Vibes of enlightenment drift through the Woodstock Fruit Festival, a weeklong late-summer gathering of close to 400 hard-core dieters in upstate New York. Or maybe that's just the banana-and-Dumpster odor of durian, the ultra-stinky tropical fruit that's like caviar to this group: pricey, fatty, finger-lickable. Fruitarians, as they call themselves, subsist almost exclusively on raw fruit, with a few leafy greens, nuts, or seeds thrown in. They're roaming the grounds of Camp Walden, in the Adirondack woods near Lake George (like the 1969 music festival, this Woodstock event isn't in the town of Woodstock), with wooden bowls in their hands, chomping on lychees, melon slices, and lettuce leaves as they pinball from lectures ("My Friend the Coconut," "How to Have Healthy Teeth on a Raw Food Diet," "The Vitamin B12 Controversy") to yoga sessions to dance parties to, most important, the all-you-can-eat fruit buffet. The assortment is staggering—crates of oranges and grapefruits, exotic melons, plums, grapes, and coconuts heaped onto a makeshift row of tables; to make a pie with it, all you'd need is a baking dish the size of a swimming pool. The crowd is ponytailed, tie-dyed, barefoot, and, yes, enlightened.

All of which makes the festival's founder, Michael Arnstein, stand out that much more starkly. Arnstein is a short, wiry, kinetic 35-year-old with a glinting smile, Coppertone skin, and a well-ironed guayabera shirt. He works as a gem dealer in New York City and lives with his wife and three children in suburban Westchester County. Though a die-hard fruitarian for almost five years, Arnstein—who was a die-hard competitive runner first—isn't particularly interested in enlightenment. The ethical upsides to this extreme vegan diet have their warm and fuzzy appeal, but if eating a spit-roasted polar bear could help him achieve a 2:10 marathon time, he'd gladly chow down. "I'm doing this," he says, "because I want to win."

Fruitarianism has been knocking around for years—Steve Jobs dabbled in it in the 1970s, later citing that fruit focus as the reason he named his fledgling computer company Apple—but only in recent years has it become popular with performance-minded athletes like Arnstein. Credit for this often goes to a Florida chiropractor named Douglas Graham, whose self-published 2006 book, The 80/10/10 Diet, put a scientific-sounding gloss on what had been a fringe eating practice for the patchouli set. For Arnstein, who since high school had been experimenting with a wide range of dietary regimens to improve his running times, the book—a gift from his wife, who also adopted the lifestyle—instantly became a bible. Before long he was adding one, then two, then three extra refrigerators to his household to contain the massive quantities of fruit he buys wholesale off loading docks—think 500 pounds every 10 days. Over the course of a day, he consumes 25 to 30 pounds of fruit, usually a single type like oranges, bananas, or apples, with a salad of greens, cucumbers, tomatoes (a fruit, remember), and peppers at dinner. ("At Whole Foods," he says, "I could be spending $200 a day.") But the expense doesn't faze him. The social awkwardness that comes with a fruit-based diet proved difficult at first ("What do I eat for Thanksgiving?" he recalls thinking. "A huge watermelon?"), but Arnstein worked through that. "You don't go to restaurants anymore," he says, but when he's forced—say, for a business dinner—he steers the gathering toward a place with a salad bar. "Some guys go to go-go bars after business dinners," he says, "but I wouldn't do that. It's the same thing. Food is no different than other bad habits." And to silence those SAD (that's shorthand for Standard American Diet) dinner companions who demand to know "why, why, why" he won't partake of restaurant fare, Arnstein, he writes on his website, will tell them he's not hungry or just ate a really big meal—the same excuses often used by sufferers of eating disorders.

Within four weeks of adopting the diet, he says, his body-fat percentage dropped from 12 to 5 and he shed 12 or 13 pounds. He eventually cut 17 minutes off his previous personal-best marathon time of 2:45. (For comparison's sake, in 2011, the average marathon finishing time for men ages 35 to 39 was just over five hours.) Before his 2008 shift to fruitarianism, he'd never clocked an overall win at a marathon; since then, he's finished first in more than a half-dozen and was the sixth American to finish the 2011 New York City Marathon, with a time of 2:28. Last year he won the Vermont 100 Mile Trail Race, and he's placed in the top five at the Leadville Trail 100 Run. And with the steadfast zeal of a true believer, he claims to reap less quantifiable benefits as well: improvements in his hearing and eyesight, enhanced mental clarity, a total absence of injury and illness of any kind. "My shit doesn't stink," he adds. "That's a joke, but it's not. My shit really doesn't stink."