Actor Jake Gyllenhaal lost out on love while making new movie Southpaw because he was too busy training to play a boxer for romance.

The End of Watch star lost 25 pounds (11.3 kilograms) for his movie Nightcrawler and director Antoine Fuqua needed the 33 year old to bulk up for his boxing movie, after it was announced Gyllenhaal would replace rapper Eminem.

The actor hit the gym twice-a-day every day to pack on 15 pounds (6.8 kilograms) of muscle, and Fuqua fears his demands may have cost his leading man a romance.

The director says, "I pretty much had him with me and my trainer every day. I took him to almost every fight. I had him train at Floyd Mayweather's gym in Vegas and we watched Floyd's fights, and the Manny Pacquiao fight. He trained in New York at Church Gym with real fighters. We literally turned him into a beast...

"I asked this guy from day one, 'I need you in the gym every day. I need you to train every day', and I said, 'The word is sacrifice, literally'... He was training like a fighter. I had him sparring, really getting hit. I put him in situations where I wanted to see what he was made of. No one but fighters understand the sacrifice it takes to be a fighter.

"I think he broke up with his girlfriend because he was just in the gym every day."

Gyllenhaal had just rekindled his on/off romance with Sports Illustrated model Alyssa Miller when he was cast in Southpaw in March (14).