Thanks for asking an important follow-up question implicitly by commenting on what the term "healthy" (used in my first post here) might mean. To make that explicit, when I refer to "healthy" people in the context of a human nutrition study, I refer to people who have longer lifespan (less mortality) and fewer documented diseases and disabilities (less morbidity) than the general than the general population baseline. (By symmetry, unhealthy people have more mortality and more morbidity than the population baseline.) Ideally, a hypothesis on nutrition and its relationship to health would be backed up by EXPERIMENTALLY OBTAINED data on both the central tendencies of groups given one treatment or another, and by similarly obtained data on individual outcomes before and after differing treatments. We have a long way to go to get data of this quality. And it may be that eventually (although this promised future seems farther away now than it did a decade ago) individuals will be able to obtain testing to allow them to receive individualized advice on diet, adapted to their individual peculiarities. Right now we are still trying to figure out very general patterns of advice, which surely won't fit all individuals equally well.

We use similar criteria for household and farm animals ("shiny coat," musculature, posture and signs of coordination and activity). Back when slavery was widespread, slave traders made their choices the same way.