US/National Football League: Eating meat could trigger positive doping tests

The NFL has advised their football players not to consume meat in Mexico or China. Doing so could be interpreted as doping, due to illegal substances regularly found in meat.
The NFL refers to Glenbuterol, which is apparently often used illegally in meat production, but is also a banned doping substance. According to a report of the LA Times, eating meat has already caused positive doping tests for several professional football players in the USA and Mexico. The American Duane Brown, the report says, could only avoid penalties by presenting receipts documenting his high consumption of meat during a holiday in Mexico.