Ari Michelson
July 19, 2016 11:45 AM

MacKenzie Walker never really gave much thought to how much she was eating, which is how the 5’5″ student ended up weighing 223 lbs. at just 13 years old.

“I didn’t really know the feeling of being full,” Walker, now 16, tells PEOPLE in this week’s issue. “I could eat and eat and eat.”

In a typical day, the Windsor, Ontario native would drink 12 to 15 cans of Coke, and have a 12-pack of tacos from Taco Bell as a late night snack after consuming unhealthy foods all day.

She reached a turning point when she was shopping for a dress for her 8th grade graduation and realized the store didn’t have options she liked for her size 22 frame.

RELATED: Cruelly Bullied for Her Weight, Alex Perrineau Turned Her Life Around — And Lost an Amazing 142 Lbs.

“Everybody got to wear the dress that they wanted for the graduation and I was just stuck with this awful peach-colored dress,” she recalls. “That kind of hit me, and I had one of those moments like, ‘Why did I let myself get here?'”

In October 2013, Walker’s father, who was also overweight, underwent gastric bypass surgery. In an attempt to lose weight along with him, Walker began eating small portions like he was eating. But even though she ended up losing around 50 lbs., she knew the diet was too restrictive to be sustainable.

She started her own Instagram account, @pinkteenweightloss13, to track her progress, and began reaching out to other Instagram members for fitness advice.

WATCH: How Gillian Faith Lost 110 Lbs.

“I started following a bunch of weight-loss accounts, and I was able to connect with different people,” she says. One of them was her coach, Chris Jones, whom she credits with her success. “He’s a big part of my life,” she says.

Walker learned to track her “macros” (protein, carbs and fat) rather than just counting calories. Her go-to meals include a lean protein like chicken or steak, a veggie like asparagus, and a carb like white rice or sweet potatoes.

“I started seeing more results and the weight started coming off easier,” she says. “I started looking better, and it was completely life-changing.”

RELATED: How I Lost 100 Lbs.: Meet This Week’s Cover Star Karyl Mullins on PEOPLE’s List

Combined with strength training, her new clean diet helped Walker to lose nearly 100 lbs. In 2015, she had surgery to remove more than 3 lbs. of loose skin around her stomach.

For more on MacKenzie, pick up the latest issue of PEOPLE, on newsstands now.

Walker’s Instagram now has over 67,000 followers, and Walker started taking on her own clients as a weight-loss coach. “Because I was so young, because I lost a certain amount of weight, I had people contacting me right away asking me questions,” says Walker.

Walker now has a self-published book and continues to do online coaching, all while attending school and working part-time at a restaurant.

“My big thing is that you cannot over-complicate [weight loss],” she says. “I want to share my story.”

Reporting by CATHERINE KAST

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