Quote from D Is For Dogs:
“All nutrients, Vitamins and Anti-oxidants within the mushroom are completely heat stable. That is the beauty of Mushrooms! They don't lose any nutritional value with heat. Since the Ergo calciferol (D2) is in the mushroom and not a separate ingredient, the same applies. This has been tested at Medallion labs multiple times with our products.”
Here is their link as well.