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Nancy Slagowitz, 49, says she has found her fountain of youth. And she didn’t discover it in an expensive pill — her miracle came in the form of a kettlebell.

The lean, married mother of two points to high-intensity interval training (HIIT), in which she uses a kettlebell, as the elixir that’s keeping her young.

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“It’s the perfect antidote [to harsher workouts],” says Slagowitz, who trains two to three times a week and says she has seen a transformation in her body and attitude. While Reebok CrossFit was punishing — “My body couldn’t handle it. I felt like Private Benjamin,” she says — her HIIT routine has had the opposite effect.

“I walk differently. I definitely feel more fit, stronger and have more energy,” says Slagowitz. “I have abs and that cut look in my arms. [Plus, I] look younger.”

Slagowitz’s turnaround lends credence to new findings that suggest HIIT can actually stop the aging process at the cellular level.

Nancy Slagowitz, 49, says she has found her fountain of youth. And she didn’t discover it in an expensive pill — her miracle came in the form of a kettlebell.

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The lean, married mother of two points to high-intensity interval training (HIIT), in which she uses a kettlebell, as the elixir that’s keeping her young.

“It’s the perfect antidote [to harsher workouts],” says Slagowitz, who trains two to three times a week and says she has seen a transformation in her body and attitude. While Reebok CrossFit was punishing — “My body couldn’t handle it. I felt like Private Benjamin,” she says — her HIIT routine has had the opposite affect.

“I walk differently. I definitely feel more fit, stronger and have more energy,” says Slagowitz. “I have abs and that cut look in my arms. [Plus, I] look younger.”

Slagowitz’s turnaround lends credence to new findings that suggest HIIT can actually stop the aging process at the cellular level.

“Your body is working hard after the workout to replenish your muscles and keep getting oxygen into the body,” says Dara Theodore, a trainer at the Fhitting Room. “It works for you beyond the 20, 30 or 40 minutes you’re doing the actual activity.”

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Jessica Bolbach, who co-owns Kore in the Meatpacking District with her mother, Candice Bolbach, recommends that older exercisers spend extra time on their form to avoid injury.

“By listening to your body and easing into the exercises … it​ will still provide the benefits [and] will prevent injury, which will allow you to do more over time,” says Jessica, 30. “In turn​,​ you’ll see even greater results and progress,​ no matter your age or fitness level.”

​Theodore says beginners of all ages should ease into a HIIT routine, and suggests starting with twice-a-week workouts with rest periods. Once the fitness improves, she says, increase this to three to four times a week.

HIIT is a particularly helpful workout for time-starved New Yorkers.

“I like [that it] hits the body from head to toe because, quite frankly, I don’t have a lot of time to work out. It’s the most bang for your buck,” says Theodore, 45, who is raising two children. The Upper East Side mother was a runner and yoga devotee until she went to the Fhitting Room. As a trainer, the incredibly cut Theodore has seen results in her physique, her energy level and even her blood work.

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“One hundred percent, I feel younger. I have genetically high cholesterol [and it] came down,” says Theodore. “And the fact that I teach class with younger trainers and we’re all on the same schedule and have the same amount of energy, I attribute that to HIIT.”

Candice Bolbach isn’t surprised by the study. The 60-something HIIT devotee who, along with her husband, takes classes four times a week at her gym says, “Not only does my body look better and feel better, but my face and skin do as well because it brings more oxygen to the blood flow. [I look] younger. That’s the truth.”

Her daughter agrees that she has seen the difference in her mother, as well as in other older clients.

“People talk about toning their trouble spots, like [their] booty. They’re the things that start to lose [definition] as you get older, but by doing this workout, those are the things you can maintain and [even] reverse,” says Jessica.

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Beyond looking and feeling younger, there are other benefits, notes Slagowitz: Her jeans are a size smaller. It was a difference she saw after a few months.

“You get lean doing this,” says the admitted foodie, adding that HIIT helps her to work off her wine-and-dining habit. “It makes it a lot more fun living in New York.”

The Workout

Dara Theodore, a trainer at Fhitting Room, one of the early HIIT proponents, says this 20-minute HIIT workout — which requires dumbbells and kettlebells at the weight of your choice — uses compound movements.

“You’re using more than one joint and accessing more than one muscle group, which makes it a more efficient and intense workout,” she says.

Start the routine with a three-minute warm-up of your choice (think jumping jacks or jogging in place).

Then, move onto the five high-intensity moves below.

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Each high-intensity move should be done for 40 seconds. In between each move, do low-intensity squat thrusts for 20 seconds, then rest for 30 seconds before starting the next move.

To do a squat thrust — “a modified burpee that’s body-weight cardio to keep your heart rate up,” says Theodore — squat with your hands on the floor, kick your legs out into a plank, bring the legs back in and stand back up. Keep it up for 20 seconds.

Do the entire routine twice.

Move 1: Kettlebell deadlift

Begin in a standing position with the kettlebell in your hands. Slowly hinge forward from your hips while pushing your butt back, lowering your arms until your hands hover by your knees. Stand back up.

“This is full body because you’re engaging from the shoulders down,” says Theodore. “Your lats are engaged, and you’re working your hamstrings and quads.”

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Move 2: Push up to renegade row

Start in a plank position with a dumbbell in each hand (resting on the floor) and your feet shoulder-width apart. Do a regular pushup, and each time you come back up, pull one dumbbell up toward your rib cage, return to the floor and then do the dumbbell pull on the other side.

“You’re engaging your lats while you’re keeping your core fully engaged,” says Theodore.

Move 3: Goblet lateral lunge

Stand with your feet together while gripping a weight with two hands. Step one leg out laterally, bending the knee, while keeping the other leg straight. Hinge forward, lowering your elbow to your knee. Come back up and repeat on other side.

“This is one of the most basic, fundamental moves. You’re using your core, glutes, abductors and inner thighs,” says Theodore.

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Move 4: Push press

Stand with your core engaged holding a dumbbell in each hand, just outside of your shoulders, with your elbows bent. Dip your knees down and then use leg power to push back up to standing position while pushing the weights straight above your shoulders. Lower your arms back to starting position.

“If doing two dumbbells at once is too difficult, it’s okay to do one arm at a time,” says Theodore.

Move 5: Russian twist

Sit on the floor with your body in a “V” position and your toes pointed. Hold the weight at your chest and using your obliques, rotate side to side.

“This is a core rotational movement,” says Theodore.