Comments Off on Carbohydrates | Last Update: April 18, 2012

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What is a carbohydrate?

Carbohydrates continue to take a beating with the resurgence of the Low Carb diet. To the lay person, carbohydrates are sugars and starches.  To a chemist, carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides.  The following table lists the some of the most common carbohydrates.

Monosaccharides Disaccharides Polysaccharides
Glucose Sucrose Starch
Fructose Lactose Glycogen
Galactose Maltose Cellulose

You are probably familiar with the carbohydrates glucose, sucrose, and starch.

Glucose is the blood sugar your doctor checks with a simple blood test and the foundation of the low card diet.  It is the carbohydrate that fuels your brain and nerves.  Your blood glucose level is controlled by your pancreas which secretes the hormone insulin when glucose levels in your blood are too high.

Hyperglycemia and Hypoglycemia

When your blood glucose level is high you are hyperglycemic.   Low blood glucose is referred to as hypoglycemia.  When your blood glucose level drops below the amount your body needs, a signal is sent to your brain producing the feeling of hunger.  Hunger pangs are your bodies way of telling you it needs carbohydrates to replenish its glucose supply.  Your body can store an unlimited amount of fat, but it can only store about 300 calories of glucose.  If you don’t eat foods with carbohydrates to replenish your supply of glucose, your body turns to its next line of defense by converting glycogen stored in your liver or protein in your muscles into glucose.

Where does glucose come from?  You can’t buy it by the bag like sucrose (table sugar).  In fact, if you eat pure glucose it does not taste sweet like sugar.   Glucose is found in all carbohydrates in the form of disaccharides and polysaccharides.

Sugars – Also Know as Simple Carbohydrates

The Monosaccharides


Glucose is found in all carbohydrates either by itself or combined with other molecules to form a disaccharide or polysaccharide (a disaccharide is formed from two monosaccharides and a polysaccharide is a chain of many monosaccharides).  Reducing carbohydrates in your diet essentially reduces your bodies many source of glucose.  That, in turn, forces your body to look for alternate sources which can result in weight loss.  Glucose in its pure form as a monosaccharide is absorbed rapidly into your bloodstream.


Fructose is the sweetest of the sugars and is absorbed directly into your bloodstream like glucose.  It is then transported to your liver where it is converted into compounds indistinguishable from those derived from glucose and sometimes into glucose itself.  High fructose corn syrup is not pure fructose, but a combination of fructose and glucose and in the United States has replaced table sugar (sucrose) in most soft drinks, cookies, cakes, and candies.


Galactose occurs as part of the disaccharide lactose which is found in milk and is seldom found by itself in nature.  

 The Disaccharides

The next three sugars are disaccharides; pairs of monosaccharides with glucose being one of the pair.


Sucrose, commonly referred to as table sugar, is the most familiar of the disaccharides.  The two main sources of sucrose is sugar cane and sugar beets.  The sucrose is processed from these two sources into the white, brown, raw, and powdered sugars available at your local supermarket.  Sucrose contains fructose which makes it a very sweet sugar.

When you eat foods containing sucrose, enzymes in your digestive tract convert it into the monosaccharides glucose and fructose which are both absorbed by your body for fuel.  When necessary, your liver can convert the fructose into glucose or your body can convert the fructose into compounds that are identical to compounds derived from glucose.   Each sucrose molecule can supply two glucose molecules.  


Lactose is the principal carbohydrate found in milk  and accounts for about 5 percent of its weight.  Human babies are born with enzymes necessary to convert lactose into its monosaccharides; glucose and galactose which can be absorbed by the babies body.

The liver converts galactose into glucose to supply a baby with energy to maintain growth. Because babies do not develop the ability to digest starches until they are several months old, milk provides a easily digestible carbohydrate to supply energy to meet his or her needs.

Lactose Intolerance

Some people eventually lose the ability to digest lactose and become lactose intolerant.  When a person who is lactose intolerant drinks milk, the lactose becomes food for intestinal bacteria rather than a source of glucose and galactose for the body.  The multiplying bacteria produce gas which irritates the intestines causing:

  • Bloating
  • Nausea
  • Diarrhea

A majority of the worlds population including peoples around the Mediterranean are lactose intolerant and do not drink milk or drink very small amounts of milk. That is one reason why most Mediterranean peoples do not eat cold cereal for breakfast.  Fortunately, for people who do enjoy milk either by itself or on there cereal, lactose free milk is readily available in most stores.  Also, some people who are lactose intolerant and cannot drink regular milk can tolerate cheese in small portions.

Being lactose intolerant is not the same as being allergic to milk.  An allergic reaction to milk, which can be fatal, is the result of the bodies immune system believing the protein in milk is a threat to the body.


Maltose is not as common as sucrose or lactose.  It is present in seeds and grains supplying plants with the energy source needed to sprout and form leaves.  By the time a plant has sprouted leaves the maltose is depleted.    Malt found in beer contains maltose.  Like other carbohydrates your body converts maltose into glucose.

Starches – Also Know as Complex Carbohydrates

Unlike the sugars, starches are composed almost entirely of dozens of glucose molecules connected together.  That is one reason many diets recommend omitting or limiting eating starchy foods like potatoes and pasta.

A single starch molecule can contain more than 1,000 glucose molecules.  Grains of rice or a potato contain a million starch molecules per cubic inch!  That is the reason why many diets restrict or eliminate flour, rice and potatoes.  Eating these foods by themselves can produce a glucose rush that can overwhelm your pancreas’s ability to produce insulin.  It may sound odd saying fats are good for you, but a pat of butter or dollop of sour cream on your baked potato provides fat for your body to digest in addition to the starch in the potato which increases the time it takes for your body to convert the starch in the potato to glucose.

Plants need starch to grow and all foods containing starch are plants.  For thousands of years people around the world relied on a source of starch for there existence.  Some examples are:

The Orient Rice
South America Corn
Europe Wheat
North America Wheat


Thousands of foods including pasta, bread, and other baked goods are produced from starch in rice, corn and wheat.

Starch in Legumes

Next to grains, the second largest source of starch is legumes; beans and peas.  Legumes are about 40 percent starch by weight.  Coming in third are tubers the most familiar of which is the potato. Like sugars, when you eat foods containing starch molecules your body converts the starch into glucose which is absorbed into your bloodstream.  Within one to four hours after eating foods containing starch, the starch is digested and is circulating in your body providing glucose for your cells.


Glycogen is not found in plants and is found in meats to a limited extent.  Glycogen is produced and stored in the liver and can provide a supply of glucose for your body in between meals.


Cellulose, like starch, is found in plants and is composed of long chains of glucose molecules.  The major difference between starch and cellulose is that the glucose bonds are so tight that the human body does not have the enzymes needed to break down cellulose molecules.  Since cellulose is indigestible, it passes through the digestive track and does not contribute any energy (calories).  Cellulose is one of the fibers which helps regulate your digestive tract.

Artificial Sweeteners and The Sugar Alcohols

If you live in the United States you are probably familiar with the artificial sweeteners aspartame (NutraSweet®) and saccharin.  A third type, cyclamate, was banned in the United States.  These “artificial” sweeteners do not occur in nature. They were developed by chemists to provide the sweetness of sugar without the calories.  A soft drink that contains sugar has about 150 calories in a 12 ounce serving versus about 1 calorie for a 12 ounce diet soft drink with an artificial sweetener.

Net Carbs

For people on low carbohydrate diets a new term “net carbs” has come into vogue.  Net carbs represents the calories coming from naturally occurring carbohydrates.  The balance of the “gross carbs” in the food is from one of the sugar alcohols; sorbitol, xylitol, and maltitol which occur naturally in foods.

Sugar alcohols are either absorbed more slowly by your body or metabolized differently than sugars.  Unlike the artificial sweeteners which have almost zero calories, the sugar alcohols contain 4 calories per gram the same as table sugar and other carbohydrates.  Like many low fat foods, low carb foods does not necessarily mean low calorie foods.