alt Hacker NewsOK, name such an approach in nutrition that doesn't already fall under regular biology.
The reason we can do science on some things without doing experiments is that there's lots of hard, unambiguous data about relatively much simpler systems.
Getting good data on the extremely complex thing that is homo sapiens is just not feasible, unless you're studying specific chemical reactions in the gut, in which case it'll take an extremely long time to figure out an actual dietary recommendation.
It sounds like your describing models. Yes we have small models. Yes we have big models. We have rat models, we have matching models, we have twin models. We have lots of models I don't know about.
I'm not sure what you're getting at. These models get better, our understanding is improved, and we will slowly uncover more truths. We already have so much more knowledge about nutrition than we did 100 years ago.