My son has questioned my eating mushrooms. I use the little white button mushrooms. Mushrooms are a fungus and he doesn't think it is wise to add any more fungus to our bodies than is necessary. What do you think?

While I have my doubts that the Vitamin D2 that can be obtained from UVB exposed mushrooms may/maynot be a good thing I think for vegetarians the fact they also may contain some B12 is probably a good reason to continue.
Vitamin B12 is the active corrinoid produced in cultivated white button mushrooms (Agaricus bisporus).

I would suggest that the regular consumption of garlic would reduce fungal problems as would eliminating sugar and cutting refined Carbs and using Coconut oil daily.

Your son is very wise.

Mushrooms contain several nutrients, including beta glucans, that are beneficial to the immune system. However, mushrooms also have a dark side, that can cause damage to an already supressed immune system.

I would suggest taking a good quality daily probiotic to balance the negative effects from the mushrooms. And its not just mushrooms, its also yeast from baked goods, brewers yeast from beer, alcohol of any kind, and even mold from a damp environment.

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Let Food Be Your Medicine And Medicine Be Your Food.(Hippocrates)