Fat burners, as with many terms employed in sport science, is a term that has a different meaning depending on the audience to which it is directed. For nutritionists and those assisting with the development of athletic performance, a fat burner is a product directed at the utilization of stored fats and their conversion into free fatty acids in the production of energy to fuel the body. In the athletic research community, there has been considerable scientific interest in whether there are mechanisms by which the body can conserve carbohydrates and at the same time consume fats; the scientific literature refers to these substances as ergogenic aids, and the process is known as thermogenesis. In the weight-loss industry, a fat burner is a more general reference to the myriad of products said to stimulate the burning of fat generally. At a number of points, these definitions intersect.
An understanding of the theory behind fat burners is founded on both the general nature of the foods consumed, and the three energy systems that are used by the body at various times, depending on the nature of the physical activity in which it is engaged. Carbohydrates, proteins, and fats are the general nutritional subdivisions of the human diet. Carbohydrates are the primary source of body fuel, as they are converted through digestion into stored sugars, which become the primary mechanism by which the body is powered. Proteins are a rarely utilized energy source; proteins are primarily a muscle building and muscle maintenance source.
Fats represent a broad range of food compounds. Those in the form of fatty acids are essential to functions such as the utilization of a number of vitamins, and others that are superfluous or even harmful to healthy physical function. For a serious athlete, careful attention is paid when food is being selected and prepared that it contains little or no harmful fats. For the non-athlete consumer, eating foods with harmful fats is something of a given; the issue becomes how can such components be "burned," or eliminated.
Fats are an attractive energy resource, as they contain a greater amount of energy potential than do carbohydrates, and there is a greater amount of fat in the human body than that which is stored as carbohydrates at any given time; conversely, fats require a greater amount of oxygen to process energy. The trick is to find how and at what point during physical activity does the body utilize fats to produce the raw material, known as adenosine triphosphate (ATP), that is used for muscle energy?
As a general proposition, the amount of fat in the body that can be contributed to energy production will depend on the intensity of the activity in question. When the activity is low intensity—such as tai chi, gentle aerobics, or walking—almost all of the required energy will be produced through the available blood glucose. At moderate activity levels, such as those present in jogging at a rate of 7.5 minutes per mile, or cycling on level terrain at a speed of 15 mph (25 km/h), the required exertion on the part of the athlete is at 60-65% of their maximum ability to consume oxygen (VO2max); the utilization of fatty acids and muscle glucose for energy production is approximately equal.
At high activity levels, such as those found in a competitive distance running, the body energy will be drawn almost entirely from glucose. It is in these circumstances that the fat burners, or ergogenic aids, have been analyzed, as the greater the amount of fat that might be used, in theory the longer and more efficiently the endurance athlete could perform.
Many of the commercially marketed fat burners include a stimulant in their composition. Ma huang (ephedra), guarana, coffee, and the ingredient bitter orange all operate to either stimulate the ability of the body to conquer fatigue or to suppress appetite. In its former function, longer workouts will necessarily mean a greater expenditure of calories. In the latter, if athletes have had their appetite suppressed, they are unlikely to consume the same amount of fat in their diet. Given that the body's primary energy source is its stored carbohydrate energy, a supplement will not create a "magic bullet" whereby stored fat will be converted to glucose, bypassing the stores of this substance in the muscles and liver. Whenever the body has increased physical demands over its available stores, it will naturally draw upon available fats, converting the fat cells into fatty acids that may be utilized.
Athletes, subject to the demands of their individual sports, should not consume more that 30% fat in their diets; often 20-25% is preferred. Of the fats consumed, the preferred sources would be fish and plant sources such as olive oil and canola oil; fats from animal sources, which are saturated fats, should be kept to a minimum, as should the trans fats found in processed foods and commercially prepared baked products such as potato chips.