Americans spend billions of dollars each year on weight loss supplements. Although we wouldn't buy a car or a dinner without knowing some details, many who purchase these supplements have no idea what is in them, or what independent reports say about their safety and effectiveness.

Even though the Food and Drug Administration has charged some companies with using banned ingredients, and the Federal Trade Commission has successfully prosecuted marketers who made illegal claims, the same products, or copycat versions, still remain available.

Some weight loss supplements contain ingredients meant to suppress the appetite. Caffeine and its herbal counterparts, guarana, bitter orange and yerba mat�, as well as ephedra (ma huang), fall into this category.

Animal studies suggest that they may slightly suppress the appetite, but the limited human studies lasted only a few months. All these ingredients are nervous system stimulants, so they commonly produce side effects like headaches, insomnia, elevated blood pressure and heart palpitations.

Product labeling can be deceptive: Weight-loss supplements labeled ephedra- or caffeine-free may contain other ingredients from the list above, which may pose the same health risks. Although ephedra (ma huang) has been banned by the Food and Drug Administration (FDA), you may still see it in some supplements.

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In a summary of more than 50 trials, this substance created a 2 to 3.6-fold increase in the risk of psychological, heart and digestive system problems.

Supplement manufacturers claim other ingredients aid weight loss by speeding up the metabolism. For example, EGCG, a phytochemical found in green tea, is being studied for its potential to reduce cancer risk. Some initial studies suggest it could slightly increase the rate at which calories burn. Now it can be found in many weight loss supplements and "weight-loss vitamins."

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However, since there are only extremely short initial studies so far, the body might adapt to EGCG, reducing its effect over time. Second, the weight loss benefit seen with EGCG amounts to about 60 to 70 calories a day.

This small difference is more likely to help prevent a gradual yearly weight gain than reverse excessive weight gain. Third, the effect of EGCG appears to depend on the dose. Supplements with amounts of 30 to 40 milligrams (mg) of EGCG, which is commonly seen in these products, may not have the same effect as a dose over 250 mg used in the studies.

Supplements may also contain ingredients that manufacturers state will block the absorption of fat or carbohydrates. Chitosan is a common example, and a few preliminary studies made it appear promising. However, several controlled studies found that chitosan had no significant effect on fat absorption. In the most recent study, men would need seven months to lose one pound of body fat. There was no fat loss for women.

Another group of ingredients are said to increase the feeling of fullness and decrease eating. Guar gum appears safe for this purpose, but 11 well-controlled studies show it has no benefit for weight loss. Psyllium can help control blood sugar and blood cholesterol, but studies do not support its reputed ability to reduce eating and assist weight loss.

There are more than 50 individual supplements and 125 combination products now available for people who want to lose weight. Yet a Harvard Medical School review of these products that set standards for product quality, safety and effectiveness concluded that none of them met all three standards.

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Future research may identify some safe and effective ingredients for weight loss, but for now it seems smarter to invest in walking shoes, a gym membership, or healthier food instead.