Losing weight might actually be a relapse for a recovering anorexic. The "relapse" from being tumor-free would indeed be a weird thing to talk about, though not

Losing weight might actually be a relapse for a recovering anorexic. The "relapse" from being tumor-free would indeed be a weird thing to talk about, though not strictly wrong if the cancer had previously been in remission. > The purpose of this study was to test the hypothesis that perfluorinated alkylate substance (PFAS) exposures are associated with body weight increases in a dietary intervention study. The phrase appears to be intended to mean the same thing as "body weight increases in a dietary intervention" which seems to describe a relapse, assuming that the purpose of "dietary intervention" is to lose weight. Edit: I guess the first sentence of this comment doesn't actually answer the question it's intending to. To the point of the question, maybe it would be weird to say that a relapsed anorexic had a "weight gain relapse" but only if it would be weird to say they are on a "weight gain treatment".