  The fraction of the longitudinal momentum of ${}^3\text{He}$ that is carried
by the isovector combination of $u$ and $d$ quarks is determined using lattice
QCD for the first time. The ratio of this combination to that in the
constituent nucleons is found to be consistent with unity at the few-percent
level from calculations with quark masses corresponding to $m_\pi\sim 800$ MeV,
extrapolated to the physical quark masses. This constraint is consistent with,
and significantly more precise than, determinations from global nuclear parton
distribution function fits. Including the lattice QCD determination of the
momentum fraction in the nNNPDF global fitting framework results in the
uncertainty on the isovector momentum fraction ratio being reduced by a factor
of 2.5, and thereby enables a more precise extraction of the $u$ and $d$ parton
distributions in ${}^3\text{He}$.
