The high purity stuff is contaminated because it’s pure. Got it. 
Water, which we typically consider to be pure, really includes a lot of salts and carbohydrates, making it slightly hypertonic. They also contribute to the flavour of water. The cytoplasm in your body’s cells has a tonicity as well. As a result, when cells are exposed to hypertonic solutions, they lose water and shrivel up. When they’re in hypotonic liquids, they absorb water and swell to the point where they burst like a balloon. The most hypotonic solution is absolutely pure water with no contaminants. As a result, if you consume enough of it, your cells will begin to absorb water. This upsets the electrolyte (electrically conductive ions) balance in your body. This is a particularly dangerous issue in the brain, where water toxicity can lead to death.
If by ‘a lot’ you really mean ‘a very, very small amount’
There is no meaningful difference between tap water and pure water, in terms of tonicity vs cytoplasm, which is orders of magnitude more salty than both.
Also, carbohydrates?
At Burning Man I bring the electrolyte salt for the camp. It’s kept in a box by the water cooler with the instructions
1/4 tsp. per liter, 1 in 3 liters is sufficient
When I worked in a lab, we didn’t call that “pure”, we called it “anhydrous”. Being free of water but loaded with benzene isn’t really pure in any useful sense.
Don’t forget to add the carbohydrates!
This thread is doomed to be periodically resurrected by people not reading the previous conversation and just saying the explodey osmosis thing. Mark my words.
Wasn’t “Explodey Osmosis” an animated series?
No, he exhorted us to Look on his works and despair.
See? Those spherical people drank the Olympic Ice, then exploded.
Just don’t drink distilled D2O (heavy water), because you’ll become radioactive and explode.
Screw spontaneous human combustion. I wanna see some spontaneous hunan fusion.
Hunan food plus isotopes = fusion cuisine
Dammit. I guess I really am hungry for Chinese food.
I heard that all copies of the alternate ending to Big were destroyed by explodey osmosis.
Oh, yeah. I forgot Add sweetener to taste because Gatorade powder is also brought in (not by me) and, used to the bulk of the sugar, people would dump an ounce or two into their liter bottle then complain how salty it was.
“Well, yeah, it’s settling a quarter inch high on the bottom. Read the directions!”
Deuterium won’t make you radioactive, no matter how much of it you consume. You’re thinking of tritium.
At truly extreme levels of purity, it does need to be handled with care; see e.g. https://www.bioind.com/media/documentation/msds/01-866(3).pdf
Fundamentally H2O is pretty reactive. The stuff we normally meet already has a lot of things dissolved in it which does tend to reduce its reactivity - not sure why.