Opal arrived at L.A.’s Best Friends Animal Society from a local shelter in August 2017. “She was brought in as a stray in her overweight condition,” says Leah Lindamood, a dog-behavior specialist at Best Friends, who eventually adopted Opal.

The 11-year-old mixed breed weighed 26 lbs. — double a healthy weight for a dog her size. But her first foster family didn’t do much to help her trim down as they struggled with the “tough love” mentality.

“When she saw human food, she would bark, so the people fostering her used to just give it to her,” Lindamood tells PEOPLE in the annual Half Their Size issue.

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In February 2018, Opal came to stay with Lindamood, who had prior experience helping dogs lose weight and set up a pup weight loss “boot camp” at her home. At first, Lindamood says Opal wasn’t able to jump on furniture and couldn’t sit well because of all the excess flab.

“All of her body movements were in compensation with being overweight,” Lindamood says. “She could barely walk up two stairs. It was hard to watch her try to waddle and move around.”

For more stories of pets and people who lost half their size, pick up a copy of PEOPLE, on newsstands Friday.

She started Opal on a strict feeding schedule twice a day and refused her when she begged for table scraps. Lindamood also switched Opal from a primarily human-food diet to portion-controlled bowls of Purina Overweight Management — a high-protein, low-fat and low-calorie dog food.

“She loves food so much that she would get so excited for her diet food,” Lindamood says. “She’ll eat anything you put in front of her. She just slowly but surely lost the weight.”

Initially, Opal was only supposed to stay with Lindamood on a temporary basis until she could get healthy, but Lindamood developed a soft spot for the pooch.

“We couldn’t give her up,” Lindamood says.

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Now, she’s a permanent part of Lindamood’s family. In order to maintain her weight, Lindamood still sticks to feeding her 1/4 cup of Purina OM twice a day.

Opal is allowed treats sparingly, and has the ultimate splurge once every three weeks when Lindamood allows her to indulge in one bite of chicken or a rawhide chew stick. Because the spry senior slimmed down, she’s also able to go on regular walks around the block.

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“Because she lost the weight, she just seems happier,” Lindamood says. “She gives love and shares all her joy.”