He actually does make this claim very specifically. In his conclusions chapter, one of the main headings is "Chew", where he states: "the bones in the human fac

He actually does make this claim very specifically. In his conclusions chapter, one of the main headings is "Chew", where he states: "the bones in the human face don't stop growing in our 20s, unlike other bones in the body." And then goes on to recommend eating "foods that require an hour or two a day of hard chewing". I seem to recall that earlier in the book he describes some research that documented this effect, but I don't have a reference.