Carbohydrates are a class of macronutrient composed of Carbon, Oxygen, and Hydrogen atoms and are the main source of metabolic fuel for the human body. Carbohydrates (or carbs) make up approximately 40-50% of the calories in the average daily diet and can be broken down into 2 major groups:
1. Simple Carbohydrates (Such as sucrose, glucose, fructose, lactose, etc.)
2. Complex Carbohydrates (Such as dextrins and starches)
Simple carbohydrates are a group of carbs that are 1-2 saccharide units in length that can be broken down into two classifications:
- The simplest form of carbohydrates often referred to as the ‘simple sugars’ because they are structurally single saccharide (sugar) units. Multiple saccharide units can be linked together to build more complex carbohydrates (like the ones below). As a classification of carbohydrates, monosaccharides include glucose (the most abundant monosaccharide in nature), fructose (sugar source in fruits), and galactose.
- The second form of simple carbohydrates, disaccharides (di - two, saccharide - sugar) are molecules composed of two monosaccharides that are joined by a covalent bond. The most common disaccharide is sucrose, which is made up of one glucose molecule and one fructose molecule that are covalently bonded to each other (chemical bonding where pairs of electrons are shared between atoms). Other disaccharides include lactose (galactose bonded to glucose) and maltose (2 glucose molecules covalently bonded)
Complex carbohydrates are a group of carbs that are 3+ saccharide units in length and can be broken down into two classifications:
- A form of complex carbohydrates, oligosaccharides are molecules composed of between 3-10 saccharide units joined by covalent bonds. Common oligosaccharides include raffinose (trisaccharide – 3 saccharide units), stachyose (tetrasaccharide – 4 saccharide units), and verbascose (pentasaccharide – 5 saccharide units) all of which are made of up covalently bound glucose, fructose, and galactose molecules. Oligosaccharides are commonly found in beans, peas, whole wheat, and bran. Human digestive enzymes cannot hydrolyze oligosaccharides but bacteria in the intestine can digest them, this is the cause of flatulence that occurs after eating these foods.
- The most complex form of carbohydrates, polysaccharides are complex molecules composed of 10+ saccharide units of either the same monosaccharide unit repeated (homopolysaccharide) or 2 different saccharide units repeated (heteropolysaccharide). Polysaccharides can also be divided into 3 different groups:
- Starch (amylose and amylopectin) - present in potatoes, legumes, and vegetables
- Glycogen - The main form of stored carbohydrate found in liver (liver glycogen) or skeletal muscle (muscle glycogen)
- Cellulose – Major component of the cell wall in plants that is not digestible by human enzymes and therefore does not provide an adequate source of energy (calories) and is classified as dietary fiber. Click here to check out my article on dietary fiber.
Calories in Carbohydrates:
Carbohydrates as a macronutrient provide 4 kcals (commonly known as Calories) / 1 gram. This means that for every gram of carbohydrates you ingest you are getting 4 Calories of energy. For example, a meal with 50g of carbs in it supplies:
50g Carbohydrates x 4 Calories (kcals) = 200 Calories from Carbohydrates.
Common Sources of Carbohydrates:
- Glucose (Simple Sugar), Fructose (Fruit), Galactose (Sugar Beets, gums, and milk (in the form of lactose)
- Lactose (Milk), Maltose (Barley & Malts), Sucrose (Table Sugar)
- Artichoke, Leeks, Onions, Asparagus
- Starch (Wheat, Rice, Rye, Corn, Potatoes, Yams, Oats, Millet), Glycogen (Stored as Muscle and Liver Glycogen in the human body), Cellulose (Cell Wall of Green Plants)
There is a general overview of the different types of carbohydrates, for a more detailed breakdown be sure to check back for more in our Nutrition Articles.
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 Gropper, S.S., Smith, J.L., & Groff, J.L. (2009) Advanced Nutrition and Human Metabolism (5th Edition) United States: Wadsworth.
 Houston, M.E. (2006) Biochemistry Primer for Exercise Science (3rd Edition) United States: Human Kinetics.
 Long Island University (May 29, 2013). "The Chemistry of Carbohydrates"