I know a guy suffers from red meat allergy. It's not that it would kill him, but it causes a reaction in headaches, soreness, tiredness, etc. The joking expres

I know a guy suffers from red meat allergy. It's not that it would kill him, but it causes a reaction in headaches, soreness, tiredness, etc. The joking expression goes "How do you know if one of your customers is a vegetarian? They'll let you know."