For several years, I ate nothing but meat and fish, and as I recall, my body did well on it.
That may be because - as I discovered later - I had an allergy to the gluten in wheat, barley and rye.
And when you think about it, flours come from grasses, and we are not cows or rabbits.
I also discover that chilies and hot spices - which I adore - do rather bad things to me.
These days, my wife feeds me a balanced diet of all kinds of things, including the dreaded vegetables, in doses larger than I would prefer. But this diet, at my age, contributes to a body that would suit a thirty year old, at least in appearance.
There's not a lot I can do about my head, though. Or my poor, poor remaining teeth.
Oats, rice (wild/brown), eggs, meat, fish, shellfish, potatoes, vegetables, fruits. Seems to work well.