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
The average American at the time was dramatically less informed than they are now about the negative impacts of calorically excessive diets on health.

The point of that doc was not to show that it’s impossible to remain healthy while only eating McDonalds … but rather to show that it’s very possible to become very unhealthy if you only eat *typical* meals at McDonald’s.

It’s possible to eat exclusively at McDonald’s and not gain weight, but those meals would not be very representative of how Americans were eating McDonald’s at the time. Certainly less-so than just going through the full menu.