  A $6.8\,\sigma$ anomaly has been reported in the opening angle and invariant
mass distributions of $e^+e^-$ pairs produced in ${^8Be}$ nuclear transitions.
It has been shown that a protophobic fifth force mediated by a
$17\,\textrm{MeV}$ gauge boson $X$ with pure vector current interactions can
explain the data through the decay of an excited state to the ground state,
${^8Be^*} \to {^8Be}\, X$, and then the followed saturating decay $X \to
e^+e^-$. In this work we propose a renormalizable model to realize this fifth
force. Although axial-vector current interactions also exist in our model,
their contributions cancel out in the iso-scalar interaction for ${^8Be^*} \to
{^8Be} \,X$. Within the allowed parameter space, this model can alleviate the
$(g-2)_\mu$ anomaly problem and can be probed by the LHCb experiment. Several
other implications are discussed.
