In a mouse model scientists saw that following a single small dose of dioxin, levels of FMO3 expression and activity were significantly increased, and circulating TMAO was increased 5x
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The amount of copper in most copper peptide serums is low enough that it shouldnt pose a major risk to most healthy people, but its still something to be mindful of.15 However, those with Wilsons disease a rare genetic condition may be best off avoiding copper peptide serums (or any skin care product containing copper), as the condition causes the element to build up in the body.16 Cancer risks GHK-Cu seemingly promotes angiogenesis, or the formation of new blood vessels.14 Though this can lead to beneficial outcomes like better wound healing, it may also lead to the growth or spread of cancer by giving solid tumors a blood supply.17 This means that if you have active or suspected cancer, it may be wise to avoid copper peptides

Because the level of, I mean they basically like, the level of vitamin C that is likely to maximize your protection against vitamin C responsive diseases is way above what someone, what the average person is getting from whole foods, but well within what you would get from whole foods if you are eating a very vitamin C rich diet, and way below what anyone who is taking vitamin C supplements is getting
In contrast, neurons rely for their GSH synthesis on the availability of extracellular cysteine that is provided by astrocytes