What's more radically different between people who are healthy and people who are not is how much they exercise rather than what they eat. There is much more worldwide cultural variation in what people eat, and much more individual variation within each culture, among healthy people than there is variation in the exercise level of healthy people. But seeking to change diet to change health sounds easier than getting more exercise, so that's what gets in the news (and on HN, over and over and over).
with controlled experiments and careful observation and follow-up on human nutrition is extremely hard. I recall a local television news documentary about a dietary study from the 1970s in which experimental subjects were confined 24/7 in a lab for many weeks (they must have been compensated handsomely to agree to participate in this experiment) and fed EXACT portions of food that were weighed with extreme precision. Oddly, I don't recall particularly what hypothesis that study was set up to test, but I do recall that it was described as the first experiment that relied on feeding subjects exactly measured portions of known foods (and excluding subjects from access to foods they chose for themselves) while involving lengthy (weeks of time) follow-up. It's still exceedingly rare for studies of human nutrition to have careful experimental designs. (And even at that, I suppose most of the subjects of that experiment are still alive, but I don't know if anyone is doing follow-up observations of those subjects in any manner.) Instead, we get inferences from Egyptian mummies (as in the submitted article) or from other uncontrolled case studies that never even had careful observation to begin with. That really isn't science;
What makes it all that much more complicated is that diet is, in fact, the best way to lose weight. Losing weight through exercise is incredibly hard and unrewarding (and often unsuccessful), but to keep the weight off after losing it, exercise is essential.
What compounds this is that 'healthy' is a particularly ill-defined catchall term. If you are, for example, morbidly obese or horribly malnourished, you probably do need to change your diet. If you get winded going up a flight of stairs, or maybe worried about general physical fitness, you probably want exercise.
What is our ideal for a healthy person?
An underwear model? someone who lives to 93?
What compounds this is that 'healthy' is a particularly ill-defined catchall term.
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.
Tim Ferriss made the assertion in the 4 Hour Body that what should be a benchmark for health is reproductive fitness ("sexyness"). This is tough to measure objectively, but not subjectively. A significant portion of our brain is devoted to processing exactly that (ever notice how you can instantly spot an attractive member of the opposite/preferred sex out of the corner of your eye?).
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.
I'll absolutely agree that its damn hard to do control what people are eating in experiments, but I disagree that exercise plays the role you say.
You can eat something in a single meal that will take SEVERAL HOURS on a treadmill to burn. It is incredibly easy for the number of calories you need to burn to pass the number of hours possible to exercise.
Doing serious scientific research
http://norvig.com/experiment-design.html
with controlled experiments and careful observation and follow-up on human nutrition is extremely hard. I recall a local television news documentary about a dietary study from the 1970s in which experimental subjects were confined 24/7 in a lab for many weeks (they must have been compensated handsomely to agree to participate in this experiment) and fed EXACT portions of food that were weighed with extreme precision. Oddly, I don't recall particularly what hypothesis that study was set up to test, but I do recall that it was described as the first experiment that relied on feeding subjects exactly measured portions of known foods (and excluding subjects from access to foods they chose for themselves) while involving lengthy (weeks of time) follow-up. It's still exceedingly rare for studies of human nutrition to have careful experimental designs. (And even at that, I suppose most of the subjects of that experiment are still alive, but I don't know if anyone is doing follow-up observations of those subjects in any manner.) Instead, we get inferences from Egyptian mummies (as in the submitted article) or from other uncontrolled case studies that never even had careful observation to begin with. That really isn't science;
http://norvig.com/experiment-design.html
that is trading anecdotes, and third- or fourth-hand anecdotes at that.