One afternoon last month, I made a nervous visit to the office of Ghiora Aharoni, an Israeli sculptor and architect of some renown. The awkward part was that I hadn’t come to interview him about his work. I was there to hear about his gut. He had just finished a 21-day cleanse, the kind with supplements, protein shakes, and endorsements by the likes of Gwyneth Paltrow. (It’s called the Clean Program.) I’d been sent to Aharoni, who turned out to be extremely gracious, by a mutual friend, Ruby Namdar, an Israeli novelist whose skeptical intelligence and Falstaffian appetites made him the last person I expected to find on a celebrity diet. Indeed, the day I learned he was on it—over a dinner of baby carrots—Ruby was very hungry and very grumpy and at a loss to explain why he was doing this to himself, other than that Aharoni had talked Ruby into joining him and three other friends in the enterprise.

I wondered, too. What draws sophisticated and healthy people like Aharoni’s friends to commercial quasi-fasts? Cleanses, whether they last a day, a weekend, or three weeks, and whether they consist exclusively of fruit and vegetable juices or just a severe restriction of solids, are quickly becoming a part of what you might call the cosmopolitan diet, consumed in the more urbane sectors of New York and Los Angeles and Austin or wherever you find Whole Foods–levels of gastronomic consciousness and sufficient disposable income. (A three-week supply of Clean Program products costs $425.) Ask around, and you’ll probably find you know someone who knows someone who’s done a cleanse of one kind or another: Blueprint, Life Juice, Master Cleanse, Organic Avenue.

Ask a doctor about cleanses, though, and she’ll probably become enraged. I e-mailed Michael Gershon, a professor of pathology and cell biology at Columbia University’s medical school and the author of a groundbreaking work on the neurology of the gut called The Second Brain, and he wrote back: “I think that people who use cleanses may have had rough anal periods (see Freud, Sigmund).” Cleanses and their cousins, colonics, have about as much medical merit, declared Gershon, as the acts of penance done by monks who’d “walk across Europe and hit themselves on the back to purge themselves of the plague.”

The Clean Program is an ordeal. Its creator, Uruguayan cardiologist Alejandro Junger, told me that his customers undertake it to lose weight, prevent the inflammation that leads to chronic disease, and feel better. I asked Marion Nestle, the nutrition professor at New York University, whether a person would shed pounds on this diet. She had me read her the calorie count on the shake packets (150 each), then said, “It would be hard not to lose weight if you could stick to it. It sounds very depriving.” (Three weeks, she added, would be about the limit for that steep a reduction in caloric intake.) As for whether cleanses forestall illness, other doctors I talked to explained that that’s at best unproven.

But none of this matters, really, because people don’t afflict themselves for their health, or not only for that. I won’t be the first to point out that cleanses look a lot like religious fasts or that people crave the transcendence that comes from self-deprivation. “If you look at the history of humanity,” as Junger put it, “every spiritual giant or teacher or enlightened being has included some sort of fasting” as a form of discipline. Moses fasted for 40 days on Mount Sinai while writing down the Torah; Jesus fasted for 40 days in the desert; Muhammad also retreated to the desert to fast and received his first revelations from the Angel Jibril while fasting during Ramadan. “They didn’t call it detox at that time,” Junger added.