The paleo diet folks often talk about evolution, and how humans are not able to evolve as fast as our environment changes, but I hear little about the evolution of ideas with respect to diet.
Sometimes science comes up with startling new revelations that just so happen to be what an old culture has been recommending all along. How did this culture come up with such an obscure idea? Well, because there's not just one culture, there's hundreds of cultures. India alone has 212 cultures recognized by it's government. Even within a culture there is great variation from family to family. Each of these subcultures is an experimental vessel for ideas. Some idea catches on in a culture, and it either helps the members of that culture succeed or not. Nearby cultures with better ideas will out-compete the ones with poorer ideas, until the cultures with poorer ideas start to copy the successful ideas into their own culture. Any culture with all the wrong ideas will die out.
Therefore it stands to reason, that any diet or medical practice that exists for centuries within a culture has some value to that culture. That benefit may be something we can understand and scientifically validate, or it may simply be a part of the cultural bond between members.
Bloodletting for example is a very ancient practice. It has fallen into disfavor due to misuse and overly wide applicaiton, but studies are starting to show that blood donation (a more controlled and sterile form of blood letting) may have positive effects on the health of some individuals. Back in the era of lead based make-up (a cultural choice that did not last long since it had a detrimental effect on the members of that culture) regular blood letting may have reduced the impact of lead poisoning by forcing a constant renewal of the blood.
How can we apply this information to our own life choices? We know that just because a choice is popular does not make it correct, but the longer a choice has been popular, and the more people and more generations that have held an idea, the more likely it is to be correct absent any other scientific evidence. Ideas long held by multiple cultures make excellent candidates for further research and experimentation.
For example, in India, a large amount of yogurt is consumed by many of it's subcultures, and has been for hundreds of years. You may wish to find out if yogurt prepared in the traditional Indian way (as some random person suggested) is good for your digestive issues. You could easily research this idea, and find: evidence that yogurt is good for most people, yogurt is bad for most people, or there is little evidence about the health effects of yogurt. As long as there is not strong evidence that yogurt is bad for you, this makes an excellent candidate for a self-experiment.
On the other hand, if one guy you know swears by drinking blue dye to cure bad eyesight, you might do research and not find any scientific study showing this is bad for you, but you should be suspicious of his claims since he is the only one to have this positive outcome, and no cultural wisdom backs up this idea.
The plural of anecdote is not data, but it is worthy of further research. Popular and old ideas are not truth, but they are worthy of your interest. In the end, even if science says that eating more broccoli prevents pimples in 95% of people, you don't know if you're part of the 95% or 5% unless you try it yourself.