An Introduction Carbohydrates

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This article provides an introduction to carbohydrates. Carbohydrates are one of the three macronutrients, a group that also includes protein and fat. This information can be used to understand the complexities of carbohydrates and their role in the body, and to support your discussions with parents on the value of this important nutrient as well as healthy food sources.


Carbohydrates play a crucial role in a healthy, balanced diet. For example, without carbohydrates, our body would lack a key fuel source. Similar to fats, the role and value of carbohydrates is often misunderstood, which can lead to a diet that restricts this important nutrient rather than including healthy food choices to provide carbohydrates in recommended amounts.

Carbohydrates are the body's primary source of energy, and provide about 4 calories per gram. This nutrient category includes sugars, starches, and fiber. The American Dietetic Association (ADA) recommends that for children 2–11 years of age, carbohydrates should make up 45–65% of total energy intake each day. Following these recommendations, approximately one half of total daily energy intake comes from carbohydrate, with the other half from protein and fat.

A principal role of carbohydrate is to supply energy in the form of glucose. Glucose is a simple sugar and is often called blood sugar, since it is the main carbohydrate found in the blood of mammals. The health and functioning of every cell relies on blood glucose.

Carbohydrates are made up of units of sugar (also called saccharides), and are classified as either simple or complex, depending on the number of sugar units they contain.

Simple carbohydrates

Simple carbohydrates are those that contain only one sugar unit (monosaccharides) or two sugar units (disaccharides). Glucose and fructose are the two most common monosaccharides. Glucose is the primary form of sugar stored in the human body for energy, and fructose is the main sugar found in most fruits. The most common disaccharide is sucrose, which is table sugar.

Simple Carbohydrate What it does Food sources

Monosaccharides

Glucose (dextrose)
"Blood sugar"
  • Monosaccharides are absorbed into the body through the walls of the intestine. They are passed into the bloodstream and stored to be used for energy.
  • Corn, rice
  • Table sugar
Fructose (levulose)
"Fruit Sugar
  • Sweetest of all naturally occurring carbohydrates
  • Absorbed int hte small intestine where it is directed to the hepatic portal vein to the liver
  • Honey, berries, melons, tree fruits and juices, sugar cane, sugar beets
Gelactose
  • Due to the enhanced nutrient properties of galactose, it is classified as a nutritive sweetener
  • Dairy products, sugar beets; also sythensized in the body
  • Present in breastmilk

Disaccharides

Sucrose
(glucose + fructose)
"Table sugar"
  • Activity of intestinal disaccharides is required for sucrose absoption as well as for lactose and maltose absorption.
  • Sugar cane, sugar beets
Lactose
(glucose + galactose)
"Milk Sugar"
  • Disaccharidase activity is required for digestion. Human milk has higher concentrations of lactose than cow's, so human milk has a sweeter taste.
  • Major sugar in milk and milk products
  • Present in breastmilk
Maltose
(glucose + glucose)
"Malt sugar"
  • Disaccharidase activity is required for digestion. Majority of maltose in the intestine is from starch digestion.

Complex carbohydrates

Complex carbohydrates are composed of more than two sugar units, and include oligosaccharides (3–10 units of simple sugar) and polysaccharides (more than 10 units of simple sugar). Complex carbohydrates can be digestible, like starches, or not digestible, like dietary fiber, based on how the monosaccharides are linked to make polysaccharides.

Starch and fiber (or cellulose) are both complex carbohydrates. Starch is broken down into simple carbohydrates to be used by the body. Because both soluble and insoluble fiber are undigested, they do not serve as energy sources. Insoluble fiber remains intact during digestion, and aids digestion by providing bulk in the stool. Soluble fiber, which forms a gel when combined with a liquid, helps regulate cholesterol levels and blood sugar. An oligosaccharide is a saccharide polymer. Oligosaccharides reach the colon almost completely undigested, where they may act as food for intestinal microbiota (also known as microflora).

Prebiotics are carbohydrates such as oligosaccharides. Prebiotics allow specific changes in the gastrointestinal microbiota, such as supporting a healthy balance of bacteria in the digestive tract. This balance in turn provides benefits to the individual including support for healthy immune system function. 

Approximately 1/3 of dietary fiber occurs as hemicellulose, and ¼ to 1/3 as cellulose. Approximately 15–20% of dietary fiber is present as pectin.

Until recently, functions of a specific type of fiber were determined by whether or not the fiber was classified as "soluble" or "insoluble". Soluble fibers, such as the type found in oat bran, have been shown to reduce blood cholesterol levels and help normalize blood sugar levels, while insoluble fibers, such as the type found in wheat bran, are known to promote bowel regularity. But the effect of fiber on the GI tract may depend more on the properties of the specific fiber – especially the viscosity and fermentability. The terms "viscous fiber" or "fermentable fiber" therefore may better describe the effect on the GI tract than the older terms. 

  • Many studies have shown a beneficial effect of viscous fibers on plasma glucose and insulin response. Viscous fibers help normalize blood glucose levels by slowing the rate at which food leaves the stomach and by delaying the absorption of glucose following a meal. 
  • Fermentable fibers are fermented by the beneficial bacteria that inhabit the large intestine, and help maintain these healthy populations. 
  • Fibers that are not fermentable in the large intestine help maintain bowel regularity by increasing the bulk of the feces and decreasing the transit time of fecal matter through the intestines.
Complex Carbohydrate What it does Food sources
Starch
  • Converted to glucose to provide energy, helps stabilize blood sugar
  • Breads, cereals, pasta, rice, potatoes
Glycogen
  • Glucose that is not used immediately is converted into glycogen for storage in the liver and muscles

Dietary Fibers

Oligosaccharides
  • cellulose - not soluble
  • hemicellulose - some soluble, some not soluble
  • pectins - soluble
  • gums or mucilages - soluble
  • lignings - not soluble
  • May function as prebiotics to feed intestinal microbiota
  • Bran, whole grains, oatmeal, barley, chicory, onions, asparagus, leeks, strawberries, legumes, root vegetables, citrus, fruits, apples
Fermentable fiber
  • Helps lower LDL and total cholesterol, helps regular blood sugar
  • Helps maintain a blanced microbiota
  • Oats/oat bran, beans, nuts, legumes, barley, some fruits and vegetables.
Non-fermentable fiber
  • Aids digestion, provides bulk in the stool, helps maintain healthy GI tract function
  • Whole grains, wheat and corn bran, some fruits and vegetables, seeds and nuts
Viscous fiber
  • Helps lower LDL and total cholesterol, helps regular blood sugar
  • Aids digestion, provides bulk in the stool, helps maintain healthy GI tract function
  • Whole grains, wheat and corn bran, some fruits

Conclusion

Carbohydrates, like fats, are an often misunderstood nutrient such that individuals may pursue a diet that restricts carbohydrates as a whole, rather than choosing foods that provide this valuable nutrient in recommended amounts. Understanding the role of carbohydrates in a healthy diet, including the value of dietary fiber, will help parents to make sound food choices for their children. As with protein and fat, a healthy diet includes a balance of nutritious sources of carbohydrates. A diet that is low or deficient in this critical nutrient is truly not "balanced", and is in need of correction to best support overall health.

References:
American Academy of Pediatrics Committee on Nutrition. Kleinman RE, ed. Pediatric Nutrition Handbook. 6th Edition. © 2009 American Academy of Pediatrics.

Dietary Reference Intakes (DRIs): Recommended Intakes for Individuals, Macronutrients
Food and Nutrition Board, Institute of Medicine, National Academies
http://fnic.nal.usda.gov/nal_display/index.php?info_center=4&tax_level=3&tax_subject=256⊤ic_id=1342≤vel3_id=5140

U.S. Department of Health and Human Services and U.S. Department of Agriculture. Dietary Guidelines for Americans, 2010. 
http://www.cnpp.usda.gov/DGAs2010-DGACReport.htm

Shils ME et al., eds. Modern Nutrition in health and Disease. Philadelphia, PA: Lippincott Williams & Wilkins, 2006.

American Dietetic Association. Position of the American Dietetic Association: Nutrition Guidance for Healthy Children Ages 2 to 11 Years. JADA 2008;108(6):1038–47.

Britannica Concise Encyclopedia http://www.britannica.com/EBchecked/topic/94687/carbohydrate

 
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