Brian Austin Green Says He Lost 20 Lbs. During His 'Rough' Battle with Ulcerative Colitis

Brian Austin Green is opening up about his "rough" battle with the inflammatory bowel disease known as ulcerative colitis.

The actor, 48, appeared on Good Morning America Thursday morning, where he spoke alongside pregnant girlfriend Sharna Burgess about his recent health issues.

Noting that he has "dealt with ulcerative colitis a few times," Green called dealing with the illness "a real rough experience."

"Sharna had never experienced it at all," he continued, before Burgess, 36, chimed in, adding, "I didn't realize how debilitating it was until I saw him and watched weight drop off him."

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.

As the Beverly Hills, 90210 alum revealed that he "lost like 20 lbs.," Burgess said that symptoms of the illness made her "scared," given that she had no experience dealing with something like it the past.

"Not having experienced this type of thing before, and still learning, I was supportive and loving and [like], 'I'm here for you,' and internally fearful for how long does this go on for?" she said.

Want to get the biggest stories from PEOPLE every weekday? Subscribe to our new podcast, PEOPLE Every Day, to get the essential celebrity, entertainment and human interest news stories Monday through Friday.

Green then noted that he has revamped his diet in an effort to better live with the disease.

"I try and avoid gluten and dairy as much as possible," he said during his GMA chat. "It's really just dietary, like, as long as I can keep things within my system that my body doesn't think I'm poisoning it with, then it doesn't fight back."

He added: "I would eat food, and literally it was like, my body didn't process any of that. So then, when you start playing catch up with, like, staying on top of being hydrated enough, that's such a battle."

Ulcerative colitis is "an inflammatory bowel disease (IBD) that causes inflammation and ulcers (sores) in your digestive tract. Ulcerative colitis affects the innermost lining of your large intestine (colon) and rectum," according to the Mayo Clinic.

The condition can develop at any age, but the disease is more likely to develop in people between the ages of 15 and 30, according to National Institute of Diabetes and Digestive and Kidney Diseases.

RELATED VIDEO: Brian Austin Green Had 'Dance Party' With Kids While Watching Madagascar

Green first revealed that he had been suffering from ulcerative colitis in a candid Instagram video post last month. "Hey everybody, so I disappeared from Instagram for a while. I had ulcerative colitis for about six, six-and-a-half weeks, which wasn't very fun," he said at the time. "Horrible timing."

He praised Burgess for standing by his side, especially while preparing for motherhood. "It's amazing to see how people can adapt to situations and how well she has adapted to this situation," he said.

As for that baby-to-be, Burgess is 33 weeks and counting down the days until her first child arrives (Green is also dad to Journey River, 5, Bodhi Ransom, 8, and Noah Shannon, 9, with ex Megan Fox as well as 20-year-old Kassius from a previous relationship).

"I always knew I wanted to have kids because that was a dealbreaker for me," Burgess said on GMA. "One of the things I fell in love with him about was watching him be a dad and the way he parents. I thought, 'It would be so incredible to raise a child with him.' "

Good Morning America airs weekdays (starting at 8 a.m. ET) on ABC.