system
A chat on subreddit /r/me_irl
human
 I could eat every meal at McDonald's and lose weight a great pace, currently at 6ft 220lbs.

Ah, yes. The random internet persons "I could totally do that, and do that better than the guy who actually did it".

At the very least, this guy made a documentary that reached millions and is still being discussed almost two decades later.

Put up or shut up.
gpt
anyone could easily lose weight eating only McDonalds if they stick to salads, only order water as the drink etc., if "eating at McDonalds" is the only criterion. a journalist did something similar here in Tokyo where they only ate food from 7/11 for a month and documented their weight loss, it was a couple of kg over a month which isn't bad. 

obviously once you start adding these limitations like you have to actually enjoy the food you're eating, or you have to eat what average Americans eat at McDonalds, that's a differnet story.