Everything You Need to Know About Carbohydrates

Complex carbohydrates are found in bread, pasta, nuts, seeds, legumes and potatoes.
Maximilian Stock Ltd./Getty Images

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 Cm(H2O)n.

For example, a simple little sugar molecule like glucose is made up of six carbon atoms, 12 hydrogen atoms, and six oxygen atoms. It is shaped like a hexagon and has the formula C 6(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, Cm(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 for good health. 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 great deal 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 begins 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.

Carbohydrates should contribute half of your daily calories. One gram of carbohydrate contains four calories whether is it is sugar or starch. One slice of bread has about 12 grams of carbohydrates. One typical chocolate bar may have about 50 grams of carbohydrates. A medium potato has about 35 grams of carbohydrates.

Although all carbohydrates have four calories per gram, some sources of carbohydrates are better for your diet than others. Fruits, vegetables, legumes, nuts, seeds, and grains are healthier than candy, sodas, and pastries. Why? The healthy carbohydrate sources also have significant amounts of vitamins, minerals, phytochemicals and fiber, all of which are vital to good health. Candy, sodas, pastries and other junk foods usually are poor sources of nutrients and sometimes we refer to these foods as having "empty calories." That means the foods have lots of calories with little or no nutrition.

Carbohydrate Requirements

Since you need half of your calories from carbohydrates, you can calculate how many you need per day. For example, let's say a person needs 2,000 calories per day. That means that 1,000 calories should come from carbohydrates (2,000 X 0.5). Since each gram of carbohydrate has four calories, then you divide 1,000 by four (1,000/4) to get 250. A person who needs 2,000 calories each day needs about 250 grams of carbohydrates per day. Of those 250 grams, about 10 percent can come from added table sugar and sweeteners. That would be about 25 grams for a 2,000 calorie per day diet. That would equal about half of a candy bar or less than one can of sugary soda. Unfortunately many people exceed that amount every day.

To meet your carbohydrate requirement each day, you need to know how many carbohydrates are in all of the foods you eat. It really is impossible to list every carbohydrate containing food here, however, here are some approximate amounts from typical examples:

  • One slice of bread - 12.5 grams total, of which 10 grams are starch and less than one gram is fiber.
  • One cup of pasta - 43 grams total, of which 36 grams are starch and 2.5 grams are fiber.
  • One medium apple - 19 grams total, of which eight grams are starch and three grams are fiber.
  • One Snickers candy bar - 63.5 total grams, of which 53 grams are sugar, two grams are fiber.
  • One cup of raisin bran cereal - 43 grams total, of which seven grams are fiber, 17 grams are starch, and 16 grams are sugar.
  • One cup of sugar frosted corn flake cereal - 28 grams total, of which 15 grams are starch, one gram is fiber, 12 grams are sugar.
  • One four-ounce glass of red wine - three grams total, of which, less than one gram is sugar.
  • One eight ounce serving of low-fat milk - 12 grams total, of which 12 grams are lactose.
  • One cup broccoli - six grams total, of which 2.5 grams are fiber and 1.5 grams are sugar.
  • One cup green beans - eight grams total, of which four grams are fiber.
  • One cup sweet corn - 31 grams total, of which 21 grams are starch, three grams are fiber.
  • Two cups lettuce - two grams total, of which one gram is fiber.
  • One cup asparagus - four grams total, of which two grams are fiber.
  • One medium orange - 15 grams total, of which three grams are fiber.
  • One half medium grapefruit - nine grams total, of which 1.5 grams are fiber.
  • One medium chocolate chip cookie - 16 grams total, of which seven grams are sugar.
  • One cup strawberries - 12 grams total, of which three grams are fiber.
  • One cup blueberries - 21 grams total, of which four grams are fiber and 15 grams are sugar.
  • One half cup marinara sauce - 14 grams total, of which less than one gram is fiber.
  • One medium tomato - five grams total, of which 1.5 grams are fiber.
  • One medium potato with skin - 29 grams total, of which three grams are fiber, 25 grams are starch.
  • One cup carrots - 12 grams total, of which 3.5 grams are fiber and two grams are starch.
  • One slice of an apple pie - 40 grams total, of which 18 grams are sugar.
  • One eight-ounce cup of orange juice - 26 grams total, of which 21 grams are from fruit sugars.
  • One cup of dry beans like pinto beans or navy beans - 47 grams total, of which 19 grams are fiber, 28 grams are starch.

Source:  USDA National Nutrient Database for Standard Reference

Nutrition Facts labels on packaged foods will also list the amount of carbohydrates per serving. It takes a little extra time and effort to look up the carbohydrate counts for all of the foods you eat, but with experience, you will begin to have a good idea of approximate calorie counts and carbohydrate counts.

Sources

Gropper SS, Smith JL, Groff JL. "Advanced Nutrition and Human Metabolism." Fourth Edition. Belmont, CA. Wadsworth Pub Co. 2005. Accessed April 15, 2016. 

National Academies of Sciences, Engineering and Medicine, Health and Medicine Division. "Dietary Reference Intakes Tables and Application." ​http://www.nationalacademies.org/hmd/Activities/Nutrition/SummaryDRIs/DRI-Tables.aspx.

Continue Reading