I am not sure Belichick (or any sane person) really believe that Guerrero is keeping Brady playing the way he is. Guerrero/Brady are selling pseudo-science diet lifestyle as facts. It is most likely placebo effect, which is an actual scientific phenomenon.
I have noticed that athletes have been able to have more success in later years (see tennis) now than ever before. I chart that up to advance in medicine in general and athletes taking better care of themselves early on and throughout their careers.
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