GNC explains that the company's fat burners are nutritional supplements that contain chemicals and herbs that stimulate thermogenesis and weight loss. Some of its brand-name fat-burning products are Nutrex Research, Inc. LIPO 6 Black Ultra Concentrate and HERS Ultra Concentrate, Cellucor D4 Thermal Shock and Grenade-Thermo Detonator. These products contain stimulants that are potentially harmful, especially if consumed in conjunction with thyroid medications, ephedrine compounds and certain herbs.
Most of the fat-burners sold by GNC include synephrine, a potentially dangerous stimulant, according to WebMD. Found in the bitter orange herb, synephrine has many of the same properties as ephedra, or ma huang, a substance banned by the Food and Drug Administration in 2004. The side-effects of these herbs, especially in people who have high blood pressure or a heart condition, include irregular heartbeat, heart attack, seizures, stroke and death. Several of the fat burners sold by GNC also include sources of caffeine, such as green tea extract, and phenylalanine, which Herbs 2000 describes as an amino acid with pain killing and stimulant effects. Taking these substances together may increase the risk of side effects. Furthermore, the Natural Medicines Comprehensive Database explains that there is insufficient evidence to rate products containing synephrine for weight loss.
According to the U.S. Centers for Disease Control and Prevention, over 34 percent of Americans are obese, and the overall health consequences of obesity are very severe. Nevertheless, the CDC recommends that Americans control their weight through a program of healthy eating, regular physical exercise and lifestyle modification rather than by relying on fad diets and unproven claims. Consult with a health care professional before using GNC fat burners, and follow all safety precautions on the label carefully.