Physicochemical characteristics of food and the digestion of starch and dietary fibre during gut transit.

Food consists of a complex group of substances that differ in chemical composition and physical structure. Collectively these substances form a matrix and it is the behaviour of this matrix which determines the digestibility, the rate and extent of nutrient release, from a food during normal gut transit. In the process of digestion both chemical and physical degradation of food occurs. Chemical degradation allows nutrient release from food, and nutrient values provided in food tables are derived by chemical analysis of food and must be combined with a knowledge of the potential nutrient availability in a particular food if a proper understanding is to be gained of nutrient utilization by the body. Nutrient availability, applied to mean nutrients in a form suitable for digestion, absorption and utilization, has been extensively studied in relation to minerals and especially iron (Hallberg et al. 1986). However, less is known about availability of other nutrients, as can be illustrated from a consideration of ‘complex-carbohydrate’ digestion. Digestion and rate of absorption of carbohydrate has been shown to be important in the control of glycaemia (Crapo er al. 1980; Simpson et al. 1981; Thorne er al. 1983) and physical form of food is an important determinant of digestion rate. The effect of physical structure, however, is difficult to predict and can act to either enhance or restrict nutrient release. For example, the action of rennin on milk protein changes milk from an emulsion to a gel and enhances digestibility, but the presence of lignin in forage and fibre restricts accessibility of cell-wall polysaccharides to microbial degradation. Thus the extent and rate of digestion during gut transit is a function of the complex interaction between physical and chemical properties of a food matrix. Measurements of physical properties of foods (Table 1) have been made in an attempt to determine their effect on digestion. However, as will be shown for starch and fibre digestion, the results often depend on methods used and may conflict with digestion results determined in vivo.