Khloé Kardashian's Trainer Tells PEOPLE Exactly How She Lost Those 35 Lbs.!

Khloé Kardashian is looking sexier than ever — thanks to her fierce dedication and hard work!

“Khloé gives as much to the gym as the gym gives to her,” her trainer Gunnar Peterson tells PEOPLE. “She’s in love with fitness.”

Peterson has been working with the reality star, 31, for three years, and he says she doesn’t skip workouts.

“She never cancels,” he says. “It can be a night where she’s short on sleep. It can be a day where she has an engagement for that day. She may have something at 8 – then we’re going at 6.”

Earlier this year Kardashian told PEOPLE she works out “almost every day,” including five-day-a-week, hour-long sessions with Peterson, who constantly switches up their methods.

“There is no typical routine,” he says. Their workouts incorporate “different intervals mixed up, whether it’s the motion of the interval or the duration of the interval or the intensity of the interval.”

“Some days it’s more heart-rate-intensive, some days it’s more lower-extremities-intensive, some days the upper extremities get a little extra love,” says the celebrity trainer, who says he uses bigger movements rather than isolated movements to tone up muscles.

Kardashian often shares photos of their intense workouts on Instagram; in many of them she lifts heavy weights.

“You have to challenge your body with an external load,” says Peterson, explaining why weight-lifting is essential to weight loss.

Peterson also incorporates boxing into their routine.

“[Boxing is] great for the heart rate, great for hand-to-eye, great for balance,” says Peterson. “It’s good for your body to contract upon impact.”

But Kardashian’s 35-lb. weight loss didn’t come from fitness sessions alone. In addition to her discipline in the gym, she’s made major lifestyle changes.

“As the changes have come, the diet has tightened up,” says Peterson. “You can’t out-train a bad diet, and you can’t out-diet a bad lifestyle. She’s made better choices when she eats. She’s been more conscious about working on sleep habits. She hydrates better.”

And while she’s lost a significant amount of weight, Kardashian shows no signs of stopping.

“She keeps getting after it and she keeps getting better,” says Peterson. “We’re always playing with [our workouts]; we’re always making it that much more challenging.”

For more on Kardashian’s weight-loss transformation, pick up the latest issue of PEOPLE, on newsstands this week.

—Gabrielle Olya, @GabyOlya

VIDEO: Khloé Works Out with Celebrity Trainer Gunnar Peterson