I think another key point is that a caveman diet was almost certainly calorie restricted (and there are claims that that in itself is good), in that they had to expend a lot more time and energy in obtaining food than we do today, and were probably a lot hungrier for a lot longer than modern humans - it was easy for them to not over-eat, because they didn't have enough food to do so. There are many factors (most notably not being able to examine the general health of any living ancient people for the obvious reason that there aren't any) that prevent us making an "other things being equal" analysis of different specific food groups consumed then and now, and all we can really do is compare current diets around the world. As others have pointed out, many Mediterranean folk live long and healthy lives on diets that are high in grains. And Asian people consume vast quantities of rice during their lives, but developed Asian countries (excluding underdeveloped ones with poverty-related health problems) have higher proportions of centenarians than just about anywhere else. There's a lot of obvious good sense in adopting a more primitive and simpler diet, avoiding refined carbs (especially sugars), eating plenty of fruit and veg, getting enough exercise, etc. But claiming that paleolithic people (a) were healthier than modern humans, and (b) were healthier because they ate no cultivated grains, is not only not supported by direct evidence but can not be supported by direct evidence, as a proper comparison is impossible. We can guess, based on dubious indirect evidence, but that's the best we can do. The purpose of fad diet books is to enrich the authors of fad diet books, and that's inevitable given the huge numbers of people who spend billions every year on weight-loss products - when all that's needed is to exercise more and ELYFB.