Lucas Irwin surprised his family with a holiday gift they just couldn’t believe: Without mentioning a peep about his determination to lose weight after years of struggle, the 300-pound young man slimmed down to a trim 170.

Keeping his year-long effort a secret from his family made his big reveal — caught on video now going viral —extra special.

“I only go home once or twice a year, usually once in the summer, once for Christmas,” Irwin, 25, told TODAY on Wednesday’s show. “And I made up an excuse not to go home over the summer just so they wouldn't see me.”

Despite his success, he was feeling the jitters before the family reunion.

“When I saw my parents for the first time, I was really nervous,” he said. “I was shaking and … it was very surreal."

They were overjoyed, and the video shows their expressions of happy disbelief as they first laid eyes on him.

“All of the sudden, down the stairs he came,” his father, Jeff Irwin, told TODAY. “And he looked just fantastic. I was more stunned than anything else. But inside, I was just beaming. I was just ... I was over the moon.”

“Just picturing it in my mind again, it brings back tears to my eyes again,” Susan Irwin, his stepmother, said on TODAY. “I am so proud of him.”

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Irwin lost his 130 pounds by following a strict eating regimen. He stayed accountable to himself by keeping a food diary, writing down everything he ate for the year.

“I lost 95 percent of the weight just through changing my diet,” Irwin said. “I only ate 1,350 calories a day. I didn't exercise at all until maybe seven or eight months in.”

Though he didn’t seek the support of his family, he did turn to the Internet for backup.

“Social media was amazing for my weight loss journey,” Irwin said. “Especially the Reddit 'LoseIt' community. You can post your progress and they would help with diet tips and that sort of thing.”

Irwin said he made the pledge to himself about a year ago to lose weight.

"It’s something that I have struggled with all my life,” Irwin said. “So I started December the 28th of 2012 and just went for a year."

Madelyn Fernstrom, the health and diet editor for NBC News, said there is some evidence to suggest that keeping your weight-loss goals private can be better than sharing them, so you are accountable to yourself, not to others. Relying on friends or family can backfire if you get off track, she said.

Irwin’s story, she said, shows the importance of self-motivation and accountability.

“When you decide to lose weight and put your mind to it, that’s the only key to success,” Fernstrom said. “A parent or spouse can be nagging you, but it is only when you decide it’s something important to you that you will get this result. That delight that his parents were expressing is that he took charge of his life.”

Irwin took control over his weight, she said, and “was walking the walk.”

“He was demonstrating, ‘I’m accountable to myself alone,’” Fernstrom said. “That indicates real control over your lifestyle.”

While watching the video, she said, she was nearly moved to tears as Irwin’s family couldn’t believe their eyes.

“It was wonderful,” she said. “They were thrilled for his success not because he was skinny, but because they saw their son as healthy, as happy and in control. As parents, they were just thrilled for his success.”

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