We have evolved to eat meat, vegetables, dairy etc. The closer these ingredients are to their original form, i.e. less processed and chemically amended purely for marketing purposes, the better. We also evolved to eat fermented foods, including (sourdough) bread and pickles of various kinds. We need protein, vitamins, minerals and various trace elements. Our bodies are pretty good at self-regulating and sometimes even filling in some deficiencies in diet, but the diet must not be synthetically limited or deficiencies can develop into problems. Our digestion and processes that feed all our cells are highly complex intertwined chains of processes. Just as you can't feed plants with large dose NPK fertilizers and expect healthy plants, you can't feed humans with a large dose of protein + one or two other thing and expect healthy humans.
The often used analogy of an engine is much miss-aligned with biological processes.