A cookie a day keeps the pounds away?

Cookies? On a diet? Apparently so.

Just ask Christina Kane, who has tried everything from the grapefruit diet to Atkins, with no success. Then she heard about Dr. Siegal's Cookie Diet, which involves eating six prepackaged cookies a day, plus one "real" meal -- say, skinless chicken and steamed vegetables.

"I thought, 'That diet looks so incredibly easy,'" said Kane, 43, a legal secretary in Washington, who started paying $56 a week for the prepackaged cookies in June, when she weighed 255 pounds. Three months later, she was 40 pounds lighter.

"If you can make it through the first week, you're in the clear," she said.

Kane is one of an estimated 500,000 people who have lost weight on Dr. Sanford Siegal's diet -- at least according to Siegal. The gist of it is simple: Eat cookies, and lose up to 10 pounds a month.

Or, in blunter terms: Consume a substance whose ingredients and nutritional value are somewhat vague and drop weight, because how can you not when you're consuming only 800 to 1,000 calories a day?

Siegal's diet isn't new; it was created in 1975, but for years was available only to patients in his Miami medical practice and at other doctors' practices that he supplied with cookies.

That changed in 2006 when he started CookieDiet.com. This year he began selling his cookies at Walgreens and GNC, and opened his first Cookie Diet store in Beverly Hills, Calif. He expects 2009 revenues to be $18 million, up from $12 million in 2008, thanks in part to endorsements by celebrities such as Kim Kardashian, Jennifer Hudson and Kelly Clarkson.

In fact, the cookie diet business has proved so lucrative that other companies have popped up: Smart for Life (six 105-calorie cookies a day; a 35-day kit costs $279); the Hollywood Cookie Diet (one 150-calorie cookie three to four times a day, plus a light dinner; $14 to $20 a box); and Soypal Cookies, marketed as "the most popular diet in Japan" (about 22 calories each; $49 a box).

The popularity of cookie diets is hardly surprising in this culture of quick fixes. Who wouldn't want to exert the minimal effort to get long-lasting results? Who wouldn't want to lose weight by consuming something verboten on most diets?

"The Cookie Diet is very appealing, because it legalizes a food -- the cookie -- that is banned from most weight-loss programs," said Jenni Schaefer, author of "Goodbye Ed, Hello Me."

"The diet gives people a false sense of control, simplifying balanced nutrition into one food: the cookie," she added.

"It's unfortunate that they're called cookies, because in some ways it denigrates them," said John Nemet, manager at the Smart for Life store in Westbury, N.Y., one of about 35 stores in the United States.

Smart for Life -- which is also run by a doctor, Sasson Moulavi, who is based in Boca Raton, Fla. -- offers an 800-calorie-a-day and a 1,200-calorie-a-day plan. It also sells shakes, muffins, soups, cereals and salad dressings, although cookies remain the staple.

"We think of our plan as a behavior-modification program, not a diet," Nemet said.

Critics of cookie diets are not convinced. Weight-loss plans that center on a diet of below 1,000 calories do not, they say, lead to long-lasting weight loss and can result in potassium deficiency, gallstones, heart palpitations, weakened kidney function and dizziness. The cookie diet particularly concerns eating-disorder activists, who have long criticized fad diets, such as the grapefruit diet, Master Cleanse and Optifast shakes.

"Generally speaking, fad diets misinform the public and fuel a fire of continued curiosity with this dieting mentality, which we know gets us nowhere," said Dr. Ovidio Bermudez, medical director of Laureate Eating Disorders Program in Tulsa, Okla. "They tend to promise a huge return for very little investment. We need to be very aware of the fact that whenever we skew our eating in any direction, the chances are that we're going to hinder our health and not enhance it."

Despite such criticism, the cookie diet thrives. Richard Kayne, chief operations officer at Smart for Life, said he expected $82 million to $95 million in gross revenue this year, up from about $30 million last year. Larry Turner, the president of Sunset Health Products, estimates that its Hollywood Cookie Diet program has grown more than 50 percent a year since its creation three years ago.