Updated August 06, 2015.

Carbohydrates are the primary source of energy for the body and include both simple sugars and larger complex carbohydrates. Your body can use carbohydrates right away or convert them into a storage form called glycogen. Excess carbohydrates can also be converted to fat.

First, a Little Chemistry

No matter how big they are, all carbohydrates are made of carbon, hydrogen and oxygen with the general formula of C_m(H₂O)_n.

For example, a simple little sugar molecule like glucose is made up of six carbon atoms, 12 hydrogen atoms, and sixoxygen atoms. It is shaped like a hexagon and has the formula C₆(H₂O)₆. A large starch molecule can be made of many little sugar molecules connected to form a long chain. The small m and n in our general formula, C_m(H₂O)_n, can run into the hundreds.

Simple sugars are made up of one or two sugar units. One common simple sugar is glucose, C₆(H₂O)₆, and it is the sugar our bodies and brains use for energy every day. Glucose is called a monosaccharide, which means "single sugar." Other monosaccharides include fructose, galactose, and ribose. Fructose is found in fruits and vegetables, galactose is found in milk and ribose is best known as a part of ribonucleic acid, which is a part of genetic transcription and is found in the cells in our bodies.

I don't want to get much deeper into the chemistry of simple sugars, but it is important to know that the single sugars glucose, fructose, and galactose can form different combinations to become disaccharides, a term that means "two sugars." These sugars include:

• Lactose (milk sugar) is made up of glucose and galactose molecules. People who are "lactose intolerant" can't digest this sugar properly.
• Sucrose (table sugar) is composed of glucose and fructose molecules. That's the white powdery or granular substance we typically refer to as "sugar" when we are cooking or baking.
• Maltose (malt sugar) is produced during the malting of cereals such as barley.

Simple sugars are water-soluble and easy for your body to digest into the individual glucose and fructose molecules. They're also quickly absorbed through the intestinal walls and into the bloodstream.

Complex carbohydrates are long chains of single sugar units. For example, the complex carbohydrate we know as starch is made up of many glucose units. These complex carbohydrates can be in the form of long chains, or the chains can form branches.

Complex carbohydrates include:

• Starch, the energy storage form of carbohydrates found in plants, especially in the seeds and roots. Starch is made up of many glucose units linked together. Starchy food examples include rice, wheat, corn, carrots and potatoes. Starches are not water-soluble and require digestive enzymes called amylases to break them apart.
• Glycogen, the energy storage form of glucose found in the muscles and livers of animals. You don't consume any carbohydrates when you eat meats; however, the amount of glycogen in animal tissue at the time of slaughter does affect the pH of the meat.
• Cellulose, the structural component of plants. Cellulose helps plants keep their shape; so in a way, cellulose acts as a plant skeleton. We are unable to digest cellulose; however cellulose is one of the principal components of dietary fiber, along with lignin, chitin, pectin, beta-glucan, inulin and oligosaccharides.

Dietary starch and cellulose are the complex carbohydrates that are essential in nutrition. Potatoes, dry beans, grains, rice, corn, squash, and peas contain significant amounts of starch. Vegetables like broccoli, cauliflower, asparagus, lettuces and other greens are not starchy. That's because the stems and leafy parts of plants don't contain much starch, but they do provide a large amount of cellulose. Since we can't digest cellulose, that means that the green and leafy vegetables contain fewer calories than the starchy vegetables.

Carbohydrates and Metabolism

The body begins the process of breaking carbohydrates down into their individual monosaccharides almost before we start to eat them. When you smell the delicious aroma of fresh-baked bread or think about that tasty chocolate that you're about to consume, your mouth begins to water. Since table sugar is water-soluble, it starts to dissolve in your mouth. Your saliva also contains a small amount of amylase, which is an enzyme that starts to break starch down into glucose while you are chewing.

Carbohydrate digestion continues in the small intestine with pancreatic amylase. Amylase breaks carbohydrates down into monosaccharides that can be absorbed into the blood stream. Once in the blood, the monosaccharides are either used for energy, stored in the liver and muscles as glycogen, or converted to fat and stored in adipose tissue.

The storage of glucose is triggered by insulin, which forces your body to store any extra blood sugar as glycogen. People with diabetes or metabolic syndrome either can't produce enough insulin, or they are not sensitive enough to the insulin they produce and need to regulate their blood sugar with medications, insulin or dietary changes.

Your body prefers to use glucose as the primary source of fuel for daily activity. Your muscles need glucose to move, and your organs need glucose to function, including your brain. While your body can make glucose from any extra dietary protein by a process called gluconeogenesis, about half of your daily calories should come from carbohydrates. Try to get your carbohydrates from healthy sources such as whole grains, fruits, and vegetables. Cookies, sodas, candy and other sweets are not so healthy.