Dietary fibre and health: an overview

Summary Many studies have found that people on diets high in fibre have reduced risks of certain diseases such as cancers, coronary heart disease, obesity and possibly diabetes. Fibre is a collective term for a group of compounds, which differ in their chemical structure and physical properties and elicit a variety of physiological effects. Some health benefits linked to fibre consumption are well established (e.g. promoting a regular bowel habit) and others are becoming more firmly established. The effects of the various non-digestible components are not always fully interchangeable, although there is considerable overlap, but they share an ability to pass undigested into the large bowel. This makes it important that fibre intake comes from a range of sources to ensure maximum health benefits. There are disagreements as to which plant-derived compounds constitute ‘fibre’ and which analytical methodologies should be used to determine the fibre content of foods. This lack of a universal definition for these carbohydrates that resist digestion in the small intestine has led to complications in establishing and communicating consistent recommendations for the type and amounts of fibre components in the diet, as well as issues related to food labelling and health claims. Furthermore, current analytical methods may typically underestimate the fibre content of foods, using the definition advised by the European Food Safety Authority. Chemical nomenclature is necessary for a coherent and enforceable approach to the measurement of fibre (and related to this, product labelling), but reconciling chemical definitions for fibre with definitions that reflect physiological effects remains a major challenge. Progress in the application of the emerging evidence regarding dietary fibre and health is constrained by a need to resolve the definition and methodology issues. Most consumers are likely to be unaware of the more recent developments and the overlapping properties of different types of carbohydrate that resist digestion in the small intestine, and the particular sources of these carbohydrates. Long overdue is the translation of the growing knowledge about the physiological properties of carbohydrates, particularly non-digestible carbohydrates, into clear and unambiguous public health messages. Many terms in common use stem from chemistry rather than physiology, and this makes derivation of health messages more difficult. Meanwhile in the UK, where the main sources of fibre are cereals and cereal products, followed by vegetables and potatoes, adults are not meeting the recommended 18 g of fibre per day (defined in the UK as non-starch polysaccharide measured by the Englyst method). The recommended amount for the UK, established in 1991, is lower than elsewhere in the world and corresponds to about 24 g according to the method used by the Association of Official Analytical Chemists (the AOAC method). Reference values in other parts of the world are typically between 25 and 35 g (AOAC fibre). A diet rich in whole grains, pulses, fruit and vegetables will assist in meeting current fibre recommendations, but a growing number of other foods now contain carbohydrate-derived ingredients that resist digestion.