  We discuss the consequences of the approximate conservation of the vector and
axial currents for the hadronic matrix elements appearing in beta decay if
non-standard interactions are present. In particular the isovector
(pseudo)scalar charge g_S(P) of the nucleon can be related to the difference
(sum) of the nucleon masses in the absence of electromagnetic effects. Using
recent determinations of these quantities from phenomenological and lattice QCD
studies we obtain the accurate values g_S=1.02(11) and g_P=349(9) in the MS-bar
scheme at mu=2 GeV. The consequences for searches of non-standard scalar
interactions in nuclear beta decays are studied, finding epsilon_S=0.0012(24)
at 90%CL, which is significantly more stringent than current LHC bounds and
previous low-energy bounds using less precise g_S values. We argue that our
results could be rapidly improved with updated computations and the direct
calculation of certain ratios in lattice QCD. Finally we discuss the pion-pole
enhancement of g_P, which makes beta decays much more sensitive to non-standard
pseudoscalar interactions than previously thought.
