In late 2018, Colin O'Brady became the first person to cross the continent of Antarctica alone and unsupported by a resupply or kite.

To save precious space and weight on his sled for food and essential gear, he completed the trek wearing just one pair of underwear.

In a new memoir about his voyage, O'Brady describes how one impulsive night of bingeing on his food rations turned messy.

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After 15 days of pulling a nearly 400-pound sled across a treacherously windy Antarctica, Colin O'Brady was hungry, his willpower depleted.

"I woke up during the night feeling overwhelmed and ganged-up on, my defenses down," O'Brady wrote in his new memoir about the expedition, "The Impossible First."

He'd embarked on a daring mission — alone and unaided by kite or resupply — trekking across the southern continent on skis. It was, at the time, a solo voyage no one had ever completed without quitting, dying, or being helped along by a kite, and O'Brady's food rations were a critical part of his strategy for what he hoped would be a successful initial crossing.

Packed up with him were special gooey 1,180-calorie "Colin Bars," created specifically for him and his trek by his sponsor Standard Process. He had carefully portioned out how much of the bars he'd be allowed to eat each day, but that fateful night in his tent, things did not go according to plan.

O'Brady ate about 2,000 calories in one 3 a.m. binge session

"I was seized by a wave of hunger that made me ravenous and a bit out of control," O'Brady wrote. "Still half-asleep, I grabbed the duffel bag with my food supplies and ripped it open, then grabbed chunks of Colin Bars and stuffed them into my mouth."

The high-fat mix of coconut oil, nuts, dried cranberries, and cocoa powder in the power bars was designed to more easily fuel O'Brady's 8,000-calorie ski days on the frigid and windswept ice. But the bars were not designed to be eaten en masse after midnight. O'Brady probably ate about 2,000 calories' worth in one 3 a.m. gorge.

The next day, he woke up feeling sick and knowing he'd have to ration what he ate that day to make up for the binge. His stomach started to gurgle even before he'd finished packing up his campsite, and he felt so uncomfortable that he pressed a hand to his belly several times as he packed his sled and prepared for another day of trekking.

"When a new wave of rumbling gurgles rolled through, my gut grabbed my mind entirely, and I tried to fight back by focusing on things outside myself," O'Brady wrote, as his indigestion worsened (eating too much, eating too fast, eating high-fat foods, and eating when you're stressed are all associated with a range of uncomfortable symptoms, including nausea, bloating, heartburn, and, eventually, diarrhea or constipation.)

There was no containing the pain any more.

After six hours of hiking, in near minus-25 Fahrenheit (minus 31.6 Celsius) temperatures, O'Brady suddenly felt what he called a "wave," one that would not stop.

He had been fairly regular until that point on the trip, going to the bathroom every morning and then staying dressed head to toe in snow gear all day until he could stop to set up camp for the night and go to the bathroom in a sheltered vestibule.

O'Brady decided it would be best to wait until the evening to take off his jacket, overalls, and base layers, get fully undressed, and go. But his body had other plans.

"I thought that I'd get relief if I was able to pass some gas, so I tried," he wrote. "Unfortunately, more than gas came out. I was relieved and disgusted at the same time."

O'Brady had brought just one pair of underwear for the 54-day trek, prioritizing space and weight on his sled for food

O'Brady had six hours more of trudging through the snow and wind before he could stop, set up camp, properly evacuate his bowels, and clean up inside his pants.

"Every step would now be accompanied by sticky chafing," he wrote.

Making matters worse, he'd packed only a single pair of underwear for the 54-day expedition.

"I bring 220 pounds of food, but one pair of underwear," O'Brady had told Business Insider, before he took off in 2018. It was a calculated move, designed to prioritize food rations and gear over comfort. "I'll be filthy at the end!"

O'Brady is preparing for his next adventure, hiking Mount Everest with his wife, Jenna Besaw, this spring. Crystal Cox/Business Insider

He had no idea all the ways that would be true.

"It was all very humiliating," he wrote in his new book, a far more raw account than the sunny, upbeat daily Instagram posts he shared on the trek.

Thirty-eight days later, a much skinnier O'Brady made it to the other side, ending the journey with a stunning final 32-1/2-hour ultramarathon finish to the edge of the Ross Ice Shelf.

The first thing he did was not to find fresh pants. Instead, he celebrated the success with tears, a beaming smile, and a call home to his wife to say "we did it."