It's not black magic, NASA for example has specified exactly what someone eats over long periods so they needed to study this. Historically, Military's have also been interested in this and have developed satisfactory minimum standards ex: http://www.apd.army.mil/pdffiles/r40_25.pdf

That does not mean such things are Ideal just that they work acceptably well for young healthy people.

It's called research. There are good well researched standards of diet for use by prisons, the military and space exploration that have enough real world use to have some level of verification.

Well, how could anyone possibly know that the diet of traditional food they eat is any better?