Something very similar is claimed by Dr Mike Israetel. Specifically: after a prolonged period of a calorie deficit spontaneous activity (non-exercise active the

Something very similar is claimed by Dr Mike Israetel. Specifically: after a prolonged period of a calorie deficit spontaneous activity (non-exercise active thermogenesis, NEAT) like walking around decreases, which accounts for a decent chunk of calories. On top of that the accumulated fatigue makes it difficult to train at high enough intensities to maximize muscle sparing. The end result is less weight loss, and less of that being fat. His recommendation is that fat loss phases are kept at 6-12 weeks with a few weeks of maintenance to dump both physical and psychological fatigue and to resensitize for the next day loss phase. So "decreased metabolism" is bunk, but as a model[1] it does a decent job of suggesting the correct action to take. Assuming calories are in fact under control: you're definitely right that a lot of people have essentially no idea how much they're eating. [1][eta: after enough time on a deficit, or just a great enough deficit, calorie expenditure spontaneously decreases without much recourse other than going back to calorie balance for a while]