Burning Carbs

Carbohydrates (also known as “carbs” or “carbos”) are a large class of foods that includes sugars and starches. They are our body's preferred source of energy for everything we do: running and jumping, eating and sleeping, and everything in between. They keep our bodies going.

They can be simple sugars (like glucose), or two or more simple sugars hooked together (like sucrose), or many simple sugar molecules bonded together into starches. Before using carbohydrates for fuel, our bodies have to break these compounds down into simple sugars. Those carbohydrates that resist breaking down move through our digestive tract as fiber and never contribute to the body's energy supply.

Breaking the bonds between the sugar molecules, our digestive system produces a supply of glucose, which enters the bloodstream and travels to our cells to supply energy. To help our cells take up the glucose from the blood, the pancreas releases the hormone insulin. Our cells use the glucose to power whatever activities we're engaged in. If there's more glucose than is needed at the moment, the extra is converted to glycogen and fat and stored in the liver, muscles, and fat cells for future use. When the blood sugar is depleted and we need more fuel, the glycogen is converted back to glucose. When we exercise for a long time, after we use up the available glycogen, we begin burning fat.

Easily digested carbohydrates, such as white bread and sugar, rapidly load the blood with glucose, which triggers a matching insulin response. This quickly drives the glucose into the cells, leaving the blood-sugar level low. We feel hungry again and, if we eat, the extra food will be stored as body fat. By contrast, carbohydrates that are combined with fiber, as they are in whole grains and in fruits and vegetables, are digested slowly and don't provoke an excessive insulin response.

Paying attention to the kind of carbohydrates we consume is vital to controlling our weight.


  1. What product forms when the bonds of sugar molecules are broken?
    [anno: Glucose is the product that forms when the bonds of sugar molecules are broken.]
  2. What does the body do with this product?
    [anno: The body uses the glucose to power activities. If there is too much glucose, the body converts the extra glucose to glycogen and fat, which are stored in the liver, muscles, and fat cells for future use.]
  3. Why are carbohydrates that contain high levels of fiber a more efficient source of energy?
    [anno: High-fiber carbohydrates are a more efficient source of energy because they digest slowly and do not cause a large amount of insulin to be released.]
  4. What other chemical changes occur in the body? Select one of these chemical changes, and describe what happens.
    [anno: Answers will vary but could include respiration. The body takes in oxygen and releases carbon dioxide.]