The building blocks of carbohydrates are sugars, starches and fiber, according to the U.S. Centers for Disease Control and Prevention. Sugars are simple carbohydrates found naturally in vegetables, fruits and milk or added to processed foods. Fiber and starch are complex carbohydrates found in vegetables, legumes and whole grains.
Simple carbohydrates are classified as monosaccharides or disaccharides, consisting of one or two linked sugars respectively, according to the Harvard School of Public Health Nutrition Source. These simple sugars have chemical structures the body can easily break down and digest to produce glucose, making them the fastest energy source. A nutrient-rich diet should include more natural sugars while avoiding added sugars, such as corn syrup and fruit juice concentrate.
Oligosaccharides and polysaccharides are complex carbohydrates, which contain at least three monosaccharides, the Nutrition Source states. These complex molecules take longer to digest because they have more chemical bonds to break down. During digestion, starch is broken down to its most basic components, giving the body another source of glucose, according to the CDC.
Fiber is an essential nutrient most often found in fibrous vegetables and grainy foods, such as potatoes, oatmeal and whole wheat bread, but the body does not digest it. Whole grains are a nutrient-rich source of fiber because they contain all three parts of the original grain: bran, germ and endosperm, the CDC states. Refined grains, such as white bread, lose a large portion of dietary nutrients, but food manufacturers can enrich these foods by adding nutrients after processing.