Scientific evidence

A hear a lot of people saying: That is good for your health and this is bad for your health, blah, blah, blah…
There are a lot of different diets around promoted as the healthiest diet ever.
So what I want to ask is…What is the the process of analyzing how healthy is a diet…
Are there any medical doctors around who know how it is done?

I think it’s analyzed statistically… imho