Leaving out the "birth to death" part, metabolic ward studies aren't unethical or unheard of, they're just terribly expensive and tend to be short in duration b

Leaving out the "birth to death" part, metabolic ward studies aren't unethical or unheard of, they're just terribly expensive and tend to be short in duration because they are so onerous. But they certainly have some advantages over, say, studies based on food frequency questionnaires.