and is an average of three studies, boosted by a factor of 1.24. this is the calculation:
1.24*(20+13+12.6)/3 = 18.848
the studies are from 1958, 1991 and 2001.
so, 1.24*(13+12.6)/2 = 15.872 is probably a better estimate.
these are indeed lumped together as "saa" (sulfur amino acids). the first study is based on a total nitrogen balance, the second on "methionine balance", with five participants, and the third....let's look at the third:
Objective: The goal of the present study was to determine the methionine requirement of men fed a diet devoid of cysteine (total SAA requirement).
Therefore, the total SAA requirements found in the present study represent the amount of dietary methionine needed to fulfill all the functions of methionine in vivo.
that's not the way this is usually presented. but, the results are actually quite useful to me as an rdi for methionine specifically, allowing me to focus on cysteine afterwards
these are the numbers from the study:
daily: 12.6*70 = 882
upper: 21*50 = 1050
using the 1.24 factor as previously to get from the ear to the rdi,
1.24*882 = 1093.68, which is about 100 mg more than the number i was using, which was constructed by turning the eu data into a ratio and applying it to the usda data
1.24*1050 = 1302, which is considerably less than the number cited in a different study (46 mg/kg)
it makes sense for me to use these numbers instead, as the rdi is higher and the upper limit is lower.
that means i'm separating cysteine from methionine in the rdis, even as i connect them in the chart. and, the next update will consequently likely also include actual cysteine precursors as well.
the above article also summarizes what i've put together fairly well:
Methionine is a nutritionally indispensable amino acid required for the normal growth and development of all mammals (1, 2), whereas cysteine is conditionally indispensable (3, 4). In addition to its required role in protein synthesis, methionine supplies the methyl group for numerous methylation reactions and the sulfur atom for cysteine formation (5–8). Through the intermediate S-adenosylmethionine, methionine is the source of the methyl groups of choline, creatine, and both DNA and RNA intermediates (1, 5, 6, 8). Cysteine is involved in the protein synthesis and biosynthesis of taurine, sulfate, and glutathione (6).
so, if i'm getting enough cysteine, what i really need methinione for is the methyl side chain - not the sulfur.