The protein restriction longevity story is more complicated than it looks.

Other research directly contradicts the healthy aging part. For example, studies found that eating more protein, especially from plants, is linked to living longer, and older men who ate less protein were more likely to die during the study. On metabolic health, some studies did show markers like blood fats and sugar control improve with less protein, but they didn't test whether that holds when calories are also increased, so the 'even when calories rise' part isn't supported. For animal lifespan, it's mixed: some mouse and fruit fly studies found longer lives with protein restriction, but a fish study found no benefit, and the mouse effect relied on a specific hormone. So the evidence doesn't paint a simple 'less protein is better' picture.