[Content warning: dieting, weight loss]
Everyone knows every study shows people can’t lose weight and keep it off. Everyone knows people who say they lost weight and kept if off. Some of that may be they do not have a sufficiently long time horizon for “kept it off”, but I think something else is going on.
No one joins a university study as their first attempt at weight loss. The people who join have tried and failed multiple times. It seems entirely possible that those people can’t lose weight, but some subset of people succeed on their first try. Tellingly, I mostly hear “I tried everything and only lost weight on the penguin diet” from penguin diet commercials. In other contexts people only announce what worked for them. Maybe they don’t want to advertise their previous failures, but it seems at least as likely that their weight loss stemmed from things all diets have (e.g. paying more attention to food, initial progress leading to a virtuous cycle) and the penguin diet happened to be the one they tried.