They are eating too much, and their regulation of hunger impulses is out of whack. There are essentially zero studies which show subjects on a controlled diet

They are eating too much, and their regulation of hunger impulses is out of whack. There are essentially zero studies which show subjects on a controlled diet under observation having more than a +/-5% different in metabolic efficiency. The burden of proof is, frankly, on those who are looking for anything except for overconsumption as the cause.