Joe Jonas hired a personal trainer to get him into shape for the Guess campaign.

The 27-year-old singer was announced as the global brand ambassador for the American fashion house in January this year and models in their Spring/Summer 2017 underwear campaign, but the star has revealed his latest fashion venture was a ''big challenge'' for him and he had to have a fitness expert with him at all times to help him follow a strict diet and fitness regime ahead of stripping down to his boxers for the commercial.

Speaking about his preparation for the photoshoot, the dark-haired hunk - who is known for being in the boyband the Jonas Brothers with his siblings Kevin, 29, and Nick, 24, - told The Metro newspaper: ''The Guess campaign was a cool thing to be part of. It was a big challenge. You have to do the dieting and training, and I was travelling so much I really had to get on it and not get distracted. I did a lot of boxing training and a trainer came on tour with me.''

The DNCE frontman had to drastically alter his eating plan and include more protein in his meals than before because he was burning ''a lot'' of calories because of his intense work out sessions.

The 'Cake By The Ocean' hitmaker added: ''I had to eat a lot of protein because you're working out so much you burn a lot of calories.''

Although Joe had to completely alter his lifestyle prior to posing for the commercial, which was directed by the Chief Creative Officer for Guess, Paul Marciano, and was photographed by Yu Tsai, he was ''excited'' to be hand-picked to represent the label.

Speaking previously about his partnership, he said: ''It feels great to be chosen by Paul Marciano as the male global ambassador for the new Guess Underwear campaign with a focus on the Guess man.

''I am excited to be part of this amazing experience and to be able to represent the brand's image.''