Here's what's in those weight-loss supplements

Research suggests that glutathione taken orally is not well absorbed across the gastrointestinal tract. In a study in which very large doses of oral glutathione were administered to humans, researchers concluded it was not possible to increase circulating glutathione to levels that could have any clinical benefit. Extracts of green tea and the increasingly popular acai berry, at least, may differ from glutathione in that there is evidence they are taken in through the gut and do make it into the bloodstream. But whether it helps to neutralize toxins or fats once there remains an open question.

Chitosan

The chemically cleansed product of grinding up the shells of shrimps, crabs and lobsters, chitosan is a powder that has been used in the water-purification industry for years: Sprinkled on top of holding tanks, it binds to lipids, or fats. Fats and oils can thus be skimmed off easily. This has led to the claim that supplements containing chitosan have amazing "fat magnet" qualities, absorbing dietary fat before it can be absorbed into the gut and flushing it away. In addition, chitosan is touted as a source of fiber, which may contribute to sensations of fullness when consumed with a meal.

But does chitosan act in the body in the same way it does in water-purification plants? Three human clinical trials found no difference in weight or serum cholesterol levels between subjects taking chitosan supplements and those taking a placebo after three-to-eight weeks. While chitosan is generally considered safe, it could be dangerous to anyone with shellfish allergies.

Hoodia

A succulent plant native to the Kalahari Desert in Africa, hoodia (Hoodia gordonii) is chewed and eaten by San Bushmen to reduce their hunger and thirst during long hunts. This back story has raised hoodia to mythic levels in the world of weight-loss supplement marketing.

In a September 2004 study published in Brain Research, scientists injected p57, thought to be the active ingredient in hoodia, into a region of rats' brains thought to govern appetite. They found reduced activity there.

In 1998, the drug maker Pfizer purchased the right to develop p57 for $21 million, but abandoned the quest for a hoodia drug in 2003. Unilever, which makes Slim-Fast meal-replacement products, recently dropped plans to fortify its products with hoodia. A former Pfizer scientist has warned that in extracting p57 from hoodia, researchers at Pfizer found that some components could not be removed that had "unwanted effects" on the liver. In an April 26, 2005, letter to the New York Times, scientist Jasjit S. Bindra warned that dieters "should be wary of using" hoodia until its safety has been better established.