system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is virtually every claim about human nutrition controversial?

First calories were bad. Then it was fat. Later it was carbs. Why is it so hard to identify the ideal human diet?
gpt
Because you can't put a human in a locked cage for 80 years and feed it an experimental diet. We can use animal models, we can use short tests, we can ask people what they ate, but you can't definitively prove nutrition requirements or long-term effects  without experimentation.