from a 1978 report by Al Aronowitz, about Mick Jagger meeting President Jimmy Carter at the White House - at an awards ceremony:

It started off with a roast beef buffet at the White House, with Leonard Bernstein one of 500 guest celebrities, and ended with a disco party at the National Gallery, with dimmed lights, crabmeat soup and 200 tables in the basement cafeteria.

. . . Jagger, accompanied by President Ahmet Ertegun of Atlantic Records and his wife, Mica, left early in search of "some eats." Jagger was wearing a knit cap and track shoes with his tuxedo. "I can't eat here; I'm a vegetarian," cried Jagger, who capped his evening by swinging through the revolving doors several times on his way out.

- however, some doubt was cast on the whole story....

There are various online references to Jagger being vegetarian, but none with any kind of real evidence. If anyone has anything specific do let us know.