Actor Jared Leto stopped eating in a bid to lose 40 pounds (18.1 kilograms) for his new movie Dallas Buyers Club.

The Thirty Seconds to Mars frontman reveals he went to great lengths to play a HIV-infected transsexual in the film, which also stars an emaciated Matthew MCConaughey.

He says, "I stopped eating. It wasn't an easy thing to do. It was 30 or 40 pounds. After a while I stopped counting."

Leto admits the drastic weight loss changed many things about him, mentally and physically.

He continues, "It changes the way you walk, the way you sit, the way you think."

In 2012, Leto had to assure fans he was healthy, after appearing shockingly thin while out in public during filming.

On, he wrote, "Thanks for all the concern... energy a bit low but I'm doing ok over here!"