Carbohydrates in human nutrition

BOX 1

RECOMMENDATIONS OF THE FAO/WHO EXPERT CONSULTATION ON CARBOHYDRATES IN HUMAN NUTRITION

The following recommendations are derived from the Consultation discussions and resulting conclusions detailed in the report. Specific recommendations are grouped under the appropriate report headings.

The role of carbohydrates in nutrition

1. That the terminology used to describe dietary carbohydrate be standardized with carbohydrates classified primarily by molecular size (degree of polymerization or DP) into sugars (DP 1-2), oligosaccharides (DP 3-9) and polysaccharides (DP 10+). Further subdivision can be made on the basis of monosaccharide composition. Nutritional groupings can then be made on the basis of physiological properties.
2. That the concept of glycaemic carbohydrate, meaning "providing carbohydrate for metabolism", be adopted.
3. Against the use of the terms "extrinsic" and "intrinsic sugars", "complex carbohydrate" and "available" and "unavailable carbohydrate".
4. That food laboratories measure total carbohydrate in the diet as the sum of the individual carbohydrates and not "by difference".
5. That the use of the term "dietary fibre" should always be qualified by a statement itemizing those carbohydrates and other substances intended for inclusion. Dietary fibre is a nutritional concept, not an exact description of a component of the diet.
6. That the use of the terms "soluble" and "insoluble dietary fibre" be gradually phased out. The Consultation recognized that these terms are currently used, but does not consider them a useful division, either analytically or physiologically
7. That the analysis and labelling of dietary carbohydrate, for whatever purpose, be based on the chemical divisions recommended. Additional groupings such as polyols, resistant starch, non-digestible oligosaccharides and dietary fibre can be used, provided the included components are clearly defined.
8. That the energy value of all carbohydrate in the diet be reassessed using modern nutritional and other techniques. However, for carbohydrates that reach the colon, the Consultation recommends that the energy value be set at 2 kcal/g (8 kJ/g) for nutritional and labelling purposes.
9. That the continued production and consumption of root crops and pulses be encouraged to ensure the adequacy and diversity of the supply of carbohydrate.
10. That the continued consumption of traditional foods rich in carbohydrate should be encouraged where populations are in transition from a subsistence rural economy to more prosperous urban lifestyles. Processed foods are likely to be a substantial part of the diet and processing can be used to optimize nutritional properties.

The role of carbohydrates in the maintenance of health

11. That the many health benefits of dietary carbohydrates should be recognized and promoted. Carbohydrate foods provide more than energy alone.
12. An optimum diet of at least 55 percent of total energy from a variety of carbohydrate sources for all ages, except for children under two years. Fat should not be specifically restricted below the age of two years. The optimum diet should be introduced gradually beginning at two years of age.
13. That energy balance be maintained by consuming a diet containing at least 55 percent total energy from carbohydrate from various sources, and engaging in regular physical activity.
14. Against consuming carbohydrate levels above the optimum, including carbohydrate-containing beverages, for purposes of recreational physical activity. Higher carbohydrate intakes are only needed for long-term extreme endurance physical activities.
15. That, as a general rule, a nutrient-dense, high-carbohydrate diet be considered optimal for the elderly, but that individualization is recommended because their specific nutritional needs are complex.

Dietary carbohydrate and disease

16. That a wide range of carbohydrate-containing foods be consumed so that the diet is sufficient in essential nutrients as well as total energy, especially when carbohydrate intake is high.
17. That the bulk of carbohydrate-containing foods consumed be those rich in non-starch polysaccharides and with a low glycaemic index. Appropriately processed cereals, vegetables, legumes and fruits are particularly good food choices.
18. That excess energy intake in any form will cause body fat accumulation, so that excess consumption of low-fat foods, while not as obesity-producing as excess consumption of high-fat products, will lead to obesity if energy expenditure is not increased. Excessive intakes of sugars, which compromise micronutrient density, should be avoided. There is no evidence of a direct involvement of sucrose, other sugars and starch in the etiology of lifestyle-related diseases.
19. That national governments provide populations in transition from traditional diets to those characteristic of developed countries with dietary recommendations to ensure nutritional adequacy and retention of an appropriate balance of macronutrients.

The role of the glycaemic index in food choice

20. That, for healthful food choices, both the chemical composition and physiological effects of food carbohydrates be considered, because the chemical nature of the carbohydrates in foods does not completely describe their physiological effects.
21. That, in making food choices, the glycaemic index be used as a useful indicator of the impact of foods on the integrated response of blood glucose. Clinical application includes diabetes and impaired glucose tolerance. It is recommended that the glycaemic index be used to compare foods of similar composition within food groups.
22. That published glycaemic response data be supplemented where possible with tests of local foods as normally prepared, because of the important effects that food variety and cooking can have on glycaemic responses.

The major dietary carbohydrates

BOX 2

CARBOHYDRATES AND OBESITY

Among the general public, obesity is a great focus of interest for aesthetic reasons. It is also a focus of concern among public health authorities because it is an independent factor of risk for diseases of lifestyle. Obese individuals are at a high risk of contracting non-insulin-dependent diabetes melitus (NIDDM), cardiovascular disease, disorders of the gastrointestinal tract and some types of cancer (Jung, 1997; FAO/WHO 1998). Therefore, the most promising approach to reducing the incidence of these lifestyle diseases is to reduce the incidence of obesity. On an individual basis, a key strategy in seeking to improve health and prevent non-communicable disease conditions related to dietary lifestyle is to encourage the prevention of excess weight gain leading to obesity or, if obesity is already present, and particularly if it is of the type that is centrally distributed in the body, to embark on a programme to lose weight permanently. As too many people already know, this is easier said than done, but it seems that, from a dietary point of view, increasing the consumption of high-carbohydrate foods, in replacement of high-fat foods, may be a good starting point. The next question relates to whether some carbohydrate foods are more efficient than others at achieving this end. This, in effect, is a question about satiety.

A key area of research in the hunt for ways of preventing excess weight gain is the attempt to find out what brings about satiation and satiety (see definitions below). This has led to studies of diets where the proportions of carbohydrate and fat are manipulated to provide high-carbohydrate or high-fat meals, and their respective impacts on energy balance are examined. Losing weight is a question of ingesting less energy than that which is expended, and this can only be achieved by a lower energy intake, an increased energy expenditure through increased physical activity, or both. It should be emphasized that this other side of the energy equation (energy expenditure) is equally important.

Carbohydrate foods seem to offer a particularly good choice on the energy intake side. In reviewing the subject, Jebb (1998) presents convincing arguments for increasing the intake of carbohydrate foods. Holt et al. (1999) reported recently on the effect of high-carbohydrate versus high-fat breakfasts on feelings of fullness and subsequent food intake, and have thus contributed to the evidence that shows that the portion size (the weight and volume of food) and the energy density (kcal/g) of meals are among the strong determinants of the satiating powers of diets, when energy levels ingested are equal. Foods that are high in carbohydrate, particularly when they are high in dietary fibre, had the most impact on reducing hunger and would, therefore, be expected to be those most helpful in reducing energy intake. By contrast, high-fat meals produced low fullness ratings and led to a faster onset of subsequent eating. In their review of the literature, Holt et al. cited other work that has shown that subjects tend to eat a constant weight of food, regardless of dietary composition. It could therefore be expected that meals with a lower energy density will have a better chance of preventing excess energy intake. But this does not constitute the magic formula that everyone looks for, and the expert report accordingly makes a recommendation of caution (Box 1, recommendation 18). It should be remembered that a calorie is a calorie, no matter how it is provided.

Definitions of terminology

appetite: the desire to eat, independent of the body's energy situation;

hunger: the sensation that arises when metabolic signals indicate the need to provide the body with food energy;

satiation: the process that leads to the end of an eating occasion;

satiety: the length of time before another meal is desired.

Carbohydrates in human nutrition

FAO and the World Health Organization convene expert consultations on nutrition to ensure that the agencies' scientific knowledge is up to date and to provide advice to member countries. The Expert Consultation on Carbohydrates in Human Nutrition was held in April 1997. In this article, the full set of recommendations made by the experts is provided. Aspects of the experts' report are highlighted, such as the importance of analytical methodologies in stimulating research on carbohydrates, recommended ways of classifying carbohydrates to facilitate food labelling, the interest of the food industry in functional foods, theories about the role of carbohydrates in controlling obesity, the glycaemic index, and some of the contributions carbohydrates make in preventing illness and protecting health.

Le r�le des glucides dans la nutrition humaine

La FAO et l'Organisation mondiale de la sant� tiennent des consultations d'experts en mati�re de nutrition afin de s'assurer que les connaissances scientifiques des diff�rentes institutions sont � jour, et de fournir des conseils aux pays membres. C'est dans ce contexte que s'est tenue, en avril 1997, la Consultation d'experts sur le r�le des glucides dans la nutrition humaine. Le pr�sent article brosse un tableau complet des recommandations �manant des experts, tout en soulignant certains aspects de leur rapport, tels que l'importance des d�marches analytiques pour encourager les recherches sur les glucides, les m�thodes pr�conis�es pour la classification des glucides afin de faciliter l'�tiquetage des aliments, l'int�r�t de l'industrie alimentaire pour les aliments fonctionnels, les th�ories concernant le r�le des glucides dans la lutte contre l'ob�sit�, l'indice de glyc�mie, de m�me que certains aspects du r�le des glucides dans la pr�vention des maladies et la protection de la sant�.

Los carbohidratos en la nutrición humana

La FAO y la Organizaci�n Mundial de la Salud (OMS) convocan consultas de expertos en nutrici�n para cerciorarse de que el conocimiento cient�fico de las organizaciones est� actualizado, y para suministrar asesoramiento a los Estados Miembros. La Consulta de Expertos sobre los Carbohidratos en la Nutrici�n Humana se celebr� en abril de 1997. En el presente art�culo se han incluido todas las recomendaciones formuladas por los expertos. Tambi�n se ponen de relieve algunos aspectos del informe de los expertos tales como la importancia de los an�lisis para estimular la investigaci�n sobre los carbohidratos, los m�todos recomendados para clasificar los carbohidratos a fin de facilitar el etiquetado de alimentos, el inter�s de la industria alimentaria en los alimentos funcionales, teor�as sobre la funci�n de los carbohidratos en el control de la obesidad, el �ndice de glicemia, y algunas contribuciones de los carbohidratos en lo que se refiere a la prevenci�n de enfermedades y la protecci�n de la salud.