After his "fat Elvis" period in 1965, John began to lose weight. Check out pictures from the '66 tour, during "How I Won The War", Sgt. Pepper's, Rishikesh, and so forth. Yep, he did the macrobiolic diet thing with Yoko. But the photo session shortly before his death, which show a naked John with a clothed Yoko, make him appear unusually thin.
Bulimic? Maybe, maybe not.
But Yoko would do well to publish John's "secret" diet plan because it's not easy for anyone to maintain their target weight for so many years.