Of the 12 dietary supplements included in the review, only two -- ephedra and chromium picolinate -- proved to be more effective than placebo.

According to Pittler, the most rigorous review showed ephedra use to be associated with modest, short-term weight loss. But the supplement is also known to speed the heart rate and constrict blood vessels, and heart palpitations were common among the participants in the ephedra studies.

Earlier this year, the FDA issued a ruling prohibiting the sale of dietary supplements containing ephedra because it presents an unreasonable risk of illness or injury.

The ruling, which requires ephedra to be removed from all unregulated products sold in the U.S., will become effective by the end of this month. It is the first time the agency has banned a dietary supplement.

The other supplement shown to be modestly effective for weight loss was the trace mineral chromium picolinate, which is touted as enhancing insulin's activity and reducing body fat. But the researchers concluded that the weight loss effect of the supplement was minimal.

The review was published in the April issue of the American Journal of Clinical Nutrition.

Other SupplementsOther supplements found to have little or no effect on weight loss included:

Chitosan -- A marine-derived starch found in the skeletons of shrimp, crab, and other shellfish. The supplement is used in products promising to keep dietary fats from being absorbed by the body. But the researchers concluded that "the evidence available in the literature indicates that there is considerable doubt that chitosan is effective for reducing body weight."

Garcinia cambogia -- One of the main ingredients in one of the best-selling supplemental weight loss aids, Hydroxycut. The researchers concluded that "the evidence for G. cambogia is not compelling."

Pyruvate -- Formed by the body during breakdown of carbohydrates, it's touted as a booster of metabolism and an appetite suppressant, but Pittler and Ernst reported that "the case of pyruvate as an aid to body-composition changes and weight loss is weak."