Lactose is found in nature only in milk.  It apparently increases the
absorption of calcium.  It is a source of galactose, which is necessary for
making galactolipids, which are important fats of the brain.  Although there
is a mechanism for making galactose from scratch in the body without eating
it, this may be a further stress on the organism, already getting abnormal
food.  There is a rough relationship between the size of an animal specie's
brain and the quantity of lactose in its milk.

Jack Newman, MD, FRCPC