Metabolism is the body’s process of using food for energy and growth. It involves two major processes: catabolism and anabolism. Catabolism is the process in which foods and other substances are broken down into simpler substances, which may release or store energy. Anabolism is the process by which simple substances are built up into more complex substances. This process uses energy.

Calories are the body’s energy source, and the body’s metabolism regulates how it uses these calories. Excess energy is generally stored in the body as fat. Therefore, when the body takes in more energy (calories) than it uses, weight is gained. If more energy is used than is taken in, the body uses its fat stores for energy, and weight is lost.

There are three primary methods in which the body uses calories, regardless of whether they are taken from stored sources or directly from food. The basal metabolic rate is the energy used when the body is at rest. The thermic effect of food (TEF) is the energy used to process (e.g., digest, absorb, transport, store) consumed food. Energy is also used during physical activity (e.g., walking, running, any other movement). The sum of all three of these is known as the total energy expenditure (TEE) of the body. A person’s metabolic rate does not remain the same at all times. Certain events (e.g., pregnancy, aging) and activities (e.g., exercise, changes in eating habits) can cause metabolism to decrease or increase.

Some energy from catabolism is stored as compounds such as adenosine triphosphate (ATP). These compounds readily release energy and can easily be stored as fatty acids in the adipose tissues. The breakdown of food into these compounds occurs as a series of metabolic pathways (chemical reactions that lead to catabolism or anabolism).

The three main energy-producing nutrients (carbohydrates, fats, proteins) are broken down in different ways to produce energy the body can use. First, they create a metabolite called acetyl CoA. Acetyl CoA can be used to create fats or ATP, two forms of stored energy. If the body has plenty of ATP, acetyl CoA is built up to make fats. 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.

Broadly defined, metabolism includes all of the physical and chemical changes that occur inside the cells of the body and that maintain life. Most often, metabolism refers to the process in which the body obtains and uses energy from food.

Metabolism consists of two contrary but complementary processes that occur at the same time:

• Catabolism (breaking down). The body creates energy by breaking down digested food or stored fat into simpler substances used for building blocks. Fats are broken down into glucose and fatty acids. carbohydrates are broken down to glucose or glycerol to use as energy for daily living. Protein is broken down to amino acids, which the body uses to rebuild or repair tissues.
  
  
• Anabolism (building up). The body uses energy from glucose and other molecules by employing these simpler molecules to build the cells, move the muscles and carry out other vital functions. For example, glucose can be used to make glycogen chains, glycerol and fatty acids can form triglycerides, and amino acids are used to make proteins.

Metabolism plays an important role in weight gain and loss. Calories are the source of energy for the body, and the body’s metabolism regulates how the body uses these calories. Excess energy is generally stored in the body as fat. Therefore, when the body takes in more energy (calories) than it uses, weight is gained. If more energy is used than is taken in, the body uses its fat stores for energy, and weight is lost.

There are three primary methods in which the body uses calories, regardless of whether they were taken from stored fat or from food. All three methods make up the total number of calories a person burns in a day. This is known as the total energy expenditure (TEE) of the body. These three methods are:

• Basal metabolic rate (BMR). Basic body processes (e.g., breathing, circulation of blood, organ function, adjusting hormone levels, growing and repairing cells) burn calories even when the body is completely at rest. The amount of energy the body uses at rest is the BMR, also called resting metabolic rate. In general, about two-thirds to three-quarters of an adult’s total energy expenditure is used by his or her BMR. This rate is typically consistent over time. It is influenced by a number of factors, including:
  
  
• • Body size and composition. The larger a body, the more calories it requires to maintain its size. The composition of a body, or its muscle-to-fat ratio, is also important. Because muscles use more energy than fat, a body with a higher ratio of muscle to fat also has a higher BMR.
    
    
  • Age. A person’s metabolism slows with age. People also tend to lose muscle mass as they age, reducing their muscle-to-fat ratio and lowering their BMR.
    
    
  • Gender. Men usually have a higher BMR than women of the same age and weight. This is due largely to the fact that men tend to have a higher muscle-to-fat ratio than women.
    
    
  • Other individual differences. Because of differences in hormone levels and certain genetic tendencies, different people store and use energy at different rates. Thyroid hormones regulate metabolism, so thyroid imbalances (e.g., hyperthyroidism, hypothyroidism) affect a person’s BMR. Certain other hormones (e.g., insulin, glucagon) play an important role in metabolism by affecting glucose levels or the transport of glucose through the body.
    
    
• Thermic effect of food (TEF). The body also requires energy to process (e.g., digest, absorb, transport, store) consumed food. This is called the thermic effect of food, and it typically represents about 10 percent of the body’s total energy expenditure. Like the BMR, the thermic effect of food is generally constant in a given individual and is not easily changed.
  
  
• Physical activity. The remainder of the body’s total energy expenditure comes from energy used during physical activity (e.g., walking, running, any type of movement). This aspect of metabolism can be altered by the individual, depending on how much he or she moves or exercises. More frequent, intense or longer-lasting exercise burns more calories.

There are many possible ways of assessing metabolic rate and function, including enzyme tests, electrolyte panel, waste product tests, glucose tests, thyroid tests, other blood tests, urine tests, liver tests, biopsies and cardiovascular tests. An individual’s physician can advise which tests may be appropriate.

A person’s metabolic rate does not remain the same at all times. Certain events and activities may cause metabolism to decrease or increase. These include:

• Age. Due to changes in hormones and a reduced muscle-to-fat ratio, metabolism slows down as people age.
  
  
• Pregnancy and breastfeeding. To support the growth of a fetus, a pregnant woman must take in more calories when she is pregnant. Breastfeeding an infant also requires more calories. These events may increase a woman’s rate of metabolism.
  
  
• Exercise. Metabolism is increased after exercising. For how long it is increased depends on the individual as well as the duration and intensity of the exercise, but the boost can last for several hours. Strengthening the muscles also increases muscle mass, boosting the body’s muscle-to-fat ratio and further increasing metabolism.
  
  
• Eating habits. How frequently a person eats affects metabolism. When calories are scarce (e.g., when fasting), the body attempts to conserve energy by slowing down metabolism. When the body receives a steady stream of calories (e.g., frequent, small meals), it does not attempt to conserve energy as much. Thus, metabolism speeds up. Metabolism also increases for several minutes immediately after eating. Therefore, eating small, frequent meals burns fat much more quickly than skipping meals or fasting.

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.

Preparing questions in advance can help patients to have more meaningful discussions regarding their conditions. Patients may wish to ask their doctor or dietitian the following questions related to metabolism:

1. What is my basal metabolic rate?
   
   
2. Could my weight problem be due to a problem with my metabolism?
   
   
3. Do I have any medical conditions that may affect my metabolism?
   
   
4. Am I at risk of developing any conditions that may affect my metabolism?
   
   
5. What kind of exercises may help me to improve my metabolism?
   
   
6. How can I improve my eating habits to improve my metabolism?
   
   
7. Are there any particular foods I could eat to improve my metabolism?
   
   
8. Does my environment have any effect on my metabolism?
   
   
9. Do I need to undergo any metabolism tests?
   
   
10. How does my metabolism of alcohol affect my existing medical conditions?