Diet pills often contain substances, such as green tea, guarana and yerba mate that claim to have “natural” stimulant properties. But what you’re really getting is caffeine or caffeine-related substances. Caffeine may temporarily boost calorie burning, but this does not mean you will lose much weight. The effects are small and temporary and caffeine tolerance tends to develop with regular use. Moreover, if a diet pill does boost metabolism, then it can also boost heart rate and blood pressure and have other potentially undesirable effects.
Some research suggests that capsaicin and related substances in chili peppers can briefly speed metabolism, increase fat burning and suppress hunger. However, the studies tend to be small, short and poorly designed and have had inconsistent results. There’s no evidence that such supplements lead to significant and lasting weight loss.
Studies on conjugated linoleic acid (CLA), a fatty acid in dairy and meat, have yielded mixed results. A 2010 report from Health Canada (which acts like our FDA) said there is insufficient evidence that CLA supplements help with weight loss. Potential adverse effects include increased inflammation, decreased HDL (“good”) cholesterol and reduced insulin sensitivity. CLA has been shown to cause liver damage in animal studies. It's unclear which formulations (there are many) are riskier.
This fiber-like substance, derived from crustacean shells, is said to block fat absorption in the intestines. Although there is some evidence that chitosan may help in the short term, studies have had mixed results and most are poor quality. Better-quality trials have found no clinically meaningful effect on weight. Plus, there are potential risks. If Chitosan does block some fat, it may also reduce absorption of some fat-soluble vitamins (A,D,E,K), as well as calcium, iron and magnesium. People with shellfish allergies should not take chitosan.
Products that contain extracts of foods such as broccoli, spinach, blueberries, garlic, sprouts, tomatoes and soybean have been touted by some companies to help you lose “10 pounds in 10 days.” These ingredients have no known weight-loss benefits and are typically present in very small amounts. Papaya and pineapple are sometimes included, too, presumably for their digestive enzymes, though these have nothing to do with weight loss either.
Besides caffeine, green tea contains polyphenols called catechins, which may promote calorie and fat burning. A review found that catechins decreased weight and helped keep it off. But not all studies confirm the benefits. The effects of green tea extracts may vary from person to person, and if there is weight loss, it’s likely to be small at best.
Cascara, senna, rhubarb root, buckthorn and aloe (often in “dieter’s teas”) are promoted for quick weight loss, under the mistaken notion that they expel undigested food and thus calories. In fact, these ingredients simply stimulate your bowels and have little or no effect on absorption of food. Chronic use may lead to dependency, decreased bowel function, dehydration and electrolyte imbalance; some deaths have been reported from laxative abuse.