Microwave ovens do convert vitamin B12 from the active to inactive form, making approximately 30-40% of the B12 contained in foods unusable by mammals.[22] Microwaving broccoli loses 74% or more of phenolic compounds (97% of flavonoids), while boiling loses 66% of flavonoids, and high-pressure boiling loses 47%[23], though the study has been contradicted by other studies.[24] To minimize phenolic losses in potatoes, microwaving should be done at 500W.[25]
Spinach retains nearly all its folate when cooked in a microwave; in comparison, it loses about 77% when cooked on a stove, because food on a stove is typically boiled, leaching out nutrients. Bacon cooked by microwave has significantly lower levels of carcinogenic nitrosamines than conventionally cooked bacon.[20] Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop.[20] Microwave blanching is 3-4 times more effective than boiled water blanching in the retaining the water-soluble vitamins folic acid, thiamin and riboflavin, with the exception of ascorbic acid, of which 28.8% which is lost (vs. 16% with boiled water blanching).[26]