There's been one mantra on dieting since day one: If you want to lose weight, don't eat as much. These days, though, there's a corollary in the world of quickie weight-loss gimmicks: If you want to lose weight, don't eat at all. And some say to keep on not eating for a day or more.

But should people really fast for days as a way to lose weight? No, according to some medical experts. Fasting for a long period of time, they say, is at least an inappropriate way to diet, and possibly a dangerous one.

"Anything that sounds too drastic usually is and can create more harm than good," said Dr. Patrick O'Shaughnessy, chief medical officer at St. Catherine of Siena Medical Center in Smithtown. "I don't consider long-term fasting to be a safe option for weight loss.''

Yet, fasting diets remain popular and have been a mainstay among celebrities for years. In addition to weight loss, some diets promise to rid the body of chemicals by giving the digestive system a break, sometimes after putting it through the ringer with the help of laxatives.


Complicated approaches

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The Master Cleanse, for example, claims to detoxify the body and melt the pounds away by requiring dieters to eat no solid food for more than a week and, instead, drink a liquid preparation. The Men's Health 8-Hour Diet recommends alternating 16 hours of fasting (calorie-free drinks only) with eight hours of eating whatever you want. And the FastDiet, which has gained popularity in England recently, allows five days a week of unlimited eating and two nonconsecutive days of fasting (except for 500 calories for women and 600 for men each day).

Marlo Mittler, a registered dietitian with the Cohen Children's Medical Center in New Hyde Park, warns prospective dieters not to be swayed by fasts.

The problem with fasting over long periods, she said, is that it disrupts the body's metabolism, which rebounds after you start eating again. That can lead to intense hunger and the return of lost weight, she said.


Detox doesn't require fasting

Relying on liquids also can lead to dizziness, nausea and headaches, Mittler said. And fasts can be especially dangerous for certain people, including pregnant women, anyone with diabetes and people with gastrointestinal disorders such as celiac disease, she said.

Then there's the idea that a brief fast will rid your body of toxins. Mittler doesn't buy it. "There's not one bit of research that says you need to detoxify," she said. "Our body gets rid of toxins on its own."

O'Shaughnessy said he believes a short-term fast, perhaps 24 to 48 hours, probably won't cause serious side effects for someone who's healthy. But he warned that longer-term fasts can cause dehydration, confusion brought on by low blood sugar and imbalances in electrolyte levels.

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Anyone considering a fasting diet should first talk to a doctor. "Many primary care physicians have added expertise in weight management," O'Shaughnessy said, and they can help you make an informed decision.