Even this is hard to do conclusively, since your body is changing as you go through life (i.e. get older). You can certainly figure out what works for you at an

Even this is hard to do conclusively, since your body is changing as you go through life (i.e. get older). You can certainly figure out what works for you at any given phase, but I think it's hard to generalize from that: certainly you couldn't start telling other people what is good based on your own experience (although some do try). Acknowledging the problems with formal studies, one advantage they have is the ability to get lots of similar input and look at medians, distributions, etc.