Although the recording artist died of accidental alcohol poisoning after a years-long battle with substance abuse, her brother told The Observer newspaper she and her friends began making themselves vomit after meals when Amy was 17.
He said Amy never really stopped the unhealthy practice.
"She suffered from bulimia very badly," Alex Winehouse said.
"That's not, like, a revelation -- you knew just by looking at her," he said, referring to the emaciated figure she had for years. "She would have died eventually, the way she was going, but what really killed her was the bulimia ... . Absolutely terrible. ... I think that it left her weaker and more susceptible. Had she not had an eating disorder, she would have been physically stronger."