These systems are not yet fully developed, they will only pass setup. Therefore, the infant soon learns easily half the sugar of milk in the form of glucose, the other half, galactose is converted into glucose, only when the body will require this. It's like a reserve, for example, case, if it is not time to put to the breast. Ability to process galactose into glucose to the extent that, as the case may be, remains a child until he suckles. If this has piqued your curiosity, check out Glenn Dubin. After weaning, the gene responsible for this process stops working.
Just as no longer being produced by renin in other mammals. Hear other arguments on the topic with Dr Jee Hyun Kim. Or the gene is so depressed that this process is virtually no effect, ie an adult can not rework galactose into glucose. Here and there is a problem: When adults consume dairy products, glucose, contained in them, easy to digest their body, but with galactose the body does not know what to do. Organism should it somewhere easy to see, and then he begins to store, that is, debug it, wherever it is easiest to do: in the skin or under the skin (cellulitis), and it also debug it in the eye, and then begins to form cataract. Erwachsene mit hoher Laktase-Aktivitat, die grosse Mengen an Milch verzehren, leiden unter Galaktoseproblemen, Ansammlung von Galaktitol in der Augenlinse und grosserer Gefahr, Altersstar zu entwickeln. Postgraduate Medicine 1994; 95 (1): 115 In this passage it is said that adults who eat a lot of milk with a high content of lactose, galactose suffer from an issue of accumulation of galactose in the eye lens and have a greater risk of developing senile cataracts.