We use mature mushroom fruiting bodies, no young fruiting body, no mycelium.
A mushroom is the "fruiting body" of a fungal organism called a basidiomycete. Fruiting bodies are high in nutrients for the human body. It contains no fillers such as Rice, Oats.
The majority of commercial mycelium producers grow it on grains like rice, oats, or sorghum. Therefore, all that grain becomes inseparable from the mycelium and remains in the final product, leading to high amounts of starch and other impurities.