Test Your Knowledge

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.