Ah yes, the Diamonds (thats the name of the husband and wife team that
wrote that book).
I've read part of the book in question (my mother had a copy), and
I've also read a bunch of stuff about what they wrote. Suffice it to
say, it doesn't appear that they have any real solid evidence for
their claims, which tend to be somewhat bizarre. Refering to them as
"researchers" would be a stretch -- they did no real original
research, except on their own bodies.
On the other hand, one could do far worse than following part of their
advice and becoming vegetarian, even if they don't advise doing so for
anything like the correct reasons. Just as you can be cured by
antibiotics even if you believe they are magic that drives out evil
spirits, you can gain advantage from a proper diet even if you do it
for the wrong reasons. The food combining nonsense they spout is
silly, but ultimately harmless even if followed -- at worst, it would
force you to inconveniently juggle what you eat and when.
Perry