John Lennon once said: "I believe in everything until it's disproved. So I believe in fairies, the myths, dragons. It all exists, even if it's in your mind. Who's to say that dreams and nightmares aren't as real as the here and now?" 

That is a view that John Lennon and the Federal Trade Commission do not share. While Lennon believed in everything until it was disproved, the FTC (and the National Advertising Division) makes marketers prove that things are real before it believes in them. 

Speaking of fairies, a practice known as "fairy dusting" occurs when a company puts a very small amount of a functional ingredient that has marketing cache in its product and then makes a claim about the ingredient, or simply highlights the fact that it is in the product. The product actually does contain the ingredient, so claiming that it does is technically true. However, it may nonetheless be misleading if the product does not contain enough of the ingredient to be efficacious, either at what the labeling or advertising claims the ingredient does or whatever the ingredient is well known to do. 

In other words, to support a claim that your product contains a well-known functional ingredient, while it may be literally true, the FTC (or others) could take the position that you are implying that the product does whatever the ingredient is well-known to do. After all, why else would consumers care? If this happens, at a minimum you may need to show that the product contains an efficacious amount of the ingredient as shown in studies. Without that, there may not be a happily ever after to the fairy tale.

While it still exists, I believe this practice of "fairy dusting" has decreased over the years.  Then again, I believe in dragons.