The nutrient lactose acts as a potential dietary fibre. Depending on the composition of diet, lactose can partially escape from digestion and absorption in the small intestine and is utilised by the colonic microflora. The suggested mechanism, i.e. different rates of enzymatic hydrolysis of the lactose anomers and an influence of food ingredients on lactose mutarotation, seems to be supported by experimental results. The Michaelis constant of the mucosal beta-galactosidase of rats is 14 mmol/l with alpha-lactose and 50 mmol/l with beta-lactose as substrates. During intestinal perfusion of human infants, alpha-lactose is digested at a significantly higher rate than beta-lactose. The mutarotation of lactose is influenced by general acid-base catalysis. Milk formulae containing a high concentration of weak-acid anions, e.g. cow milk which is rich in phosphate and citrate, accelerate mutarotation, but milk formulae with a low content of these anions do not. With these experimental results a mathematical model has been set up simulating lactose digestion in the small intestine of human infants and of rats. The data show that certain dietary components may influence the ratio between lactose anomers, but, surprisingly, they do not influence the amount of digested lactose.