Hayden Panettiere suffered from agoraphobia after her brother passed away

She told PEOPLE: "I just ballooned out. It didn't matter what I did, what I ate. I know stress and cortisol running through your body can do that.

"Now I think my body was protecting itself, shielding itself from the world."

Jansen died of an undiagnosed enlarged heart, and Hayden is convinced that his death was the cause of her weight gain.

She said: "He was my only sibling, and it was my job to protect him.

"When I lost him, I felt like I lost half of my soul ... I will always be heartbroken about [him], I will never be able to get over it. No matter how many years go by, I will never get over his loss."

Hayden's struggles snowballed and, at one stage, she was barely able to leave her house.

The actress shared: "I didn't recognise myself. My agoraphobia came out, which is something I've struggled with in the past."

Hayden first found fame as a child, and she's always been very self-conscious about her appearance.

The 'Heroes' star - who has also enjoyed success as a singer - said: "Having grown up in this industry, you're terrified if you don't look decent when you walk out the door.

"That started happening for me when I was 16, where you're already at an age where you don't feel great about your changing body - and you certainly don't need grown men commenting on [our] insecurities."