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Metabolism involves all the ways in which the body obtains and uses energy (calories) from food. People often think of calories as a certain amount of food, but calories are actually a measure of the energy produced by food or burned by activity. The calorie commonly referred to in discussions of nutrition and metabolism is actually a kilocalorie (large calorie, equal to 1,000 small calories). That is the amount of energy needed to heat a liter (slightly more than a quart) of water by 1 degree Celsius (about 2 degrees Fahrenheit).

Three types of nutrients provide energy to the body. They are carbohydrates, fats and proteins. During digestion, the body breaks these nutrients down into four types of substances that can be absorbed into the blood. These substances are glucose (and other monosaccharides), glycerol, fatty acids and amino acids. 

Energy-yielding nutrients may provide different amounts of energy (calories). For example, the approximate number of calories provided by a gram (0.04 ounces) of the following nutrients are:

• Carbohydrates – 4 calories
• Fats – 9 calories
• Protein – 4 calories

Some of the energy produced when food is broken down is captured by high-energy storage compounds such as adenosine triphosphate (ATP). ATP is the major form of energy used for cellular metabolism. These compounds readily release energy and can easily be stored as fatty acids. ATP is used to store and transport chemical energy within cells. Thus, energy produced by catabolism (breaking down) can be used to fuel anabolic (building up) functions of the body.

A series of metabolic pathways (chemical reactions that lead to catabolism or anabolism) determine how energy produced by catabolism comes to be stored in high-energy storage compounds. The breakdown of substances in a metabolic pathway requires the use of many enzymes and coenzymes. The enzymes are used to facilitate metabolic reactions and the coenzymes support and assist the enzymes.

The series of chemical reactions that occur during metabolism may differ depending on the type of nutrient involved, such as:

• Carbohydrates. The digestive tract absorbs carbohydrates as simple sugars, mostly glucose. Glucose is further broken down into a metabolite called pyruvate. Pyruvate can then be converted through an anaerobic (without oxygen) process into lactic acid for short bursts of energy. Or, pyruvate could be converted through an aerobic (with oxygen) process into acetyl CoA for a slower but much more long-term source of energy. Lactic acid may be recycled back into glucose (in a process called the Cori cycle), but acetyl CoA cannot be remade into glucose.
  
  
• Fats and oils. These are broken down into glycerol and fatty acids. Glycerol can be either built up (anabolism) to make glucose or broken down (catabolism) to form pyruvate. The pyruvate created from glycerol can be used to form lactic acid or acetyl CoA in the same manner as with glucose. Fatty acids are broken down in a process known as beta oxidation (also called fatty acid oxidation), which produces acetyl CoA. During beta oxidation, oxygen combines with fragments of the fatty acid to release electrons into the electron transport chain. Unlike glycerol, fatty acids cannot be used to create glucose.
  
  The electron transport chain produces ATP for energy storage. Electrons released into the electron transport chain are passed between a series of protein “carriers” until they join with oxygen and hydrogen to create water. Each time an electron is passed from one carrier to another, ATP is generated.

• Proteins. These are broken down into amino acids. Amino acids are converted to building blocks for compounds such as hormones, nucleic acids (part of DNA), digestive enzymes and antibodies. Although their primary uses in the body are for the protein development and repair, amino acids can also be used for energy. Different amino acids can be broken down into different metabolites. Some are broken down into pyruvate, some into acetyl CoA, and others enter the Krebs cycle directly.
  
  The Krebs cycle (or citric acid cycle) takes place inside the mitochondria. In this process, acetyl CoA is broken down into two molecules of carbon dioxide and coenzyme A (CoA), and electrons are released into the electron transport chain.

Acetyl CoA can be used to create either fats or ATP. If the body has plenty of ATP, acetyl CoA is used to make fatty acids. If the body’s ATP levels are low, acetyl CoA enters a metabolic pathway – either the electron transport chain or Krebs cycle – to produce more ATP. Since all three of the main energy-producing nutrients can be broken down into acetyl CoA, they can all be used to generate fat, which the body can use as energy.

When ATP is used, it is converted into adenosine diphosphate (ADP). A substance called creatine phosphate is then used to change ADP back into ATP. This is called the ATP CP energy system.

Alcohol is also metabolized in the body. Unlike food, it has no nutrients and cannot be stored in the body for later energy use. The metabolism of alcohol can interfere with the metabolism of food because while the liver is busy metabolizing alcohol, it stops converting other compounds (e.g., glucose, glycerol, fatty acids, amino acids) into energy.