Why does this happen?
Bearing in mind that beer is about 95% water and 5% alcohol, the liver converts this 5% into about the same amount of water. The reason is that alcohol affects, the pituitary gland, specifically the part that regulates the amount of water in our body.
The pituitary gland produces a hormone called vasopressin, one of whose jobs is to reduce urination when we start to become dehydrated. ADH (vasopressin) is released to send a signal to the body to reduce urine production and conserve water.
Alcohol reduces vasopressin production, which in turn increases urine production. Because alcohol affects the brain very quickly, urine production also increases very quickly just after alcohol consumption.