One Direction have gone on a health kick.

The 'Kiss You' hitmakers have been put on a strict fitness regime and healthy eating plan by their record label Columbia in a bid to whip them into shape before the North American leg of their 'Take Me Home' tour, which kicks off in Mexico City this weekend.

An insider told the Daily Mirror newspaper: ''It's important the lads are in the best possible shape for these dates. Bosses don't want half measures. The boys spend a lot of time travelling and there can be a tendency to eat junk food and play lots of video games.

''Their trainers have insisted they eat healthier food and it helps that they had healthy catering for the UK leg of their tour. It's a great starting point for all the hard work they'll have to put in ahead of their stadium tour next year.''

The heartthrobs are being put to the test by personal trainers Mark Jarvis and Jimmy Wallhead, who are determined to bulk up the boys and define their muscles using a combination of circuit training, low-carb diets and boxing.

The source added: ''But, thanks to [trainers] Mark and Jimmy's new regime, they've been working out lots more. Harry [Styles] has really bulked up his arms, and Liam [Payne] has a sturdy set of abs.

''Niall [Horan], Louis [Tomlinson] and Zayn [Malik] have been working out with separate trainers and they're getting into it, too.''