Ashlee Simpson says ''dedication'' is the secret to Jessica's weight loss.

The 34-year-old star has heaped praise on her sister after the 39-year-old singer lost 100lbs since giving birth to he daughter Birdie Mae just six months ago.

Asked about her sibling's fitness regime, she told Us Weekly: ''Dedication! It works. I mean, she's been working so hard and she looks amazing.''

Her kind comments come after Jessica - who has revealed she weighed 240lbs when she welcomed her baby into the world in March - opened up on her diet changes.

She recently said: ''It's been a lot of hard work. I have to say that I have worked very hard. Everything that I've been eating is mostly made with cauliflower.''

The mum-of-three - who also has Ace, six, and Maxwell, seven, with her husband Eric Johnson - admitted she didn't think she'd ''gain as much'' as she did during her other two pregnancies.

However, she added: ''When you're pregnant, I gain a lot, I mean, a lot. I didn't expect to gain as much with my third. I thought I'd learned my lesson, but apparently that's just the way God made me, very hungry and pregnant.''

The 'With You' hitmaker previously said she was working ''really hard'' on her body by walking lots and eating healthily because she wanted to feel ''good'' in her own skin.

She said at the time: ''I am working really hard right now. It's not easy at all, but I am determined to feel good.

''I have been doing a lot of walking - getting my steps in not only burns calories but it also helps me clear my head and get focused. I am eating healthy too - I discovered I really like cauliflower. Who knew it could be a substitute for almost anything?''