Those words might be used to describe a scenario in which one had been losing weight until an abrupt change of course. It's not a strictly identical idea to "weight gain".
"adults with obesity first lost at least 8% of their body weight and then completed at least 26 weeks on a specific diet"
Or someone with cancer having it reoccur as a "tumour free relapse"?
> 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".
Some definitions I found:
- suffer deterioration after a period of improvement.
- a deterioration in someone's state of health after a temporary improvement.
- to slip or fall back into a former worse state
- a recurrence of symptoms of a disease after a period of improvement