  The mass varying neutrino scenario is a model that successfully explains the
origin of dark energy while at the same time solves the coincidence problem.
The model is, however, heavily constrained by its stability towards the
formation of neutrino bound states when the neutrinos become nonrelativistic.
We discuss these constraints and find that natural, adiabatic, stable models
with the right amount of dark energy today do not exist. Secondly, we explain
why using the lightest neutrino, which is still relativistic, as an explanation
for dark energy does not work because of a feedback mechanism from the heavier
neutrinos.
