[almaIV]

> Do you have any reason to believe the experts know better what is good for one's health?

Yes. You don't need to look at the effects of a particular diet because you can be reductionist as a proxy. For example, examining the effect of a meat-heavy diet on people is difficult, as you said. But if you know what meat does in a diet (correlates with high cholesterol, e.g.) you can examine the effect of high cholesterol on a diet, and if you find out it's bad you could determine that eating in a way which contributes to high cholesterol is generally a bad idea.