I think the true effectiveness of the “eat only one thing” diet is boredom. I mean there’s a lot you can do with potatoes, but even something that versatile you’re running out of stuff inside a week. In pretty short order you’re hating mealtime and eating a lot less.
At least he wasn’t eating potatoes grown in a Martian Quonset hut, after being left for dead.
https://modernfarmer.com/2015/10/can-you-grow-plants-on-mars/
I had my best weight loss by eating Wheaties every single morning and sometimes would have it for a second meal later in the day...I grow to crave it...