Lactose, the main carbohydrate in milk and other dairy products, is found only in the milk of mammals (Figure 4.1). It is not known why milk contains a special carbohydrate; one hypothesis is that lactose solubility may be matched best with milk synthesis and expression and may provide appropriate energy while minimizing osmotic load (Mustapha et al. 1997a). As far as is known, lactose has no special nutritional importance for adults; however, it is the most important source of energy during the first year of a human’s life, providing almost half the total energy requirement of infants.