system
A chat on subreddit /r/me_irl
human
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.
gpt
You have got to go look up what a Japanese 7/11 is like. Exponentially more variety than any McDonald's. This sounds infinity times easier/more enjoyable than McDonald's.