  A lattice computation of the leading-order hadronic contribution to the muon
anomalous magnetic moment can potentially help reduce the error on the Standard
Model prediction for this quantity, if sufficient control of all systematic
errors affecting such a computation can be achieved. One of these systematic
errors is that associated with the extrapolation to the physical pion mass from
values on the lattice larger than the physical pion mass. We investigate this
extrapolation assuming lattice pion masses in the range of 200 to 400MeV with
the help of two-loop chiral perturbation theory, and find that such an
extrapolation is unlikely to lead to control of this systematic error at the 1%
level. This remains true even if various tricks to improve the reliability of
the chiral extrapolation employed in the literature are taken into account. In
addition, while chiral perturbation theory also predicts the dependence on the
pion mass of the leading-order hadronic contribution to the muon anomalous
magnetic moment as the chiral limit is approached, this prediction turns out to
be of no practical use, because the physical pion mass is larger than the muon
mass that sets the scale for the onset of this behavior.
