Perhaps my original statement was a bit strong. I guess I should say that our level of understanding is not proportionate to the types of conclusions we accept when it comes to food.
Compared to our understand of fluid dynamics, our understanding of nutrition is almost non-existent. Are eggs good for you? Should I drink cows milk? Do multivitamins do anything?
Any answer that a nutritionist would give you be couched in a whole list of qualifiers (if you have low cholesterol, if you're over the age of 65, etc) since you're asking questions about the interactions between very complicated systems like any good scientist should. If someone did give a binary answer to any of the questions you asked, they're probably wrong or trying to sell you something. Don't interpret that as lack of understanding, there is quite a bit known about human nutrition in the modern era.
Compared to our understand of fluid dynamics, our understanding of nutrition is almost non-existent. Are eggs good for you? Should I drink cows milk? Do multivitamins do anything?