Well damn, it looks like my longer comment deleted while I was getting verified. TL;DR Our bodies are not great at absorbing nutrients unless we evolved to do it. Wheat is loaded with vitamins and minerals, but they’re pretty much unusable by humans (and only somewhat more usable with processing like soaking, sprouting, fermenting, etc.). The same can generally be said about vitamin pills. OTOH animals are wonderful at converting those nutrients into usable forms. There are many peoples that lived for millenia on an almost all-animal diet, yet the same can’t be said of plant diets.