Metabolism

Every time you swallow a bite of sandwich or slurp a smoothie, your body works hard to process the nutrients you've eaten. Long after the dishes are cleared and the food is digested, the nutrients you've taken in become the building blocks and fuel needed by your body. Your body gets the energy it needs from food through a process called metabolism.

What Is Metabolism and What Does It Do?

Metabolism (pronounced: muh-tah-buh-lih-zum) is a collection of chemical reactions that takes place in the body's cells to convert the fuel in the food we eat into the energy needed to power everything we do, from moving to thinking to growing. Specific proteins in the body control the chemical reactions of metabolism, and each chemical reaction is coordinated with other body functions. In fact, thousands of metabolic reactions happen at the same time - all regulated by the body - to keep our cells healthy and working.

Metabolism is a constant process that begins when we're conceived and ends when we die. It is a vital process for all life forms - not just humans. If metabolism stops, a living thing dies.

Here's an example of how the process of metabolism works in humans - and it begins with plants: First, a green plant takes in energy from sunlight. The plant uses this energy and a molecule called cholorophyll (which gives plants their green color) to build sugars from water and carbon dioxide. This process is called photosynthesis, and you probably learned about it in biology class.

When people and animals eat the plants (or, if they're carnivores, when they eat animals that have eaten the plants), they take in this energy (in the form of sugar), along with other vital cell-building chemicals. The body's next step is to break the sugar down so that the energy released can be distributed to, and used as fuel by, the body's cells.

After food is eaten, molecules in the digestive system called enzymes break proteins down into amino acids, fats into fatty acids, and carbohydrates into simple sugars (e.g., glucose). In addition to sugar, both amino acids and fatty acids can be used as energy sources by the body when needed. These compounds are absorbed into the blood, which transports them to the cells. After they enter the cells, other enzymes act to speed up or regulate the chemical reactions involved with "metabolizing" these compounds. During these processes, the energy from these compounds can be released for use by the body or stored in body tissues, especially the liver, muscles, and body fat.

In this way, the process of metabolism is really a balancing act involving two kinds of activities that go on at the same time - the building up of body tissues and energy stores and the breaking down of body tissues and energy stores to generate more fuel for body functions.

• Anabolism (pronounced: uh-nah-buh-lih-zum), or constructive metabolism, is all about building and storing: It supports the growth of new cells, the maintenance of body tissues, and the storage of energy for use in the future. During anabolism, small molecules are changed into larger, more complex molecules of carbohydrate, protein, and fat.
• Catabolism (pronounced: kuh-tah-buh-lih-zum), or destructive metabolism, is the process that produces the energy required for all activity in the cells. In this process, cells break down large molecules (mostly carbohydrates and fats) to release energy. This energy release provides fuel for anabolism, heats the body, and enables the muscles to contract and the body to move. As complex chemical units are broken down into more simple substances, the waste products released in the process of catabolism are removed from the body through the skin, kidneys, lungs, and intestines.

Several of the hormones of the endocrine system are involved in controlling the rate and direction of metabolism. Thyroxine (pronounced: thigh-rahk-sun), a hormone produced and released by the thyroid (pronounced: thigh-royd) gland, plays a key role in determining how fast or slow the chemical reactions of metabolism proceed in a person's body.