  I consider the $\gamma W$-box correction to superallowed nuclear
$\beta$-decays in the framework of dispersion relations. I address a novel
effect of a distortion of the emitted electron energy spectrum by nuclear
polarizabilities and show that this effect, while neglected in the literature,
is sizable. I estimate its size in the approximation of a linear energy
dependence, and using two models that are expected to give the lower and the
upper bound. The respective correction to the $\beta^+$ spectrum is estimated
to be $\Delta_R(E)=(1.6\pm1.6)\times10^{-4}{E}/{\rm MeV}$ assuming a
conservative 100\% uncertainty. The effect is positive-definite and can be
observed if a high-precision measurement of the positron spectrum is viable. If
only the full rate is observed, it should be included in the calculated ${\cal
F}t$-values of nuclear decays. I argue that this novel effect should be
evaluated in state-of-the-art nuclear models and included in the analyses of
nuclear beta decay experiments to ensure the correct extraction of $V_{ud}$
from decay rates, and of the Fierz interference term from precision
measurements of decay spectra.
