How Food Works

Carbohydrates

You have probably heard of "carbohydrates" and "complex carbohydrates." Carbohydrates provide your body with its basic fuel. Your body thinks about carbohydrates like a car engine thinks about gasoline.

The simplest carbohydrate is glucose. Glucose, also called "blood sugar" and "dextrose," flows in the bloodstream so that it is available to every cell in your body. Your cells absorb glucose and convert it into energy to drive the cell. Specifically, a set of chemical reactions on glucose creates ATP (adenosine triphosphate), and a phosphate bond in ATP powers most of the machinery in any human cell. If you drink a solution of water and glucose, the glucose passes directly from your digestive system into the bloodstream.

The word "carbohydrate" comes from the fact that glucose is made up of carbon and water. The chemical formula for glucose is:

You can see that glucose is made of six carbon atoms (carbo...) and the elements of six water molecules (...hydrate). Glucose is a simple sugar, meaning that to our tongues it tastes sweet. There are other simple sugars that you have probably heard of. Fructose is the main sugar in fruits. Fructose has the same chemical formula as glucose (C₆H₁₂O₆), but the atoms are arranged slightly differently. The liver converts fructose to glucose. Sucrose, also known as "white sugar" or "table sugar," is made of one glucose and one fructose molecule bonded together. Lactose (the sugar found in milk) is made of one glucose and one galactose molecule bonded together. Galactose, like fructose, has the same chemical components as glucose but the atoms are arranged differently. The liver also converts galactose to glucose. Maltose, the sugar found in malt, is made from two glucose atoms bonded together.

Glucose, fructose and galactose are monosaccharides and are the only carbohydrates that can be absorbed into the bloodstream through the intestinal lining. Lactose, sucrose and maltose are disaccharides (they contain two monosaccharides) and are easily converted to their monosaccharide bases by enzymes in the digestive tract. Monosaccharides and disaccharides are called simple carbohydrates. They are also sugars -- they all taste sweet. They all digest quickly and enter the bloodstream quickly. When you look at a "Nutrition Facts" label on a food package and see "Sugars" under the "Carbohydrates" section of the label, these simple sugars are what the label is talking about.

There are also complex carbohydrates, commonly known as "starches." A complex carbohydrate is made up of chains of glucose molecules. Starches are the way plants store energy -- plants produce glucose and chain the glucose molecules together to form starch. Most grains (wheat, corn, oats, rice) and things like potatoes and plantains are high in starch. Your digestive system breaks a complex carbohydrate (starch) back down into its component glucose molecules so that the glucose can enter your bloodstream. It takes a lot longer to break down a starch, however. If you drink a can of soda full of sugar, glucose will enter the bloodstream at a rate of something like 30 calories per minute. A complex carbohydrate is digested more slowly, so glucose enters the bloodstream at a rate of only 2 calories per minute (reference).

You may have heard that eating complex carbohydrates is a good thing, and that eating sugar is a bad thing. You may even have felt this in your own body. The following quote from The Yale Guide to Children's Nutrition's explains why:

If complex carbohydrates are broken down to monosaccharides in the intestines before they are absorbed into the bloodstream, why are they better than refined sugar or other di- or mono-saccharides? To a great extent it has to do with the processes of digestion and absorption. Simple sugars require little digestion, and when a child eats a sweet food, such as a candy bar or a can of soda, the glucose level of the blood rises rapidly. In response, the pancreas secretes a large amount of insulin to keep blood glucose levels from rising too high. This large insulin response in turn tends to make the blood sugar fall to levels that are too low 3 to 5 hours after the candy bar or can of soda has been consumed. This tendency of blood glucose levels to fall may then lead to an adrenaline surge, which in turn can cause nervousness and irritability... The same roller-coaster ride of glucose and hormone levels is not experienced after eating complex carbohydrates or after eating a balanced meal because the digestion and absorption processes are much slower.

If you think about it, this is incredibly interesting because it shows that the foods you eat and the way you eat them can affect your mood and your temperament. Foods do that by affecting the levels of different hormones in your bloodstream over time.

Another interesting thing about this quote is its mention of insulin. It turns out that insulin is incredibly important to the way the body uses the glucose that foods provide. The functions of insulin are:

To enable glucose to be transported across cell membranes
To convert glucose into glycogen for storage in the liver and muscles
To help excess glucose be converted into fat
To prevent protein breakdown for energy

According to the Encyclopedia Britannica:

Insulin is a simple protein in which two polypeptide chains of amino acids are joined by disulfide linkages. Insulin helps transfer glucose into cells so that they can oxidize the glucose to produce energy for the body. In adipose (fat) tissue, insulin facilitates the storage of glucose and its conversion to fatty acids. Insulin also slows the breakdown of fatty acids. In muscle it promotes the uptake of amino acids for making proteins. In the liver it helps convert glucose into glycogen (the storage carbohydrate of animals) and it decreases gluconeogenesis (the formation of glucose from noncarbohydrate sources). The action of insulin is opposed by glucagon, another pancreatic hormone, and by epinephrine.

What you can begin to see from this description is that there are actually lots of different things happening in your body around glucose. Because glucose is the essential energy source for your body, your body has many different mechanisms to ensure that the right level of glucose is flowing in the bloodstream. For example, your body stores glucose in your liver (as glycogen) and can also convert protein to glucose if necessary. Carbohydrates provide the energy that cells need to survive.

For more information on carbohydrates, glucose and insulin, check out the links at the end of this article.