Great question — answering as the UphillTech team:
Yes , you are fine bringing a bag of protein powder in your carry-on. The TSA requires food (which it would probably be considered) to be wrapped and to be sent through the X-ray machine at the security checkpoint, but so long as it is not a liquid or gel it isn't banned. Note that, especially since it won't be in its original container, they may want to sample it for testing . The same goes for any powder (e.g. powder makeup, sugar, dietary fiber supplements, talcum, Gatorade). It goes without saying that powders which are hazardous materials (e.g. explosive or corrosive) are banned of course.
Full answer by choster: https://travel.stackexchange.com/a/14299 (CC BY-SA 3.0).
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