Eh, granola and “health food” are not necessarily good for you. Read the label. If it’s ultra processed, high carb, has seed oils or soy, put it down. Or you’ll get the gut of metabolic syndrome. Which the corona just loves.
Lol. Human bodies are more sophisticated than that. Humans can get by on a variety of foods. The problem of the Chinese peasants and solders were that they ONLY ate wheat bread and wheat gruel. Occasionally they got other food. But as a rule all they ever got to eat was wheat. Completely lacking the aminoacid Lysine and vitamins like C. If they'd eaten beans with their wheat and some occasional carrots, they'd have been fine. They just didn't. Or didn't do it often enough. Ancient Egypt had chronic and widespread problems with scurvy. Which is ridiculous considering that they had no practical barriers to fixing this. They just never connected the dots.
An extreme high protein diet has it's own set of problems.
Humans can't win. Whatever extreme diet you eat you will get problems. Whatever diet you eat you will get problems. The best we can do is eat a varied diet. High protein diets gives you high blood pressure and puts a lot of strain on the heart and kidneys. AS long as you live you will be healthy. But you will NOT live as long.
When it comes to strengthening the immune system. We can't really do much. We can fuck it up in various ways, ie eating extreme diets. But if we manage to refrain from that, then we're good.
