1- They don't specify exposure levels other than Chronic factory exposure which I assume to be relatively extreme
2- Diacetyl is formed in beer, but is also readily reabsorbed by yeast. It has a low flavor threshold and is generally an off flavor, so there is likely very little in beer (comparatively)
3- While the study found an association between chronic diacetyl exposure and certain brain protein processes, it did not prove a cause-and-effect relationship
So I think that if you brew butter popcorn beer 8 hours a day for 20 years you might have a problem, maybe.
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- Kaiser Wilhelm