Last November, Phil Libin stopped eating. The idea came to him over coffee with his friend Loïc Le Meur. A fellow entrepreneur, Le Meur was in the middle of a three-day fast, and he couldn’t stop talking it up. The health benefits! Weight loss! Longer life! It sounded insane. “I decided I would go home and research this fasting thing just so I could prove to him that he was being an idiot,” says Libin.

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But the more he Googled, the more the internet revealed that a growing number of people — and not just any old people, but scientists — thought there were benefits to the practice. “I was like, holy crap,” he remembers. “Unlike everything else, this is the first theory that felt complete to me.”

At the time, Libin was fat, and he thought this was a problem. His uniform startup t-shirt stretched taut against the bulge of his belly. He weighed around 258–260 pounds, nearly the heaviest he’d ever been.

Since his last year of high school at New York’s Bronx Science, Libin had been pudgy. When he was in his twenties, starting and building software companies, he didn’t care. “It was a spectacularly bad idea, but I just had decided that I’m going to totally not worry about my health until I’m 45,” he remembers. “But then as soon as I turn 45, then I’m going to take it really seriously.”

By last fall, Libin’s personal deadline was approaching. He was due to turn 45 in January. In recent years, he’d tried to address his weight through diets and exercise, and he’d often succeed in losing 20 or even 30 pounds, but then inevitably, the weight would creep back. He’d been to his doctor for testing and blood work; the doctor told him that if he kept things as they were, he’d have type two diabetes within five years.

Also, he wanted to start a company. It’d been more than a year since Libin had left the board of Evernote, the digital note-taking company he’d joined as CEO in 2007. As a partner with General Catalyst, he’d been investing for a while. As much as he enjoyed it, he was restless—but he wasn’t sure about becoming a CEO again. “I thought, I gotta at least prove to myself that I can govern myself, before I’m ready to govern a company.”

So, Libin tried not eating for a day. “I was, like, hungry, but it wasn’t a disaster,” he recalls.

The following week, Libin decided to try a 72-hour fast. The first day was hard. The second day was even harder. But on the third day, he felt amazing! “I got out of bed feeling better than I’d felt in 20 years,” he says. “I was in a very good mood, and felt really clear.”

Seven months later, Libin fasts regularly between two and eight days at a time. He has lost 81 pounds, and now his T-shirt hangs loosely beneath well-tailored sports jackets, his features assuming a new definition on his face. And while the weight loss may have been the original goal, Libin is addicted to the way he feels when he’s not eating. “My mood is radically and sustainably better than it was before. I would describe it as almost like a very mild state of euphoria,” he says. “At this point for the past couple months, I haven’t been fasting to lose weight. I’ve been fasting because I’ve craved the feeling of fasting.”