In tokamak plasmas subjected to substantial toroidal-field ripple, sufficiently energetic members of the bulk-iron population can diffuse rapidly away from the hot central region by the superbanana process. The result is a truncation of the ion velocity distribution, causing a marked reduction in the fusion reactivity. This tail-depletion effect may account in part for the anomalously low central ion temperature deduced from the neutron flux emitted from dense deuterium plasmas in the Alcator device, and also may have relevance to similar observations in the beam-heated TFR plasma.