Justin Bieber reveals secret to his underwear model body

Justin Bieber has revealed the secret behind his muscular physique in his new campaign for Calvin Klein, insisting he works hard to maintain his svelte frame by working out with his trainer, Patrick Nilsson, five days a week.

The 20-year-old singer - who has been unveiled as the face of the brand's latest underwear adverts alongside model Lara Stone - works hard to stay in shape with his trainer Patrick Nilsson, but insists he doesn't eat healthily all the time. 

He said: "I work out with my trainer, Patrick Nilsson, five days a week for about 45 minutes each. We do three days splits: back and triceps one day, chest and biceps on day two, shoulder and legs on day three, and abs every other day.

"I'm not on a special diet, but I do try to watch what I eat. I do eat unhealthy sometimes - I am a 20-year-old guy!"

Justin first sparked rumours of a team-up with Calvin Klein after posting a photo of himself sporting the label's iconic briefs on Instagram in July this year.

He told US Weekly: "I've wanted to be involved with Calvin Klein for a while now. Earlier this year, I put up a picture on Instagram of me in their boxer briefs with the #mycalvins tag, and people went crazy for it and it got their attention."

The 'As Long As You Love Me' hitmaker revealed his partnership with the brand by posting several steamy images of himself and the blonde beauty - shot by famed fashion photographers Mert Alas and Marcus Piggott - on the picture-sharing site yesterday (06.01.15).

Alongside one the images, Justin teased there may be more to come from his modelling career in 2015. 

He wrote: "This is just the start of the year #mycalvins #beallyoucanbe #learnfromyourfaults #risetosuccess #dontbeashamedredeem #thebestisyet2come (sic)."