Your question suffers from a few problems.

It starts out rather vague:

• "A historian" - are you sure it was a proper historian, or just someone thinking they were (like that sports journalist talking about ancient aliens all the time)?
• "on TV" - which of thousands of channels?
• "a couple of years ago" - 'a couple' as in 'two', or as in 'could be anywhere from one to five'?
• "some city" - again, plenty of those

But that vagueness might be helped by the relative specificity of what follows:

when it went under siege, the people in the city had a diet of only a type of groundnut for food

If someone recognises this tale, they might be able to say where it originated (in fact or fiction). I don't recognise it, but I do think it's very unlikely. A city under siege usually has huge stores of food prepared. If they don't, or if they run out, the people starve. There is no way to get food into a city under siege (that's the whole idea of a siege!) and there is no way to grow enough food for an entire city inside the city walls. This makes me pretty sure this story is not true.

And then there's this:

that resulted in having the best health for the people.

Now I'm confident this story is bollocks. Nuts belong in a healthy diet. Eating only nuts (or only anything) is not healthy (NHS UK: "eat them in moderation"). What I think is going on here is that someone on TV either made up that siege or hugely twisted something that did happen in order to support their nut-only diet fad. It's not history and it's not good dietary advice.