Carbohydrate foods often contain vitamins and minerals plus other compounds, such as phytochemicals and antioxidants, which may have health implications. Consuming a wide variety of carbohydrate foods is therefore recommended as this is more likely to be a nutritionally adequate diet with the health benefits commonly ascribed tocarbohydrate foods (83).
Food choice depends not only on nutrition and health considerations but also on factors such as local availability, cultural acceptability and individual likes and needs. There is no one measure which can be used to guide food choices in all cases. The chemical composition of foods (e.g. fat, sugars, dietary fibre content) should be an important factor influencing food choice.However, simply knowing the chemical nature of the carbohydrates in foods, for example, does not reliably indicate their actual physiologic effects. Foods which are good choices in some situations may not be the best choices in others. Likewise, foods which are poor choices in some situations may be good choices in others.
Two indices of carbohydrate foods based on their physiologic functionshave been proposed. A recently suggested satiety index (84) measures the satiety value of equal energy portions of foods relative to a standard, which is white bread. The factors which control food intake are complex and satiety needs to be distinguished from satiation. Nevertheless, investigation of satiety indices of foods is considered an interesting area of future research, which, ifvalidated, may aid in the selection of appropriate carbohydrate foods to promote energy balance. A more established index is the glycemic index which can be used to classify foods based on their blood glucose raising potential.
Definition of glycemic index (GI)
The glycemic index is defined as the incremental area under the blood glucose response curve of a 50g carbohydrate portion of a test foodexpressed as a percent of the response to the same amount of carbohydrate from a standard food taken by the same subject. The italicized terms are discussed below because the methods used to determine the glycemic index of foods and to apply the information to diets may profoundly affect the results obtained.
Incremental area under the curve
A number of different methods have been used tocalculate the area under the curve. For most glycemic index data, the area under the curve has been calculated as the incremental area under the blood glucose response curve (IAUC), ignoring the area beneath the fasting concentration. This can be calculated geometrically by applying the trapezoid rule. When a blood glucose value falls below the baseline, only the area above the fasting level isincluded. Sample data are shown in Table 1. The data for Standard #1 are used in the diagram in Figure 2 to illustrate the details of the actual calculation.
TABLE 1 Sample blood glucose responses to the ingestion of 50g carbohydrate
|Nature of the monosaccharide components |
| |Fructose |
| |Galactose |
|Nature of the starch |
| |Amylose |
| |Amylopectin |
| |Starch-nutrient interaction |
||Resistant starch |
|Cooking/food processing |
| |Degree of starch gelatinization |
| |Particle size |
| |Food form |
| |Cellular structure |
|Other food components...