Many producers of mushroom fruiting bodies grow their mushrooms on grain, the issue with this is that the mushroom then absorbs the grain, resulting in a starchy product. But in nature medicinal mushrooms grow on wood!
When we consume mushrooms, we are expecting the nutrients from the mushroom itself, not the grain.
Ironically, some companies are growing grain spawn and selling it as a nutritional supplement, and although there is some mycelium, if you break up the mycelium, you will find that you will predominantly have grain in the product. So, mushroom powders made from grain-grown mycelium or rice-grown mycelium are primarily containing grain powder. What you are left with, therefore, is the material on which the mycelium is cultivated.
What is called mycelium and is marketed as mushroom by companies is often mainly grain spawn — a product that is cultivated on grain or rice in a laboratory.
You can tell a mycelium product by its mildly sweet taste, a bit like breakfast cereal, these products, as you would expect, are mostly starch, and they have few if any of the active compounds which you expected to get when you purchased them. A mushroom grown on wood-substrate and extracted from the fruiting bodies will have a distinctive, powerful mushroom smell and flavour.