In tokamak plasmas subjected to substantial toroidal-field ripple, sufficiently energetic members of the bulk-ion population can diffuse rapidly away from the hot central region by the superbanana process. The ion velocity distribution is truncated at the velocity above which the escaping fast ions cannot be replaced sufficiently rapidly by Coulomb collisions. This tail-depletion effect causes a marked reduction in the fusion reactivity and can give rise to an anomalously low central ion temperature as deduced from the thermonuclear neutron flux. The application of this phenomenon to the Alcator and TFR plasmas is discussed.