i went looking for upper limits for histidine & isoleucine and found a good write-up on histidine:
ergothioneine is one of the items in the bottom list of iffy molecules. it's down there because i had previously read that humans can't synthesize it, which appears to be more of an open question than a stated fact. if we are able to synthesize it, we would synthesize it from histidine. so, i'll file that away mentally, for later - but i should be clear that humans have not been proven able to generate this amino acid, and the general opinion at this time appears to be that we can't. further, while we can absorb it, and appear to even transport it actively, and even know what codons are responsible for the transport, we don't actually know what our body does with it. it's an antioxidant in vitro, but we haven't demonstrated as much in vivo. garlic, wheat, eggs & beans are good sources for ovo-vegetarians. note that mushrooms are nutritionally useless because we can't do anything with chitin, but they are quite high in this compound (along with heavy metals and whatever else is in the shit they grow in, because they don't have an excretion system). there's a write up on this molecule here:
urocanic acid is another molecule produced by histidine in humans, which was once thought to act as a natural sunscreen to block the effects of uvb radiation on dna in the skin, but that has since been debunked. this molecule seems to react to uv light, and seems to have something to do with immune response, but it's not clear how or why. there's a write-up on this in chapter 5.2.2 (p. 98 in the text, 121 in the pdf) in this book:
this study suggests 57 mg/kg as a concern point:
upper limits:
1.5*57*50/980 ~ 435%
.5*57*50/980 = 145%