Chris Hemsworth did a 4-day fast to reverse aging. His trainer played a cheeky joke to test his willpower.

• Chris Hemsworth tried a 4-day fast for his show about aging, "Limitless."
• Some research suggests fasting can boost longevity, but there are downsides.

Chris Hemsworth put his willpower to the test by trying a four-day fast.

The diet was an experiment to test the health benefits of foregoing food — one of many extreme challenges the Marvel star tried for his National Geographic docuseries on longevity, "Limitless."

Hemsworth worked in collaboration with Dr. Peter Attia, a longevity specialist and host of the podcast "The Drive," to design his fast. Research suggests fasting may help extend your life. However, you may not need to go hungry for days at a time to see the benefits.

While Hemsworth is no stranger to shorter periods of intermittent fasting, using it to lose weight to prepare for roles, this was more intense. This time, he was committed to four days without a meal.

Luke Zocchi, his longtime friend and personal trainer, told Business Insider that he had initially planned to join the fast in solidarity, along with Hemsworth's assistant. But the director of the show had other plans, pulling them aside and instructing them to raise the stakes by eating in front of Hemsworth.

Two days into the fast, as Hemsworth was preparing to compete in an underwater hockey match for the show, he told Zocchi and the crew how much he appreciated their support in fasting. He also mentioned he had a particular craving for pepperoni pizza.

"After he had this big emotional talk with us, we jumped in the car, went down to a pizza shop and we bought 10 pizzas," he said.

Surrounded by hidden cameras, they waited for Hemsworth to finish swimming and return to find them chowing down.

"I honestly thought he was going to cry," Zocchi said.

Still, Hemsworth showed discipline, braving an even bigger feat than being a superhero: he resisted the siren song of pepperoni pizza on an empty stomach.

Zocchi said a second season of "Limitless" is forthcoming, although no details have been made public just yet. In the first six-episode series, Hemsworth tried challenges including a Navy SEAL-style "drown-proofing" exercise and a bone-chilling swim in Arctic waters.