Comment by FloorEgg

There is a third option: looking at the diets of your closest ancestors with the best longevity.

There may be a misconception that there is one single best diet for everyone, when in reality we people (over generations) evolve with our diets, and your best diet and my best diet may be completely different.

The problem with using science as a guide is that there are just too many variables and not enough time, data and money to isolate them all sufficiently.

However that is distinct from the idea that too much of something like refined sugar might be unhealthy for just about everyone. So science does have an important role to play, I just don't think it's advanced far enough to fully answer the question for everyone.