Most of these guidelines are based on the same sort of flawed population-level studies that can't control for many other aspects. For example, the "Mediterranean diet" is consumed in countries that have all sorts of other specific lifestyle differences that are just as likely to influence health and longevity - siesta, a general tendency not to stress or work in exaggerated amounts, a month or more of vacation per year, significant sun exposure, good social healthcare, and many others. All of these together have certain (mild) effects on longevity and health, but there is virtually no way to isolate any one from all of the others.

There are also many studies that suggest other kinds of diets are beneficial, and many populations that consume significantly different diets and have even better health than the average person around the Mediterranean. For example, inuit populations also display generally low obesity and risks of heart attack, stroke, diabetes etc - while consuming almost the opposite of a Mediterranean diet or of plant heavy diets.