I'm not sure what specifically you could be referring to. In fact, for vegans it can be easier to sometimes take supplements that have iron and vitamin C in one package, rather than carefully preparing a meal so that you're eating both plant-derived iron and vitamin C at roughly the same time.
> 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.
