An ultra-swimmer tried to eat as much as a tiger shark and ended up consuming 40,000 calories in a day

Ultra-athlete Ross Edgley needs a lot of fuel for marathon swims, but tiger sharks have huge appetites, too. He tried to keep up for Sharkfest.

• Ross Edgley tried to out-eat a tiger shark by fasting, then eating over 40,000 calories in one day.
• "Shark vs. Ross Edgley" is part of National Geographic's Sharkfest.
• Despite eating an enormous amount of food, Edgley only matched 28% of the shark's estimated daily diet.

Olympic swimmers have been known to eat 10,000 calories a day, but they have nothing on a tiger shark, which can consume more than that in a single bite.

That didn't stop ultra-athlete Ross Edgley from trying to out-eat the fish that have been affectionately dubbed "the garbage cans of the sea."

For "Shark vs. Ross Edgley," part of National Geographic's Sharkfest that debuted on June 30, the ultra-marathon swimmer went up against a variety of different species to see how he'd fare against their speed, agility, and stomachs.

He didn't fair well in speed, achieving only about 25% that of a mako shark. And for agility, he tried jumping completely out of the water like a great white but only made it about 75% of the way.

This isn't surprising given how well-adapted sharks are at moving through water compared to humans. But what about the stomach? Perhaps eating as much as a tiger shark is where Edgley could gain a lead in the competition.

"I thought I was going to do pretty well," Edgley told Business Insider of the tiger shark challenge. "I can put away quite a bit of food." He's been known to eat 15,000 calories a day while training for long-distance swimming events.

Despite that practice, it turns out that it's far more difficult to eat like a shark than he could have imagined.

Edgley tried to fast and feed like a tiger shark

Researchers have tracked tiger shark migrations, finding that they can swim around 4,660 miles a year — just a little less than twice the width of Australia.

To keep their energy up for those long swims, the apex predators will eat just about anything, from turtles to dolphins to plastic bottles. But if food isn't readily available, they might have to go a few days without a meal.

To approximate this for the show, Edgley went 24 hours without water, sleep, or food while swimming, weightlifting, running, and performing other forms of exercise. "That's up there with one of the worst things I've ever done," he said in the show.

His next move was to eat a little over 40,000 calories in a single day by chowing down on pizza, fish and chips, blueberry lemon cheesecake, and four tubs of ice cream. By the end of the experiment, Edgley had lost almost 14 pounds after the fast and then regained 22 pounds after his feeding frenzy.

"I was so happy with that" amount of calories, Edgley said. He thought for sure it put him in competition with the tiger shark.

During the entire fasting and feeding, Edgley was under close medical supervision to ensure his safety, he wrote on Instagram. "Do not try this at home," he warned in a TikTok video.

Tiger sharks can take 6-pound bites

For the show, marine biologist Mike Heithaus and Edgley wanted to know how many calories a tiger shark could eat in a day and compare it to the 40,000 calories that Edgley ate.

So they ran an experiment.

Heithaus made a gelatin "lollipop" that mimicked the rubbery, high-calorie fat of sea mammals that sharks will sometimes eat to see just how much a tiger shark can swallow in a single bite.

While Heithaus and Edgley were diving, an 11-foot, 460-pound tiger shark took a 6.4-pound bite out of the gelatin. Heithaus estimated it was the equivalent of 25,000 calories. Extrapolating that, Heithaus estimated a tiger shark might take about 10 bites a day for a total of around 250,000 calories per day — far more than what Edgley consumed.

But something to keep in mind is that Edgley weighs a lot less than a tiger shark. Even taking into account their respective body weights, though, Edgley only ate 28% of what a tiger shark could consume in a day.

"It was humbling," Edgley said. Despite all of his training, he barely made a dent in a tiger shark's typical diet.

Still, he said it's been a fun way to get people interested in sharks, which was the main point all along.

"That's what's been amazing," he said, "seeing on social media, even kids just going, 'Oh my God, he tried to eat like a tiger shark. He had seven liters of custard.'" His TikTok videos for the challenge have racked up tens of thousands of views.

Edgley has been passionate about shark conservation since a basking shark unexpectedly joined him on a 157-day swim he took around Great Britain in 2018. He hopes his exploits help people fall in love with the animals. And he wouldn't mind trying to keep up with even more shark species. "I'll put my goggles on again and find some custards," he said.