> shows that you are using a given diet that the human body is not meant to sustain?

In an industrialized society, you will have a difficult time finding a way of eating that can be justified as both optimally healthy and "as nature intended." By that measure the choices are bad, or less bad.

If you're an ethical Vegan, e.g. someone who does not believe in exploiting animals for your food, then the diet being "bad" like that could be legitimately remedied by supplements.