Non-thermal nuclear fusion reactivities are computed semi-analytically for the deuterium tritium, DD and deuterium helium 3 (DHe) reactions for distributions consisting of a Maxwellian bulk and an isotropic hot tail. The energy of the hot tail is optimized in order to maximize the reactivity enhancements. Fusion reactivity enhancements at constant total energy of the distribution are estimated. Under conditions relevant to fusion energy research, the reactivity enhancements are marginal, except for the DHe reaction, with the condition that electron temperature must be decreased to make the regime with high enhancements accessible. Only when the Maxwellian bulk temperature is low can enhancements of several orders of magnitude be obtained.