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The light-quark mass dependence of the nucleon axial isovector charge ($g_A$)has been studied up to next-to-next-to-leading order, $O(p^4)$, in relativisticchiral perturbation theory using extended-on-mass-shell renormalization,without and with explicit $\Delta (1232)$ degrees of freedom. We show that inthe $\Delta$-less case, at this order, the flat trend of $g_A (M_{\pi})$exhibited by state-of-the-art lattice QCD (LQCD) results cannot be reproducedusing low energy constants (LECs) extracted from pion-nucleon elastic andinelastic scattering. A satisfactory description of these LQCD data is onlyachieved in the theory with $\Delta$. From this fit we report $g_A(M_{\pi\rm{(phys)}}) = 1.260 \pm 0.012$, close to the experimental result, and$d_{16}= -0.88\pm 0.88$ GeV$^{-2}$, in agreement with its empirical value. Thelarge uncertainties are of theoretical origin, reflecting the differencebetween $O(p^3)$ and $O(p^4)$ that still persists at large $M_{\pi}$ inpresence of the $\Delta$.

Light-quark mass dependence of the nucleon axial charge and pion-nucleon scattering phenomenology