Alex O'Loughlin was 6 years old when he started studying Shobukai karate in his native Australia. On the first day, he slipped into his pint-size gi with its crisp white belt and bowed before entering the dojo. Inside, he met the sensei, watched the other kids practice their lessons, and couldn’t wait to be part of the action. “I picked it up very quickly,” he remembers. By the time he was 10, he’d earned a brown belt. “It was a huge part of my life, man,” he says. “I wouldn’t miss it.” But even more important, it was the beginning of a commitment to fitness that has served the 34-year-old actor well throughout his life—not to mention as the star of CBS’s turbocharged cop series Hawaii Five-0. Over the years, O’Loughlin has dabbled with other forms of exercise. “In my early 20s, I did a lot of weights. I was huge—closer to 200 pounds,” says the 6'1″ actor, who clocks in today at a solid 175. “For lunch I’d have a chicken, a loaf of bread, a gallon of milk, and a bag of chocolate cookies. Then I’d go to the gym and bench 240 pounds. That’s a young man’s thing.” Today, he pulls up to a roadside cafe in Malibu for his MF interview and jumps out of his black Porsche Cayenne, looking tan and happy in a gray T-shirt and khaki cargo shorts, a plastic gallon jug half-filled with water dangling from his hand. It’s been a big year for O’Loughlin. “An epic year,” he says. After two previous TV series disappointments—2007’s vampire-detective drama Moonlight and 2009’s even shorter-lived Three Rivers—he’s just wrapped his first season of Hawaii Five-0 and returned to the mainland.

But the actor has also discovered that island living has its perks. For one thing, he’s finally taken up surfing—a sport he tried when he was 18 but quit after getting caught in a steep, fast break. But this year, he conquered that fear and now frequents the beaches of Diamond Head, Tongs, and Velzyland on the North Shore. “Surfing is my great passion in Hawaii,” he says. “I have a bunch of boards—all of which I ride badly.” His co-star, Scott Caan, who plays Danny “Danno” Williams on the series, reports otherwise. A longtime surfer, he says that O’Loughlin is progressing “on a very advanced level.” Of course, there’s nothing like sports to bring out competition among cast mates. “Scotty is a machine,” says O’Loughlin. “I’ll never be as good as him. And I’ll never match his jiu-jitsu, either. But if we’re in a ring together—fists and stand-up fighting—I’d lick him standing up.”

Caan laughs when he hears this. “We have a great respect for each other,” he says. “We are completely comfortable working together and going for each other’s throats without worrying that we’ve crossed a line. Alex is a natural leader. He takes control and makes things happen on-set—and in his personal life, too. He’s The General, and I think that comes across in this role.” Of course, even for The General, chasing bad guys every week—pretend or not—can take its toll on the human body. “I’m damaged a bit,” O’Loughlin admits. His litany of first-season injuries includes tendinitis in his bicep, and a torn shoulder. So he’s spending his time off getting his core energy up, stabilizing his back, and healing his shoulder. “I’m going to come in like a bull at the top of Season 2,” he promises.

When it comes to body transformation (and maintenance), O’Loughlin turns to L.A.-based trainer Jeff Blair, whom he met two years ago while playing Jennifer Lopez’s baby daddy in the romantic comedy, The Back-up Plan, which required him to look lean and ripped, fast. “He put me on this nutrition program—and he changed my life.” These days, O’Loughlin makes sure to drink about two gallons of water a day (which explains the jug he carries). Breakfast consists of a half-cup of oatmeal, a banana, a few almonds, and a splash of 2% reduced-fat milk. Lunch and dinner include a balanced combo of protein, veggies, and carbs—and he has a simple paradigm for portion control. “If you look at your open hand, the size of your palm is the amount of protein you should take—about two-thirds of a chicken breast. Your splayed four fingers equal the amount of space you should have for vegetables. And the triangular area between your index finger and thumb is for carbohydrates—brown rice or whole-meal pasta. The key is to have a very low-sodium diet and understand that 80% of [the process] happens in the kitchen, not in the gym.” When it comes to workouts, though, the actor is hardly a slouch. “I like to circuit train,” he says. “If you do one exercise after another without breaking, you bump your metabolic rate up to such a place that you create what’s called a ‘furnace effect’—your metabolism is firing at such a high rate that you can go to sleep and you’ll burn calories through the night. That’s my ideal place to be.”

Check out the August issue of Men's Fitness for more on Alex O'Loughlin's fitness regimen, on newsstands now.