Michael "the Fruitarian" Arnstein applies temporary tattoos before the 100-mile 2011 Western States Endurance Run.

In 2009 Arnstein launched thefruitarian.com to promote the diet, and last year he founded the Woodstock Fruit Festival—partly because he "wanted to socialize with people who ate like me," but also because he wanted to "bring forth people who've eaten this way for decades." Attendance at this year's festival doubled from 2011, and Arnstein expects it to double again in 2013. "I really believe we'll have well over 1,500 people coming to our event by 2014—after that we'll have multiple-session events and probably some big produce sponsors," he says.

"I think it's the next thing," he says of fruitarianism. "I say that because it works. Fruit is the perfect food. It's got amino acids, it's got protein, it's the best and cleanest source of carbohydrates, and it has a little bit of fat. It's the Garden of Eden diet, it's the perfect end of the road for diets. This is it, man!"

While nutritional experts would never suggest that fruit is forbidden, they argue there can indeed be too much of a good thing. "The definition of a bad diet is one that excludes a major food group," says Dr. Felicia Stoler, a registered dietitian, an exercise physiologist, and the host of TLC's Honey We're Killing the Kids. By that measure, fruitarianism—which excludes several—is really bad. "You can't get all your nutrients from fruit," Stoler says flatly. Although fruit does contain a small amount of protein, according to Jeffrey Morrison, M.D., the author of Cleanse Your Body, Clear Your Mind, "they're not complete proteins. Fruit doesn't have branch-chain amino acids and glutamine, which are the main building blocks of muscle. This is an example of a great idea"—avoiding processed foods, upping one's fruit intake—"gone to excess."

Arnstein has heard charges like these before. "Doctors tell me that it's not going to work," he says, "but until this point, they've all been wrong." Arnstein, who supplements his 90-plus-percent-fruit diet with just vitamin B12, argues that "science has no bearing on understanding this. To run 100 miles continually tests the body beyond any normal measures. If I was protein-deficient, I would break down.

"People these days," he says, doctors included, "don't even know what a healthy person looks like."

But Susan Albers, a clinical psychologist who specializes in eating disorders and the author of Eating Mindfully, says "extreme" diets like this one need to be approached with caution. "When people cut out major food groups, it can often cause cravings," she says, which can provoke stressful cycles of bingeing and guilt. Another dietitian, Nancy Clark, whose specialty is nutrition for athletes, worries that the lifestyle upheavals required by the diet—fruitarianism can involve almost constant grazing—could overwhelm a person. Albers agrees. "If the diet becomes an obsession, or your life becomes inflexible because of it, it becomes very isolating," she says. "It becomes the center of your life."

Thomas Billings, a 59-year-old San Francisco computer consultant, had that precise experience during his nearly 10 years as a strict fruitarian in its hippie heyday of the 1970s. His descriptions of the early stages mirror Arnstein's: improved health and vigor and perceived mental clarity, bordering on euphoria. "I achieved the 'holy grail' of fruitarianism," he writes on his website, beyondveg.com. "I was on 100 percent raw fruit for around 2 years." For the remaining years, Billings' diet consisted of 75 percent raw fruit, which is widely considered the minimum for bona fide fruitarianism; vegetables, nuts, and seeds filled in the other 25 percent. But the diet took its toll, with emaciation, fatigue, muscle cramps, and gum disease creeping in. The problem? "Not enough nutrition," says Billings, whose weight dropped to 88 pounds at one point. Billings now follows a 60-to-80-percent-raw lacto-vegetarian diet, but he's become a critic of fruitarianism, which he now sees as a kind of cult. "People can replace religion with this philosophy," he says. "Dietary purity becomes an analogue to spiritual purity."