My cat lost weight on diet food. Is this a healthy change or a concern?

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A lot of vets have a hard time assessing an overweight cat during an exam, mainly because some cats have a short temper and we are working against the clock in a lot of these cases. So sometimes we cannot feel for the ribs, spine, etc. It is very possible that Jasper was overweight to begin with and now is more ideal weight. Without a photo attached it is hard to tell, but as long as his other faculties are intact (i.e. eating well, normal stools), then I would not worry about the weight loss. The weight control food is limiting his calorie intake, and with his activity level, he is likely just going back down to his normal standard weight.
He should level off with he weight loss once he has reached his ideal weight soon, if this is the case.
Here is a website link so that you can assess his BCS (body condition score) yourself at home https://www.wsava.org/WSAVA/media/PDF_old/Body-condition-score-chart-cats.pdf

If however, you are still concerned about his weight loss, and he seems overly hungry, then I would be suspicious of underlying thyroid disease. A thyroid test would be next on the agenda if he meets these criteria. Thyroid disease is typically easily managed in cats and he is of the age where we do see more of this disorder.

i hope that Jasper's weight loss is attributed to his new food and that he starts to level off with his weight soon! Best wishes and take care.