This issue comes up frequently in nutrition circles, for obvious reasons. And the answer may seem obvious as well: Of course you can judge whether someone's diet is helping them or hurting them by how they look. But I'm not so certain.
There are a couple of arguments on the "yes" side. One is that in addition to morbidity and mortality info (such as is available), we (especially the "we" in the Paleosphere) give some consideration to the issue of how attractive traditional people eating traditional diets look. Even before Weston Price codified some of the markers of health/attractiveness in the groups he studied, explorers and missionaries remarked repeatedly upon the "vigorous" and "handsome" appearance of the so-called primitives they journeyed among.
Another argument relies on insights into sexual selection from evolutionary biology: Attractiveness is a serious proxy for health in this view. It's how members of a species determine who owns the highest-quality genetic material, and whether it's worth it to hit that.
And deteriorations in health are very often accompanied by unmistakable deteriorations in attractiveness, no matter how undiminished the beauty of the sufferer's soul remains.
There may be more arguments for the "yes" column, and that's one of the things I'm looking for in this question, I guess. I'd like to hear what you all think those are.
Here are the arguments I've got in the "no" column. The first and most persuasive to me is that attractiveness as a proxy for biological fitness -- defined as how well-adapted an organism is to its environment -- is about the organism's adaptedness to its entire environment, not just to its current diet. The entire environment includes things like air quality, sleep, sunshine, social well-being, plus a whole raft of things we've not yet identified as having an impact on health.
Making it even more unreliable as a marker for the healthfulness of the subject's current diet is the fact that we now know that what your parents did before you were born will have an impact on your health -- and what their parents did, and so on, and so on. And in the modern industrial world, many of us may be paying for the sins of people we haven't even met. (I could have bundled this with my expansion of the word "environment," but that graf was getting too long.)
While we're on evolutionary biology, Michael Rose explains very clearly the problem of adaptive pressure ceasing once a subject is past the reproductive stage. This very simple but clever insight may be behind the whole process of ageing -- we may get old simply because there's nothing telling our bodies what to do after we spawn. And this may be why middle age often hits women so much harder than men in the attractiveness department (when does the reproductive stage end for men?).
Finally, there's a less abstract argument: We all know people who look great no matter what they shove in their pie-holes. I'm chuckling as I write this, thinking of one slender woman in particular who recently got some notoriety when she was mentioned by name in a recent post by a prominent blogger (who was writing about a recent prominent kerfuffle, lol). I first "met" her as a fellow zero-carber, when she was puting herself out on FB and other places as an example of how fit ZC could make a person. She's now a VLCer and decries her former diet as being unhealthy, and tells people how much healthier she is now that she's including more carbs in her diet. But by her own admission, she looked much the same ten years ago, when she was eating processed junk food; five or so years ago, when she was a vegetarian, and I can vouch that she looks no different on VLC than she did on ZC. I don't doubt her report that she feels better, but my point is you couldn't tell by looking at her.
Okay, I've run my keyboard enough for one question. What do you guys think -- do looks tell the tale of a diet's healthfulness? Is "attractiveness" a fair index?
P.S. Although this question was inspired by it, I don't mean this to be a referendum on the discussion of whether it's socially appropriate to weigh in publically on various individuals' hot/notness, which came up in a different question. I mean this in the much more general sense implied by the title of my question.