Re: natural foods. This word combination has no shared definition, but if you mean, loosely, foods without additives, then we can say with certtainty that Gogh et al. (Famous artists of the past who had bipolar) pretty much ate food without artificial colors, flavors, etc., simply because these additives had not been invented during their lifetimes. And these people still suffered.

I think a better approach is to eat a lot of what you like, within a balanced meal plan, because enjoying your food and drink will help your mood directly.