nothing prevents muscle breakdown. gluconeogenesis hits your intracellular amino acid pool first, so glutamine (and maybe taurine) supplementation may stop that hitting you too hard. if it wants to hit your muscles, it will, but that's secondary.
BCAAs may prevent fatigue during very long runs, but due to tryptophan chemistry in your brain. full range amino straight after a run can reverse (and overcompensate) for the muscle breakdown.
yours
the pedant