It’s one of those occasions when zero point energy is relevant.
The bonding in D20 and H2O is exactly the same the nucleus is not important in forming bonds just the electrons.
But because of the larger mass of D the D-O bond has a lower zero point energy and so is much more difficult to break.
So it’s this that is the potential problem - difficult to break O-D and C-D bonds that would be formed if you managed to drink enough heavy water.
I’ve also been led to believe that heavy ice sinkd in normal water (i’ve been meaning to try it).
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