Have you had vegemite? Whole grain breads? Congratulations, you've had brewing by-products.
Brewing by-products includes things like yeast cakes which are extremely high in a wide variety of vitamins, some of which are difficult to obtain in other feeds outside of soy (which is getting more and more expensive). The whole grains still contain proteins and carbohydrates, which means they're great animal feed and can be used in whole grain products.
I'm not aware that there's ever been any sort of food poisoning or scare traced back to spent grains, and a brief google search reveals none. On top of that, if you're doing something to your grains that makes them poisonous, odds are your beer isn't safe for consumption, either. There really is no need for the FDA to even look into this, as it's effectively been done for centuries with no problems.