From a nutritional point of view, carbohydrates are quantitatively the most important nutrients in the human diet; according to dietary recommendations 50-60 per cent of our total energy intake should come from them. Many food products contain significant amounts of carbohydrates. Nevertheless, surprisingly little attention still is given to the choice of carbohydrates with respect to their physiological properties when it comes to the development of new products. It was only in the early 1980s that the scientific community began to focus on the physiological diversity of carbohydrates. Since then, knowledge of the physiological roles of different types of carbohydrates and their involvement in health and disease has developed considerably and has challenged many long-held beliefs about sugars, starches and dietary fibre.
The challenge in the classification of traditional carbohydrates has been to align the chemical divisions of carbohydrates with those reflecting their physiological properties. Nowadays, with increased research the health benefits of carbohydrates have expanded beyond their nutritive functions making classifications even more complex. The term 'functional carbohydrates' has been coined to describe carbohydrates that have an inherent physiological functionality in addition to the more common nutritional value of their traditional cousins. As the diversity of carbohydrates has grown it has become increasingly difficult for nutritionists and food developers alike to keep track of the range of carbohydrates now available, their characteristics and benefits. Present classification approaches are limited when it comes to making clear the full benefits of the latest functional carbohydrates.
Present classification: Defined by chemical nature
One of the most traditional forms of carbohydrate classification is according to their degree of polymerisation (DP), ie, the number of monosaccharide units. Carbohydrates are divided into sugars (monosaccharides and disaccharides), oligosaccharides, and polysaccharides as well as hydrogenated carbohydrates (polyols). Each of these groups can be further divided into sub-groups depending on the make-up and quantity of the monosaccharide units. Although this method of classification is comprehensive in terms of defining carbohydrates by their chemical structure, it raises difficulties for food ingredients manufacturers and nutritionists because physiological properties, like the rates of digestion, could vary widely within a given group. For example, the oligosaccharides category comprises malto-oligosaccharide, which is highly digestible, as well as fructo-oligosaccharide, which is nondigestible.
Defined by carbohydrate availability
Because the chemical nature of carbohydrates in foods does not completely describe their physiological effects, carbohydrates have further been classified in terms of their availability to the human metabolism. This approach offers a useful reference point as to the digestibility of a specific carbohydrate and its subsequent physiological availability. 'Available' or 'glycaemic' carbohydrates are — rapidly or slowly — digested and absorbed in the small intestine (this category includes glucose, sucrose, fructose and processed starches) and 'non-available' or 'non-glycaemic' carbohydrates (eg, dietary fibres like inulin or oligofructose) are those that largely escape digestion and absorption in the small intestine and are completely or partially fermented in the large intestine, leading to virtually no blood glucose response. This approach has its limitations, in that it does not take into account the different speeds of digestion of available carbohydrates.
Defined by glycaemic impact
The development and refinement of the glycaemic index (GI) from 1981 onwards has helped to gain a greater understanding of the glycaemic impact of different foods on blood-sugar levels. Used to classify foods based on their blood glucose raising potential the glycaemic index (GI) reflects the blood glucose response of a carbohydrate as result of its rate of intestinal digestion and subsequent release into the bloodstream (in comparison with a reference carbohydrate or food) and assigns different categories as low (=55), medium (56-69) and high (=70). The limitation of this approach is, for instance, that the GI of an individual food can vary depending on the way it has been cooked or processed (or even its degree of ripeness). But the GI is an important step in the right direction when it comes to simplifying the complex world of carbohydrates and their physiological response for consumers as well as food-industry professionals.
Is thinking beyond traditional classifications needed?
The obvious answer is: yes! Although largely effective in classifying traditional carbohydrates, the approaches detailed above have their limitations when it comes to giving complete pictures of the features and benefits of next-generation carbohydrates.
It is clear that as more is discovered about the energy metabolism of functional carbohydrates, they will have the potential to play an increasingly significant part in healthy, weight-managed diets. Characterised by their low-glycaemic properties with related low insulin levels and their support for normal glycaemia and lipid metabolism, functional carbohydrates are vital tools offering new opportunities in creating healthy and innovative food and drink products. As can be seen, the next generation of carbohydrates has a wealth of characteristics that goes well beyond the present classification formats. For the time being, nutritionists and food developers would do well to think rely on scientific data rather than on traditional formats.
Dr Antje Jungclaus is manager of nutrition communication at BENEO-Palatinit.