To understand why this happens, we need to look at metabolic pathways. These are chemical reactions that convert molecules into the body. When everything in the body is working the way it should, galactose is converted into a specific pathway (pathway 1). In this pathway, galactose is converted into a metabolite called Gal-1p. People with Galactosemia are missing an enzyme called GALT, so Gal-1p cannot be processed any further. Instead, Gal-1p is backed up and clogs the pathway, meaning galactose cannot go where it should go. When this happens, galactose overflows into a different pathway (pathway 2) where it should not go.
When galactose travels down pathway 2, it does not know how to properly process the galactose. An enzyme that is not supposed to be involved called Aldose Reductase steps in and converts the galactose. Aldose Reductase changes galactose into toxic galactitol causing harm to the body. Galactitol can cause build up in the blood, tissues and organs including the brain. Finding new ways to block toxic galactitol may help reduce damage to the body.