- Do we actually know anything about nutrition, given the every-five-year swings between "eggs are cardioprotective/eggs are a heart attack on a plate," "fat is the Devil/carbs are the Devil," etc? If so, what is it and how do we know it?

Actual scientists are not making those swings, the media is in its chronic churn. The data says something like "a reasonable amount of eggs, in moderation, are probably good for you, but eating too many is probably bad for you" and "all humans need fat/carbs/protein in reasonable proportions, which no doubt vary according to genes and lifestyle in ways that we're still working out".