From what i understand, the lactase enzyme was evolved in adult humans when we began breeding livestock that produced milk and drinking this milk as a source of nutrition. It is strange for one species to drink the milk of another species i am sure, but milk would contain all the necessary nutrients for healthy growth of a calf for several weeks before it transitions to eating grass. The quantity of lactase found in humans decreases or is gone completely from the age of 5 onwards (indicating this is only to digest milk from breastfeeding). Is it safe for a non-lactose intolerant person to drink extremely high quantities, or even entirely live off of cows milk and for how long? Would it also be safe for a lactose intolerant person despite the symptoms they will experience?