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Broadly defined, metabolism includes all of the body’s physical and chemical changes. It 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. Catabolism is:
  
  
• • Breaking down food or stored energy (fats and carbohydrates) to glucose, to use as energy for daily living
    
    
  • Breaking down protein 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.

The amount of energy the body uses at rest is the basal metabolic rate (BMR), also called resting metabolic rate (RMR). This rate is based on how many calories the body burns as well as a number of factors that vary by the individual, including:

• Body composition (muscle-to-fat ratio). Muscles raise BMR, whereas fat slows it.
• Physical activity.
• Genes.
• Thyroid activity.

Regulation of metabolism begins in the brain, where the hypothalamus signals the pituitary gland to make and release thyroid-stimulating hormone (TSH). TSH then causes the release of the thyroid gland's two major hormones: thyroxine (T4) and triiodothyronine (T3).

Thyroid disorders such as Graves’ disease, other types of hyperthyroidism, Hashimoto’s thyroiditis and other sources of hypothyroidism may affect many aspects of metabolism and lead to major health problems if not controlled.

Metabolism slows naturally with age, primarily because people generally lose muscle mass and gain fat. Individuals can increase their metabolism and help prevent or reverse obesity by:

• Strengthening the muscles, increasing muscle mass
• Getting more exercise in general
• Losing body fat
• Eating frequent small meals

People often think of calories as measuring an amount of food, but they actually measure 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 (about a quart) of water 1 degree Celsius (about 2 degrees Fahrenheit).

The three main categories of food are broken down in different ways. These reactions release energy and produce carbon dioxide and water as waste products expelled by the body. The three categories are:

• Carbohydrates. The digestive tract absorbs carbohydrates as simple sugars, mostly glucose. The body relies on a ready supply of glucose for energy. Glucose not immediately used for energy is stored in the liver and muscles as glycogen. The body can break down the glycogen into glucose for energy as needed. Excess glucose is converted to fatty acids and stored in fat tissue.
  
  
• Fats. These are broken down primarily into fatty acids. The body can use fats (e.g., to build cell walls) or store them in fat tissue for possible later use.
  
  
• 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. After proteins are processed, leftover substances (carbon, hydrogen and oxygen) are converted to glucose.

The categories of food provide different levels of energy (calories). Approximate number of calories provided by a gram (0.04 ounces):

• Carbohydrate provides 4 calories.
• Protein provides 4 calories.
• Fat provides 9 calories.
• Alcohol (ethanol) provides 7 calories.

When a person eats, the breakdown of food begins in the mouth as saliva starts turning complex carbohydrates (starches) into glucose. Acids and enzymes in the stomach break down proteins into amino acids and fats into fatty acids. Next, the small intestine absorbs the glucose, amino acids and fatty acids into the bloodstream, along with vitamins and minerals.

Carbohydrates, the easiest to break down, are absorbed first, followed by proteins. The body takes longest to break down and absorb fats. This order explains why eating simple carbohydrates can cause blood sugar to spike, and why fatty foods cause a feeling of fullness.

Alcohol is quickly absorbed in the stomach and small intestine. Unlike food, however, it has no nutrients. Nor can it be stored. The liver metabolizes the alcohol, and while it is doing so it does not convert glucose into energy. Many alcoholic beverages such as beer, wine and cocktails contain carbohydrates that must be figured into a diabetic meal plan.

Normally the body breaks down most of its food into glucose and uses that sugar for energy. An organ in the abdomen called the pancreas releases two of the main hormones that regulate glucose:

• Insulin lowers blood glucose levels by letting glucose into the body’s cells.
  
  
• Glucagon raises blood glucose levels by prompting the liver to release glycogen.

Insulin is an anabolic (building-up) hormone. Levels of insulin are high (and levels of glucagon are low) up to four hours after a meal. Insulin:

• Tells the liver to stop breaking down energy stores, such as glycogen and triglyceride reservoirs
  
  
• Enables cells to use glucose
  
  
• Enables excess glucose to be stored in liver, muscle and fat cells

Glucagon is a catabolic (breaking-down) hormone. Levels of glucagon are high (and levels of insulin are low) four to 16 hours after a meal. Glucagon:

• Forms glucose in the blood from breaking down glycogen and other forerunners such as lactate and amino acids
  
  
• Breaks down fat (triglycerides), to free fatty acids released into the blood and used by the liver and skeletal muscles for energy.

Diabetes involves an inability to make or use insulin. These problems with insulin prevent the body from properly transporting and metabolizing glucose. The result is high levels of blood glucose (hyperglycemia), which can causes diabetic complications such as damage to the nerves (neuropathy) and blood vessels (angiopathy).

In addition to thyroid conditions and diabetes, there are many other metabolic disorders, ranging from other endocrine diseases to metabolic syndrome to lipid storage disorders (such as Tay-Sachs disease) to amyloidosis, in which proteins build up in the kidneys, heart and other organs.

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