[not_clear]

When I had first heard about this, it was the only anti-MSG argument that sounded even remotely plausible, but it seems that nowadays commercial MSG is made by bacterial fermentation, which means that they would also have the preference for L-oriented amino acids.

Wikipedia confirms this

    http://en.wikipedia.org/wiki/Monosodium_glutamate#Production

The referenced papers on the process (from 1957!) indicates that they were reporting L-glutamate producing bacteria specifically:

    https://www.jstage.jst.go.jp/article/jgam1955/3/3/3_3_193/_article

Here is an overall history of MSG production, including chemical synthesis (now obsolete). Even when they were doing this, they screened out the D-glutamate and re-synthesized it to maximize L-glutamate production (might not have been 100% effective, as apparently it involved doing optical screening of the crystals)

    http://ajcn.nutrition.org/content/90/3/728S.full

Disclaimer - I am Japanese American, so grew up with Ajinomoto MSG, dashi, tomato sauce, parmesan, Doritos, etc. as part of my diet, so am biased toward skepticism on this topic.

[not_clear]

Interesting - the metafilter referenced elsewhere on the thread had a pointer to a paper about D-glutamate vs L-glutamate. Apparently, bacterial fermentation generates a fair amount of D-glutamate, but this is true of "naturally occurring" fermentation (such as in cheese) too.

The abstract claims foods to which MSG is added directly contained lower amounts of D-glutamate than foods where the MSG comes from fermented foods. I wonder why? Maybe due to better optimization / control for L-glutamate production in the bacteria?

http://onlinelibrary.wiley.com/doi/10.1002/chir.530060410/ab...