lets take spiderman for example. he got super human stats. version A of spiderman only eats junk food. version B of spiderman eats healthy. avocado, boiled chicken, rice and all that. do you think it will effect his stats?
It should, in the same way it would affect a human.
It is just that it may not be noticeable - in the same way a cheetah that eats cat food and soy wafers in captivity will seem to be as fast as one that eats gazelles in the wild against a human runner.
But you'd have to go into indie comics to find a super powered character that isn't in peak condition regardless of diet.
Healthy eating is more important for long term health than short term. As long as you're getting your basic macronutrients and not getting scurvy, you can get 90% of your calories from Cheetos and still function fine.
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