As someone who drinks several cows worth of milk a week, i've tried to drink a gallon of milk within an hour, and needless to say, it doesn't work.
As pip said, your body needs to digest some of the protein content in the milk before releasing it into the intestinal tract. Now, this takes a bit of time to happen, and the stomach physically cannot expand to store a gallon of milk for the time to digest.
This causes your body to expel the excess milk until it has more managable levels.
You'll also find that the high fat and lactose content causes some serious stomach upsets beyond space constraints.
Find the Jackass episode where 3 people tried to drink a gallon of milk inside an hour for a more graphic illustration of how things like that really don't work.
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