that is a matter of great debate!
among other ideas, i do enjoy one very simple adage:
1. eat food (ie not processed pseudo-foods)
2. not too much
3. mostly plants
it doesn't go quite as far as it needs to re 'well rounded' or 'balanced' but, a good start.
bahaha world without zinc https://www.youtube.com/watch?v=U1iCZpFMYd0