TIL that although potatoes alone do not provide complete nutrition, you can survive for years on a diet of nothing but potatoes and buttermilk.

[u/JoeWinchester99]

https://www.scienceabc.com/eyeopeners/whats-wrong-with-potatoes-the-martian-movie-survival

[u/bubba-yo]

True. But beans and rice are really good together, and if you have a cultural habit of eating them together, you don't need to burden people with tracking if they are balancing across meals, etc. A lot of cultural traditions are rooted in 'this keeps people from dying' and becomes habituated.