What individual has lived longest on a single natural foodstuff, be it fish, meat, vegetable, fruit, or legume?

I can't find anything on this interesting question.

23 years on potatoes ,apparently.

https://www.thesun.co.uk/living/1474652/a-potato-addict-who-lived-on-nothing-but-crisps-for-23-years-overcame-her-obsession-after-doctors-warned-it-could-leave-her-infertile/

 

Not saying it is the longest (or necessarily true). But it was an interesting story.

Can't think of a single one that wouldn't lead to health issues.
But I would think it depends on your definition of 'living'.
You could be in a coma indefinitely on just IV, but I don't think that counts.