When Shanna Nash was trying to give up smoking two years ago, she was concerned about weight gain. But instead of chewing gum or swearing off sweets, she decided to go the route of a friend who lost 15 pounds on the "master cleanse."

For up to 10 days, fasters drink as much as three liters a day of a concoction of freshly squeezed lemon juice, water, maple syrup and cayenne pepper with the idea that it will flush their body of toxins and excess pounds -- quickly.

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Nash, a jewelry designer from Brooklyn, N.Y., says on the first day or two she experienced intense food cravings, light-headedness and couldn't stray too far from a toilet. On the fifth day, something clicked. She felt energized and was noticeably losing weight. A few more days in, though, the all-liquid diet began to take its toll. Increasingly irritable, she couldn't stop staring at people while they ate.

It's not hard to see why extreme detoxifying diets like the lemon juice diet are so popular. People, including Beyoncé, who told Oprah Winfrey she lost 20 pounds on the cleanse for her Dreamgirls role, want a slimmer body so badly they'll do just about anything.

This is nothing new: Body cleansing and detoxifying formulas have been around at least since the 19th century, when European spas promoted the idea of ridding the body of disease-causing poisons. The master cleanse was popularized in the 1970s with the publication of the book "The Master Cleanser" by Stanley Burroughs. However, spas and citrus elixirs cannot rid the body of toxins.

"We have organs that naturally detoxify us," says Dawn Jackson Blatner, a registered dietitian with the American Diabetic Association, pointing to the liver and kidneys, which filter toxins from the blood, as well as the spleen, skin, lungs and digestive system. "There really isn't a need for special potions to help that along."

What detoxifying diets like the master cleanse can assist with is almost certain weight loss. At about 115 calories per serving with a minimum of six servings per day, cleanse dieters consume about 690 calories daily, compared with the 1,800 daily calories most women need to maintain their weight.

Notably, however, most of the loss is water weight; within 24 to 48 hours of resuming a regular diet, many people will likely gain back two to three pounds, says Blatner.

Another popular detox method used for weight loss is a colonic irrigation, a process in which the gastrointestinal tract is flushed out with water. Colonics are most commonly performed in clinics by licensed practitioners though some may try at-home versions.

Popular belief is that toxins accumulated in the digestive tract are cleared out in the process. "You cleanse the colon and remove this nasty, brown, smelly stuff, and it looks like the colonic is working," says David Levitsky, Ph.D., professor of nutritional sciences and psychology at Cornell University. But the idea that toxins are building up there, perhaps for years, is a myth, Levitsky says.

In addition, a flush disturbs the colon environment by removing helpful bacteria that aid in digestion and contribute to a healthy immune system. "The colon is an ecologically balanced environment like the rain forest," says Dr. Don C. Rockey, chief of digestive and liver diseases at UT Southwestern Medical Center. "Disrupting that -- like chopping away all the trees -- is not a great idea."

While detox diets might not actually help your body remove toxins, some can be healthy. A raw food regime, for example, is largely made up of vegetables and grains, which are high in nutrients and fiber. However, it is simplistic to divide cooked and raw foods into a nutritional black and white. While some foods do lose vitamins when cooked, the amount is insignificant, says Blatner, pointing out that some foods, like tomatoes, become more nutritious when cooked.

It is true, however, that foods or drinks that contain alcohol, red meat and gluten, for example, could sensibly be avoided in the name of better health and weight loss, as Oprah Winfrey did during her "21-day cleanse" using Kathy Freston's Quantum Wellness plan. Vegan-like diets such as this one tend to be high in fiber and low in saturated fat.