Role of Carbohydrates
http://www.extension.iastate.edu/nutrition/sport/carbohydrate.html
Iowa State
The role of carbohydrates in the body include:
o providing energy for working muscles
o providing fuel for the central nervous system
o enabling fat metabolism
o preventing protein from being used as energy
Carbohydrates are the preferred source of energy or fuel for muscle contraction and
biologic work.
Foods containing carbohydrates are in the grains, fruit, and milk groups. Vegetables have
a small amount of carbohydrate.
After carbohydrates are eaten, it is broken down into smaller units of sugar (including
glucose, fructose and galactose) in the stomach and small intestine. These small units of
sugar are absorbed in the small intestine and then enter the bloodstream where they travel
to the liver. Fructose and galactose are converted to glucose by the liver. Glucose is the
carbohydrate transported by the bloodstream to the various tissues and organs, including
the muscles and the brain, where it will be used as energy.
If the body does not need glucose for energy, it stores glucose in the liver and the skeletal
muscles in a form called glycogen. If glycogen stores are full, glucose is stored as fat.
Glycogen stores are used as an energy source when the body needs more glucose than is
readily available in the bloodstream (for example, during exercise). The body has limited
storage capacity for glycogen (about 2000 calories), which is why carbohydrates are
commonly referred to as the ‘limiting fuel’ in physical performance.
Carbohydrates spare the use of protein as an energy source. When carbohydrate
consumption is inadequate, protein is broken down to make glucose to maintain a
constant blood glucose level. However, when proteins are broken down they lose their
primary role as building blocks for muscles. In addition, protein breakdown may result in
an increased stress on the kidneys, where protein byproducts are excreted into the urine.
Finally, glucose is essential for the central nervous system. The brain primarily uses
glucose as its energy source, and a lack of glucose can result in weakness, dizziness, and
low blood glucose (hypoglycemia). Reduced blood glucose during exercise decreases
performance and could lead to mental as well as physical fatigue.
Carbohydrates in the Body
All living cells contain glucose. For glucose to enter the cells it needs help from a
hormone called insulin. Insulin acts as gatekeeper and is released once carbohydrate is
ingested. It signals the cells to absorb the glucose. The glucose is then used for energy,
stored in the liver and the muscles as glycogen, or stored as fat.
Glycogen stores are essential for athletic performance because they serve as an energy
reservoir when blood glucose levels are decreased due to high intensity exercise or
inadequate carbohydrate intake. Glycogen stores become depleted as the intensity and
duration of the exercise increases. It is imperative for the athlete, whether a sprinter or
endurance athlete, to restore glycogen by consuming carbohydrates on a regular basis.
A well-nourished adult can store approximately 500 grams or 2000 kcal of carbohydrates.
Of this, approximately 400 grams are stored as muscle glycogen, 90-110 grams as liver
glycogen, and 2-3 grams circulate in the blood as glucose. When the body needs more
glucose than is available in bloodstream to support energy demands, glycogen stores are
used to raise blood glucose levels. However, it is important to note that the glycogen
stored in muscle is used directly by that muscle during exercise, it cannot borrow
glycogen from other resting muscles.
Carbohydrates serve as a primary energy source in the diet and provide 4 calories per
gram of carbohydrate. Carbohydrates exists in many forms, which can be divided into
primarily two groups; simple and complex carbohydrates.
Simple carbohydrates are also known as simple sugars and have a smaller structure.
Some common simple carbohydrates include sucrose (sugar found in candy, soda,
juice etc.), and lactose (sugar found in milk)
Simple carbohydrates are readily broken down and absorbed in the intestine into
the blood stream. This rapid increase in blood glucose leads to a rapid release of
insulin that is needed to help transport the sugar into the cells.
Energy is rapidly produced but only lasts for a short period of time.
Complex carbohydrates are also known as starch and fiber and have a larger structure.
Some common foods containing complex carbohydrate include bread, pasta, and
whole grains.
Complex carbohydrates take a longer time to digest and absorb. This creates a
slow and steady increase in blood glucose and a slow and steady increase in
insulin levels.
Energy is slowly produced, not as quickly as simple carbohydrates; however, the
energy lasts for a longer period of time.
Fiber is a complex carbohydrate, and can be classified as soluble or insoluble.
Soluble fiber can be broken down and provides energy. Soluble fiber is found in
many fruits and vegetables including green leafy vegetables, celery, carrots,
apples, pears, among others. Insoluble fiber cannot be digested in our digestive
system, and does not provide energy. Insoluble fiber is commonly found in whole
grain cereal, bread, and rice. Insoluble fiber has an important role in health by
promoting gut motility and satiety.
The most important carbohydrate for an athlete is the complex carbohydrate, starch.
Starch is the major energy source in the diet and is broken down into glucose in the body
that can be stored in the liver and in the muscles as glycogen. Glycogen can then be used
to maintain blood sugar at a constant level for optimal energy supply. Common starches
include bread, cereal, pasta, and other various grains, but also corn and peas. Make Half
your Grains Whole Grains in order to consume more complex carbohydrates.
Name:__________________________Period:_______Class:_________Date:_________
1. What are 2 roles that carbohydrates play in the body?
2. What does the body do with excess carbohydrates? How might this affect a person’s
weight?
3. Why might a person who is on a low-carb diet experience dizziness?
4. Why is insulin production essential to absorbing carbohydrates?
5. A common misconception is athletes must “carb load” before a particularly taxing
event. What does the body do with excess glycogen stores? Why is carb loading a myth?