The simulation and analysis of neutron signals of plasmas heated by neutral beam injection (NBI) requires fast computer codes for evaluating non-thermal velocity distributions. In such calculations effects of sawtooth activity are usually neglected. In this comment, the consequences of such an approximation are discussed. The time-dependent bounce-averaged Fokker - Planck equation is solved numerically. The solution is used to calculate the effect of sawtooth activity on the time evolution of the fusion reactivity.