Fat burning—also called fat oxidation or fat metabolism—is the complete breakdown or oxidation of fat molecules to generate energy for bodily activity.
Energy for the human body is provided by the oxidation—or using oxygen to break bonds between atoms—in carbohydrates, fats, and proteins from food. Energy for future use is stored primarily as fat. The energy is measured in kilocalories (kcal), which are equivalent to the calories listed on food labels. Although different types of carbohydrates, fats, and proteins supply varying amounts of energy, on average, fat provides about twice the number of kilocalories of energy per gram (g) as carbohydrates and protein: 9.5 kcal/g for fat, compared with 4.2 kcal/g for carbohydrates and 4.3 kcal/g for protein.
Fatty acids are the major type of lipids (fats) that are burned for fuel. The other important lipids in the body are phospholipids, which form membranes and function as regulatory signals, and steroids, such as cholesterol and hormones. Fatty acids come from food and from fat stores in the body. They can also be made from carbohydrates and from some amino acids in protein.
Fatty acids are chains of carbon atoms attached to an acid. The chains differ in length and in the number of bonds between the carbon atoms. In saturated or hydrogenated fatty acids, the carbon atoms are all joined by single bonds. Most animal fats—including those in eggs, butter, whole milk, cheese, and coconut oil—are saturated. In unsaturated fatty acids, two or more of the carbon atoms are joined by double bonds. These are referred to as monounsaturated and polyunsaturated fatty acids, respectively. Oleic acid in olive oil is a monounsaturated fatty acid. Fats from chicken, fish, and vegetables tend to be polyunsaturated. Saturated and unsaturated fats are nearly equal in caloric value.
About 98% of total lipids in the diet are fatty acids in the form of triacylglycerols or triglycerides (TG), which consist of three fatty acids joined by a glycerol molecule. About 95% of digested fat from food is stored as TG in fat cells called adipocytes, to be burned for energy as needed. Typically about 50,000–60,000 kilocalories of energy are stored as TG in adipocytes throughout the body. Another 2,000–3,000 kcal are stored in skeletal muscle cells as intramuscular triglycerides (IMTG). Some TG also circulate in the blood as lipoproteins, including particles called chylomicrons.
TG from food are primarily digested by enzymes in the small intestine to monoglycerides and free fatty acids (FFA). These are coated with bile acids to form globules called micelles, which diffuse into intestinal cells where they are converted back into TG and then into chylomicrons that travel through the bloodstream. An enzyme called lipoprotein lipase (LPL) on blood vessel walls breaks down TG in chylomicrons and other lipoproteins into monoglycerides and FFA, to use as fuel in active tissues and to convert back to TG in adipocytes and liver cells for storage. Thus, LPL controls the distribution of storage fat throughout the body.
FFA are mobilized from storage sites for fat burning under the control of the hormone epinephrine (adrenaline), which is released during exercise. Epinephrine binds to two types of receptors on fat cells—those that stimulate and those that inhibit an enzyme called hormone-sensitive lipase (HSL). In a process called lipolysis, HSL breaks apart TG in adipose tissue, releasing FFA into the blood. HSL is more responsive to epinephrine stimulation during aerobic exercise, thereby mobilizing more fatty acids for fuel. Furthermore, physical training enhances HSL sensitivity to epinephrine stimulation, so that HSL is activated at lower epinephrine concentrations. However, fat cells in some parts of the body have more inhibitory receptors, suggesting that it may be harder to mobilize FFA from these areas. Obesity decreases HSL responsiveness to epinephrine stimulation, so that higher epinephrine concentrations are required to mobilize fat stores.
Epinephrine released during exercise also activates HSL to initiate the release of FFA from IMTG stored in muscle cells. The released FFA are transported directly to the mitochondria for oxidation. Carnitine is responsible for transporting FFA into the mitochondria. Carnitine also plays a role in removing toxic by-products of fat burning from the mitochondria. Therefore, carnitine is concentrated in muscle tissues that utilize FFA for fuel.
Actual fat burning takes place in the mitochondria, in a cyclical process called beta-oxidation. With each cycle of beta-oxidation, two carbons are removed from the end of the fatty acid carbon chain in the form of a molecule called acetyl-coenzyme A (acetyl-CoA). Each cycle generates five molecules of ATP (adenosine triphosphate). The high-energy bonds in ATP provide the energy required for cellular metabolism and muscular contraction. The acetyl-CoA formed by beta-oxidation enters the citric acid cycle, where it is further broken down to carbon dioxide and water, with the generation of additional ATP.
Many factors, in addition to epinephrine, control fat burning. Growth hormone (GH) increases the mobilization of FFA from fat tissue. It also inhibits the uptake of glucose (carbohydrate) by active tissues, so that fat is burned instead. The hormone estrogen may increase fat metabolism, both during exercise and while at rest, by several mechanisms, including the stimulation of GH production. Some evidence suggests that interleukin-6 (IL-6), which is elevated during exercise, may selectively stimulate lipolysis in skeletal muscle without affecting adipose tissue.
The amount of adipose fat that is burned while at rest is related to the size of the fat cells, with larger cells having higher lipolytic activity. Fat burning during exercise depends, in part, on the intensity and duration of the exercise. During low-intensity exercise, the majority of kilocalories come from fat and the proportion of fat to carbohydrate burned generally increases over the course of an exercise session and with increasing intensity, up to a point. At high exercise intensity, the percentage of energy from fat decreases, although the total amount of energy from fat increases as total energy expenditure increases. As exercise duration increases, carbohydrates become depleted, and the muscles resume fat burning.
For the general population, maximum fat burning rates occur at exercise intensities of about 47%–52% of maximum oxygen consumption. In trained athletes, maximum fat burning occurs at about 59%–64% of maximum oxygen consumption. However, there is a great deal of variation among individuals and some people may reach maximum fat burning only at much higher exercise intensities. Fat burning also varies with the type of exercise; for example, more fat is burned while running than while cycling.
It has been estimated that as much as 50% of fat burned during moderate to intense exercise comes from IMTG. Muscle contraction appears to stimulate IMTG metabolism. Most of the remaining burned fat comes from adipose tissue, with a small amount from TG in the blood.
Dietary factors appear to influence fat metabolism in complex ways. Fat burning typically increases when food consumption decreases or exercise levels increase, but fat burning is not related to the amount of fat in the diet. In contrast, increased carbohydrate consumption increases the percentage of carbohydrates that are burned as fuel. This decreases the proportion of fat burned, since the body prefers to burn carbohydrates rather than convert the carbohydrates to storage fat.
Although there are individual variations in fat burning, the most significant differences are between males and females. Although there are no apparent gender differences in fat metabolism while the body is at rest, during low- and moderate-intensity exercise, women derive a higher proportion of their energy from fat than do men. This may be a result of hormonal differences and gender differences in the percentage and distribution of body fat. Women generally have a higher proportion of body fat than men and are more likely to have a gynoid or “pear-shaped” body type, with fat stores in the hips and thighs. Men are more likely to have an android or “apple-shaped” body type, with more fat deposition in the abdominal area. Obesity also affects fat burning and alterations in fat metabolism are known to contribute to obesity.
Exercising after fasting longer than six hours—before breakfast, for example—appears to optimize fat burning. Ingestion of carbohydrates prior to exercise significantly decreases the rate of fat burning, although slowly digested carbohydrates, such as oatmeal, may increase fat burning. Fat oxidation rates appear to decrease following high-fat meals.
Large amounts of saturated fats in the diet increase the risks for atherosclerosis, heart disease, and stroke. Daily fat intake should be limited to no more than 30% of total calories. Athletes and other highly active individuals are advised to limit fat to 25% of total calories. Only one-third of dietary fats should be saturated, with another one-third monounsaturated and one-third polyunsaturated. Olive oil, nuts, peanut butter, and avocados are good sources of monounsaturated fats.
The only way to lose body fat is with a calorie deficit—burning more calories than are consumed in beverages and food. It is the number of calories burned—not the type of fuel—that ultimately results in fat loss. Although exercising on an empty stomach burns more fat, it will not result in overall fat loss unless total calorie intake is reduced. A well-balanced diet of whole grains, lean protein, fruits, vegetables, and water, along with regular exercise, remains the most reliable fat-burning program.
Exercise machines or workouts geared toward low-intensity “fat-burning zones” are unlikely to increase fat loss. Since fat loss ultimately depends on the number of calories burned in excess of calories consumed, boosting exercise intensity may be as or more effective than attempting to burn fat with lowintensity exercise.
Despite decades of research, the complex effects of exercise on fat burning are not well understood. For example, studies suggest that although exercise improves the fat-burning capacity of muscle, exercise of moderate duration (60 minutes or less) appears to have little effect on fat burning over a 24-hour period. Likewise, dietary fatty acid composition does not appear to affect 24-hour fat burning.
See also Carbohydrates ; Exercise ; Fat .
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Margaret Alic, PhD