Fat Burning

Definition

Fat burning—also called fat oxidation or fat metabolism—is the complete breakdown or oxidation of fat molecules to generate energy for bodily activity.

Purpose

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.

Description

Fatty acids and triglycerides

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.

Digestion, storage, and mobilization

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.

Fat burning

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.

Exercise

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.

Demographics

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.

Preparation

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.

KEY TERMS
Acetyl CoA—
Acetyl-coenzyme A; a molecule with various functions in metabolism, including transporting carbon (acetyl) groups from the beta-oxidation of fatty acids to the citric acid cycle for further breakdown.
Adipocyte—
Fat cell.
Adipose tissue—
Tissue made of fat cells.
Albumin—
A protein in the blood that transports fatty acids to muscle cells.
ATP—
Adenosine triphosphate; a nucleotide with three phosphate groups that supplies cellular energy by undergoing enzymatic-facilitated hydrolysis to adenosine diphosphate (ADP) and adenosine monophosphate (AMP).
Beta-oxidation—
The step-wise breakdown of fatty acids via the removal of two-carbon fragments.
Calorie—
A unit of food energy.
Carbohydrate—
Sugars, starches, celluloses, and gums that are a major source of calories from food.
Carnitine—
A substance that transports fatty acids into the mitochondria of muscle cells.
Chylomicron—
A lipoprotein that is high in triglycerides and is common in the blood during fat digestion and assimilation.
Enzyme—
A protein that promotes a biochemical reaction.
Epinephrine—
Adrenaline; a hormone that both stimulates and inhibits lipolysis.
Estrogen—
A female sex hormone.
Free fatty acids (FFA)—
A large family of lipids that are oxidized as fuel and consist of chains of carbon atoms of various lengths attached to an acid.
Growth hormone (GH)—
A polypeptide hormone, secreted by the pituitary gland, that regulates growth and fat burning, among other functions.
Hormone—
A substance, such as a protein, that is produced in one part of the body and travels through the bloodstream to affect another part of the body.
Hormone sensitive lipase (HSL)—
An enzyme that releases free fatty acids from triglycerides in response to the hormone epinephrine.
Interleukin-6 (IL-6)—
A small molecule produced by various cell types that has a variety of functions, including metabolic regulation during exercise.
Intramuscular triglycerides (IMTG)—
The form in which fatty acids are stored in muscle cells.
Kilocalorie (kcal)—
A unit of energy equivalent to a calorie.
Lipids—
Fats, including fatty acids for cellular energy, phospholipids for cell membranes, and steroids such as cholesterol.
Lipolysis—
The process of breaking down fats.
Lipoprotein—
A large class of protein-lipid complexes.
Lipoprotein lipase (LPL)—
An enzyme that breaks down triglycerides in lipoproteins to monoglycerides and free fatty acids.
Mitochondria—
The organelles in cells that produce energy.
Monounsaturated fat—
A fat that contains one double bond per molecule, such as in canola and olive oils.
Polyunsaturated fat—
A fat that contains two or more double per molecule, such as fats from fish and vegetable oils.
Protein—
Chains of amino acids that form the structural components of cells, as well as enzymes, hormones, and antibodies, and which can serve as energy sources.
Receptor—
A molecule, such as a protein, inside or on the surface of a cell, that binds a specific messenger molecule.
Saturated fat—
Hydrogenated fat; fat molecules that contain only single bonds, especially animal fats.
Triglycerides (TG)—
Lipids formed from glycerol and three fatty acids, which circulate in the blood as lipoproteins and are burned as fuel for energy.

Risks

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.

QUESTIONS TO ASK YOUR DOCTOR

Results

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.

Research and general acceptance

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 .

Resources

BOOKS

Berg, Eric. The 7 Principles of Fat Burning: Get Healthy, Lose Weight and Keep It Off! Alexandria, VA: KB, 2010.

Geissler, Catherine, and Hilary J. Powers. Human Nutrition, 12th ed. New York: Churchill Livingstone, 2011.

Harpaz, Mickey, and Robert Wolff. Menopause Reset!: Reverse Weight Gain, Speed Fat Loss, and Get Your Body Back in 3 Simple Steps. Emmaus: Rodale, 2012.

Laferriere, Rachel. 365 Ways to Boost Your Metabolism: Everyday Tips to Achieve Your Maximum Fat-Burning Potential. Avon: Adams Media, 2010.

Lopez, Mario, and Jimmy Pena. Extra Lean: The Fat-Burning Plan that Changes the Way You Eat for Life. New York: New American Library, 2010.

Venuto, Tom. The Body Fat Solution: Five Principles for Burning Fat, Building Lean Muscles, Ending Emotional Eating, and Maintaining Your Perfect Weight. New York: Avery, 2009.

Williams, Melvin H. Nutrition for Health, Fitness & Sport, 10th ed. Boston: McGraw-Hill, 2012.

PERIODICALS

Carey, D.G. “Quantifying Differences in the ‘Fat Burning’ Zone and the Aerobic Zone: Implications for Training.” Journal of Strength & Conditioning Research 23, no. 7 (2009): 2090–95.

Cooper, J.A., et al. “Influence of Dietary Fatty Acid Composition and Exercise on Changes in Fat Oxidation from a High-Fat Diet.” Journal of Applied Physiology 109, no. 4 (October 2010): 1011–18.

“The 14 Best Fat-Burning Foods.” Muscle & Fitness/Hers 12, no. 4 (July/August 2011): 44.

Grothe, Jenny. “Eat for Your Goal Fat Burning vs. Lean Muscle Building: Six Great-Tasting Options to Help You Burn Fat and Gain Lean Muscle Any Time of Day.” Muscle & Fitness/Hers 12, no. 4 (July/August 2011): 60–64.

Melanson, E.L., P.S. MacLean, and J.O. Hill. “Exercise Improves Fat Metabolism in Muscle but does not Increase 24-h Fat Oxidation.” Exercise and Sport Sciences Reviews 37, no. 2 (April 2009): 93–101.

Schoenfeld, Brad. “Does Cardio After an Overnight Fast Maximize Fat Loss?” Strength and Conditioning Journal 33, no. 1 (February 2011): 23–25.

The Single Best: Fat-Burning Secret.” Health 25, no. 2 (March 2011): 16.

Van Proeyen, Karen, et al. “Beneficial Metabolic Adaptations Due to Endurance Exercise Training in the Fasted State.” Journal of Applied Physiology 110, no. 1 (January 2011): 236–45.

WEBSITES

Doheny, Kathleen. “5 Fat-Burning Strategies.” WebMD Feature. February 6, 2009. http://www.webmd.com/diet/features/5-fat-burning-strategies#1 (accessed January 18, 2017).

Vella, Chantal, and Len Kravitz. “Gender Differences in Fat Metabolism.” University of New Mexico. http://www.unm.edu/~lkravitz/Article%20folder/genderdifferences.html (accessed January 18, 2017).

ORGANIZATIONS

American Heart Association, 7272 Greenville Ave., Dallas, TX, 75231, USA, (800) AHA-USA-1(242-8721), http://www.heart.org .

Margaret Alic, PhD

  This information is not a tool for self-diagnosis or a substitute for professional care.