Energy Metabolism


Anaerobic Metabolism

Oxygen Not Required

Cannot continue indefinitely

Only carbohydrate used

High-intensity activities

How is Food Energy Transformed in the Body?    

The chemical energy in your breakfast cereal cannot be directly used by your body's cells.  First, a compound called adenosine triphosphate (ATP) must be formed.  ATP is a nucleotide composed of adenosine (made from adenine and ribose) and three phosphates (Pi):

Adenosine-Phosphate (Pi)        Phospaate (Pi)       Phosphate (Pi)

The symbol       indicates that the endmost phosphates are connected by high-energy bonds, which is where ATP holds the energy from nutrients.  Later, ATP releases this energy to produce heat and to meet the needs of the body's cells -for digesting food, moving muscles, transmitting signals along nerve cells, as well as a thousand other biological activities.  The sum of all the energy transformation processes in the human body is known as metabolism.

The energy from food is used continually to produce more ATP.  this may occur anaerobically or aerobically.  Anaerobic metabolic processes take place in the cytoplasm, or fluid portion, of each cell and do not require or use oxygen.  Aerobic metabolism occurs in the cell mitochondria - specialized structures sometimes referred to as the powerhouse of the cell - and oxygen is required.  Because humans require oxygen to sustain life, it would seem that all metabolic processes must be aerobic; however, some metabolic processes use anaerobic metabolism.  Carbohydrates are particularly versatile because they can be broken down either aerobically or anaerobically.  

What is the Role of Carbohydrates in Energy Metabolism?

Carbohydrates are vitally important in sustaining the body's metabolic machinery.  Not only are they unique in their ability to generate ATP aerobically or anaerobically, but they also play a critical role in the oxidation of lipids.  The expression "fats burn in a carbohydrate flame" refers to the essential contribution of carbohydrates in keeping aerobic energy systems running so that fats may be broken down completely.  Incomplete breakdown of lipids results in accumulation of ketone bodies, which can have serious health complications.

What are Ketones? 

Ketones are the result of the body burning fat for energy or fuel.  For a person with diabetes, ketones are often the result of prolonged high blood sugar and insulin deficiency.  Without the right about of insulin, glucose starts to build up in the blood stream and doesn't enter the cells.

After dietary carbohydrates are digested and absorbed, they are reduced to glucose, the form in which carbohydrate circulates throughout the body and is used by the cells.  Glycolysis is the breakdown of glucose.  It occurs very rapidly in the cytoplasm of the cell whether or not oxygen is present.  Glycolysis is a 10-step process  that ends with the production of pyruvate.  

What is Pyruvate?

Pyruvate forms in the body when carbohydrates and protein convert into energy.  Several foods, including red apples, cheese, dark beer, and red wine, contain small amounts of pyruvate.  Pyruvate provides energy to the body and is also an antioxidant.

During anaerobic metabolism, when the body's energy demands are greater than the rate at which energy can be generated aerobically, lactic acid is formed from pyruvate.

Anaerobic glycolysis cannot continue indefinitely, in part because the production of lactic acid inhibits a key enzyme (phosphofructokinase) and ultimately slows metabolism.  In addition, lactic acid accumulation causes pain or burning in the working muscles, which slows down all but the hardiest among us.  Finally, anaerobic glycolysis releases only about 5% of the potential energy from glucose, so its ATP-generating capacity is limited.  Fortunately, the energy that remains in glucose can be extracted via an aerobic process called the citric acid cycle.

What is the Citric Acid Cycle?

Aerobic metabolic process that fully breaks down carbohydrate, fat, or protein to generate ATP, form carbon dioxide and water, and remove hydrogen atoms for the electron transport system.

 Nutritionists and exercise physiologists frequently note that "fats burn in a carbohydrate flame."  This observation refers to the crucial role of carbohydrate in keeping the citric acid cycle operating.  When carbohydrates are insufficient (eg in anorexia nervosa, fasting, and type 1 diabetes), oxaloacetate may be used to form glucose for cells that require or prefer glucose as a fuel.  The result is insufficient oxaloacetate to combine with acetyl CoA to form citrate.  So, although fats are a tremendous source of potential energy, they require carbohydrates to release their energy most efficiently.

What is the Role of Protein in Energy Metabolism:

The proteins that make up our bodies include muscle fibers, blood components, hormones, enzymes, and parts of cell structures.  In the average adult, about 25% of total stored energy is in the form of protein.  The body keeps its stored protein under fairly tight control an does not increase stores simply in response to an increased intake of dietary protein.  Individuals who wish to increase their protein stores, such s bodybuilders trying to build muscle mass, require not only a good diet but also tissue-building drugs or intense physical training to do so.

Energy metabolism should never be the primary function of proteins.  Amino acids, which are the building blocks of proteins, are better used for the growth and maintenance of body tissues.  

How do we know which nutrient is being used for fuel?

Fuel Metabolism and Endurance Exercise

Carbohydrate, protein, and fat each play distinct roles in fueling exercise.


  • Provides a highly efficient source of fuel—Because the body requires less oxygen to burn carbohydrate as compared to protein or fat, carbohydrate is considered the body’s most efficient fuel source. Carbohydrate is increasingly vital during high-intensity exercise when the body cannot process enough oxygen to meet its needs.

  • Keeps the brain and nervous system functioning—When blood glucose runs low, you become irritable, disoriented, and lethargic, and you may be incapable of concentrating or performing even simple tasks.

  • Aids the metabolism of fat—To burn fat effectively, your body must break down a certain amount of carbohydrate. Because carbohydrate stores are limited compared to the body’s fat reserves, consuming a diet inadequate in carbohydrate essentially limits fat metabolism.

  • Preserves lean protein (muscle) mass—Consuming adequate carbohydrate spares the body from using protein (from muscles, internal organs, or one’s diet) as an energy source. Dietary protein is much better utilized to build, maintain, and repair body tissues, as well as to synthesize hormones, enzymes, and neurotransmitters.


  • Provides a concentrated source of energy—Fat provides more than twice the potential energy that protein and carbohydrate do (9 calories per gram of fat versus 4 calories per gram of carbohydrate or protein).

  • Helps fuel low- to moderate-intensity activity—At rest and during exercise performed at or below 65 percent of aerobic capacity, fat contributes 50 percent or more of the fuel that muscles need.

  • Aids endurance by sparing glycogen reserves—Generally, as the duration or time spent exercising increases, intensity decreases (and more oxygen is available to cells), and fat is the more important fuel source. Stored carbohydrate (muscle and liver glycogen) are subsequently used at a slower rate, thereby delaying the onset of fatigue and prolonging the activity.


  • Provides energy in late stages of prolonged exercise—When muscle glycogen stores fall, as commonly occurs in the latter stages of endurance activities, the body breaks down amino acids found in skeletal muscle protein into glucose to supply up to 15 percent of the energy needed.

  • Provides energy when daily diet is inadequate in total calories or carbohydrate—In this situation, the body is forced to rely on protein to meet its energy needs, leading to the breakdown of lean muscle mass.

How do dietary fats act as key regulators of Energy Balance?

You are probably comfortable with the idea that caloric intake must be in balance with caloric output for body weight to remain stable.  Now, consider an additional dimension to the energy balance equation.  For body weight to remain stable, the nutrients that are burned to sustain metabolism and activity must be oxidized in proportion to their presence in the diet.  In other words, the person who consumes a diet that is 45% fat must be oxidizing at least that much fat, or excess will be stored.  

Looking at it this way, obesity can develop from a failure to balance fat intake with fax oxidation.  

The factors that determine which nutrients are oxidized and in what proportion are dietary intake, energy needs, weight status, and probably heredity.  An important pint is that nutrient oxidation depends not only on which nutrients are consumed by also on which nutrients are burned, and this may be different for lean and obese individuals.


Aerobic Metabolism

Oxygen Required

Theoretically, can continue forever

Carbohydrate, fat and protein use

Low-to moderate-intensity activities