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Generally speaking, a non owner of a trademark may use a trademark nominatively i.e. reference can be made to the actual trademarked product or its source. In addition to protecting product criticism and analysis, US law permits nominative usage by competitors in the form of comparative advertising. The question of confusion though remains. So with such nominative use one must make sure that a customer is not confused about the source of the product that is making the reference (i.e. the referring product is unrelated to the trademarked product)
Generally speaking you can mention a trademarked diet in a book but personally I would not prominently put it in the first line. For example how about: Eating with Metabolic Syndrome: recipes for Metgenics FirstLine Therapy Diet. Please note that this example is just my personal opinion and not by any means a legal determination that such example is not infringing.
You should also include a prominent disclaimer on the cover that your book is not related to Metgenics FirstLine Therapy and is not endorsed by it.
Finally, every time you mention their name don't forget to add a circle-R (trademark sign) next to it
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