculation for a product, available carbohydrate is calculated from the carbohydrate contents of the ingredients. In lab testing, available carbohydrate is obtained by subtracting fat, protein, fibre, moisture and ash from 100%. The precision of this method is therefore limited as it can add together a number of smaller inaccuracies.
6) total carbohydrate
Total carbohydrate differs from available carbohydrate only in that it includes fibre.
Fibre is a carbohydrate that cannot be digested by enzymes in the digestive system. This is a simple enough definition but attempting to measure dietary fibre in the lab is tricky, because we cannot create a digestive system in a lab method. And the simpler lab methods for dietary fibre may not pick some forms of fibre, especially soluble ones.
Dr. David F. Bartley