Zac Efron hasn’t made a secret of his toned body, but the heartthrob took six-pack abs to a whole new level while preparing to play a marine in an upcoming flick – and put them on display while out and about in California recently.

“Zac put on 18 lbs. for his role in The Lucky One,” says a source. “Since then, he’s lost weight but he’s keeping up a fitness routine.”

Efron, who bulked up with the help of trainer Logan Hood, kept his diet in check by following healthy eating tips from nutritionist David Allan. Also a factor in his toned physique: During mealtimes the actor, 23, turned to Z.E.N. Foods, an L.A.-based meal delivery service.

“He loves being outdoors, hiking and exercising outside,” adds the source. “He’s always been in shape.” [Source]

For more about Efron and his amazing body transformation, pick up the new issue of PEOPLE, on newsstands Friday.