A buddy of mine did the milk challenge with 1%. He finished too, but was REALLY sick for the next 24 hours or so, including about 2 hours where he didn't dare leave the bathroom for fear of not making it to the toilet in time. Didn't puke though, which is the main challenge. Its not the volume, its the lactic acid.
The internet suggests it's more that the stomach is only about a half gallon, and that the excess liquid and the high fat content trigger the vomiting reflex.
My keyboard is acting up, so if you see any random 'u's thrown in after 'i's, just iugnore them.
Playing Pokémon X
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4- or 5-IV Pokémon available for trade listed on my Giant League Wiki page.