L-ascorbic acid (E300)

You are right that L-ascorbic acid is not only safe, but even very beneficial for histamine intolerance. According to the SIGHI classification, it is graded 0 (well tolerated). Contrary to your description, however, research suggests that vitamin C affects the DAO enzyme indirectly - acting as its cofactor and aiding in the degradation of histamine by accelerating its oxidation. At the same time, it stabilizes mast cells, thereby reducing the release of endogenous histamine into the bloodstream. However, when supplementing, it is necessary to pay attention to the purity of the preparation, because synthetic forms produced by fermentation or supplements containing dyes and aromas can cause a pseudo-allergic reaction in sensitive individuals, although the ascorbic acid molecule itself is harmless.