From an elemental point of view, all carbohydrates are created equal. That’s also true from a dietary point of view, as all digestible carbohydrates are broken down into glucose, the body’s primary source of energy 1. However, there are some meaningful differences between various carbohydrates based on how the basic building blocks are assembled.

Water + Carbon = Carbohydrates

Carbohydrates are constructed from carbon, oxygen and hydrogen atoms. The proportion is roughly a 1:2:1 ratio of carbon to hydrogen to oxygen. Since two hydrogens and an oxygen yield one water molecule, this proportion is called hydrate of carbon, from which the term carbohydrate is derived. Your body breaks down carbohydrates into glucose, which fuels the synthesis of adenosine triphosphate, or ATP, the body’s energy currency. When this complex three-phase process is complete, the carbon, hydrogen and oxygen atoms are recycled as water and carbon dioxide.

Simple Carbohydrates: Monosaccharides and Disaccharides

Monosaccharides, or simple sugars, are the smallest type of carbohydrate, containing three to seven carbon atoms. The most common monosaccharide is glucose, followed by fructose and galactose. Each one has six carbons. Free monosaccharides are not found in food in significant amounts. What you do find are disaccharides, which are two monosaccharides bonded together. Sucrose, or table sugar, combines glucose and fructose and is the most common disaccharide. The other common disaccharides are lactose and maltose. Lactose, the natural sugar found in milk, combines galactose and glucose, while maltose combines two glucose units.

Complex Carbohydrates: Oligosaccharides and Polysaccharides

Oligosaccharides are short chains of three to nine monosaccharide units. Trisaccharides, which have three monosaccharide units, are the type found most commonly in nature. Tetrasaccharides have four units, while pentasaccharides have five. Polysaccharides are long chains of 10 or more monosaccharide units. Some polysaccharides include hundreds or thousands of “links.” The polysaccharide glycogen is the major form of stored carbohydrate in human and animals. Its highly branched chains of glucose molecules are found in the liver and skeletal muscle tissues. From a nutrition point of view, the other important polysaccharides are starch, the storage carbohydrate of plants, and cellulose, which makes up plant cell walls. Both are also entirely made of glucose units.

Carbohydrate Digestion

Dietary starches and disaccharides are broken down by digestive enzymes into monosaccharides, which are then absorbed by cells in the intestinal wall. Shorter chain saccharides are generally digested more quickly, although this depends in part on what other foods are consumed in the same meal. Oligosaccharides are digested by bacteria in the intestine. Cellulose passes through your body undigested because you don’t have the correct enzyme to break the unique bonds between the glucose units.