Matt Damon says cutting one thing out of his diet got him down to his 'high school' weight

• Matt Damon, 55, says he made a big diet change to prepare for his latest film, "The Odyssey."
• He said cutting out gluten helped him slim down to 167 pounds, a weight he hadn't been at since high school.
• Experts say gluten-free diets are not universally necessary for weight loss or better health.

Matt Damon, 55, says one diet change left him lighter than he has been in years.

During an appearance on Wednesday's episode of the "New Heights" podcast, Damon spoke about how he prepared for his latest role in "The Odyssey."

"I was in really good shape. I lost a lot of weight. He said he wanted me like lean but strong. It's a weird thing," Damon told hosts Jason and Travis Kelce, referring to the film's director, Christopher Nolan.

To achieve that physique, Damon said he cut one thing out of his diet.

"I literally, just because of this other thing I did with my doctor, stopped eating gluten," Damon said. "I used to walk around between 185 and 200. I did that whole movie at 167. And I haven't been that light since high school. So it was a lot of training and a really strict diet."

The actor said he works with a trainer, and compared the physical preparation to how the Kelce siblings would gear up for a football season, with training becoming part of his daily routine.

"You know, it's like just part of your day. It's part of your job, right? And it's like yeah, you get really routinized about it and really kind of build your day around all that stuff," Damon said.

The actor added that he hasn't had gluten since. "I'm done. I'm done. I'm gluten-free everything," Damon said.

A gluten-free diet eliminates gluten, a protein found in grains such as wheat, barley, and rye. It is often adopted for medical or digestive reasons, including to manage symptoms of celiac disease.

For most people, gluten isn't necessarily harmful.

"Evidence suggests that, for general health, the emphasis should be on a whole, minimally processed, plant-based diet, which can include gluten-containing grains," Grace Fjeldberg, a registered dietician with the Mayo Clinic Health System, told Business Insider in 2021.

Despite its popularity, a gluten-free diet doesn't necessarily result in weight loss and isn't a universal approach to better health.

Damon is no stranger to getting into peak shape for a role.

In a 2016 BBC interview, Damon said that getting back into shape for his return to the Bourne franchise was "brutal," after his last appearance in 2007's "The Bourne Ultimatum."

"For the first Bourne movie I was 29 and I thought that was hard work getting into shape," Damon said.

"Now I'm 45 and it's just brutal. We shot this bare-knuckle fighting scene on my 45th birthday and it was a lot of work to get there," he added.