  We investigate the finite nuclear mass corrections in the helium atom in
order to resolve a significant disagreement between the $2^3S - 2^3P$ and $2^3S
- 2^1S$ transition isotope shifts. These two transitions lead to discrepant
results for the nuclear charge radii difference between $^4$He and $^3$He. The
accurate treatment of the finite nuclear mass effects is quite complicated and
requires the use of the quantum field theoretical approach. We derive $\alpha^6
m^2/M$ correction with the help of nonrelativistic QED and dimensional
regularization of the three body Coulombic system, and present accurate
numerical results for low lying states. The previously reported $4 \sigma$
discrepancy in the nuclear charge radius difference between $^3$He and $^4$He
from two different atomic isotope shift transitions is confirmed, which calls
for verification of experimental transition frequencies.
