Complex sugars are carbohydrates made up of long chains of simple sugar molecules linked together. Unlike simple sugars such as table sugar or fruit sugar, which contain just one or two sugar units, complex sugars contain dozens to thousands of glucose molecules bonded in chains that your body must break apart before it can use them. This slower breakdown is what gives complex sugars their key nutritional advantage: a more gradual, steady release of energy.
How Complex Sugars Differ From Simple Sugars
All carbohydrates are built from the same basic building blocks: single sugar molecules like glucose, fructose, and galactose. Simple sugars contain one or two of these units. Table sugar, for example, is just one glucose molecule bonded to one fructose molecule. Your body splits that bond almost instantly, sending sugar into your bloodstream quickly.
Complex sugars work differently. They’re chains of sugar units (usually glucose) connected by chemical bonds called glycosidic linkages. Short chains of fewer than about a dozen units are called oligosaccharides. Longer chains, sometimes containing thousands of units, are polysaccharides. These chains can be straight or branched, and the type of branching determines how your body handles them. Starch, glycogen, cellulose, and dietary fiber are all complex sugars, but each behaves very differently once you eat it.
The Three Major Complex Sugars
Starch
Starch is the most important complex sugar in the human diet, accounting for more than 50% of our total carbohydrate intake. Plants produce starch as their energy storage system, packing it into potatoes, grains, corn, and legumes. Chemically, starch is a mixture of two components: amylose, which forms straight chains coiled like a spring with six glucose units per turn, and amylopectin, which has branching side chains. Your body breaks both down into usable glucose using a family of enzymes called amylases.
Glycogen
Glycogen is the animal equivalent of starch. Your body converts excess blood sugar into glycogen and stores it primarily in two places: about 70% in skeletal muscle and much of the rest in the liver, where it makes up 4% to 8% of the liver’s weight. When you skip a meal or exercise, your body draws on glycogen reserves first, converting it back into glucose. Structurally, glycogen looks like a more densely branched version of amylopectin, with branches every 8 to 12 glucose units. This heavy branching allows your body to break it down quickly when it needs fast energy.
Cellulose
Cellulose is the structural fiber in plant cell walls and technically also a chain of glucose molecules. The critical difference is in how those molecules are linked. Cellulose uses a type of bond (called a beta linkage) that human digestive enzymes simply cannot break. This means cellulose passes through your system undigested. That’s not a waste, though. Cellulose is the main component of insoluble dietary fiber, and it plays a major role in digestive health.
How Your Body Digests Complex Sugars
Digestion of complex sugars starts in your mouth. Saliva contains an enzyme called salivary amylase that immediately begins chopping starch into smaller fragments. This enzyme works best at the near-neutral pH of your mouth and continues working briefly after you swallow, until stomach acid deactivates it.
The real heavy lifting happens in the small intestine. Your pancreas releases its own version of amylase (chemically identical to the salivary version) that breaks starch down into two-unit sugar fragments. Enzymes embedded in the lining of the small intestine then split those fragments into individual glucose molecules, which pass through the intestinal wall into your bloodstream. The entire process, from swallowing to absorption, takes considerably longer than digesting simple sugars, which is why complex carbohydrates produce a slower, more gradual rise in blood sugar.
Complex Sugars and Blood Sugar Control
When you eat any carbohydrate, your digestive system breaks the digestible portion into sugar that enters your blood. As blood sugar rises, your pancreas releases insulin to help cells absorb that sugar for energy or storage. Simple carbohydrates cause a fast spike in blood sugar and a correspondingly large insulin release. Complex carbohydrates take longer to digest, producing a more gradual rise instead.
The glycemic index (GI) is a scale that measures this effect. Foods rated 55 or below are considered low-glycemic, those between 56 and 69 are medium, and anything from 70 to 100 is high-glycemic. Most intact complex carbohydrates fall in the low to medium range. A bowl of steel-cut oats, for instance, releases its glucose slowly, keeping blood sugar relatively stable. Highly processed complex carbs like white bread, however, can score just as high as simple sugars because the processing has already broken down much of the structure your body would normally need time to digest.
Dietary Fiber: The Complex Sugar You Don’t Digest
Fiber is a complex sugar (or group of complex sugars) that resists digestion in the small intestine. It comes in two main forms, and each does something different in your body.
Soluble fiber dissolves in water and forms a thick, gel-like substance in your digestive tract. This gel slows stomach emptying and delays the absorption of nutrients, which blunts the blood sugar spike after a meal and lowers insulin demand. Soluble fiber also increases bile excretion, which pulls cholesterol out of your system and reduces LDL (“bad”) cholesterol levels. Good sources include oats, beans, lentils, and fruits like apples and pears.
Insoluble fiber does not dissolve in water and is barely fermented. Instead, it speeds up the passage of food through your digestive tract, adding bulk to stool and promoting regularity. Research shows a strong inverse relationship between insoluble fiber intake and the risk of type 2 diabetes. You’ll find it in whole wheat, nuts, seeds, and vegetables with edible skins. Foods labeled a “good source” of fiber contain at least 2.5 grams per serving, while those labeled “excellent source” provide more than 5 grams.
Resistant Starch: A Special Case
Some starch behaves more like fiber than like a typical complex carbohydrate. Called resistant starch, it passes through the small intestine undigested and reaches the large intestine intact, where gut bacteria ferment it. This fermentation produces short-chain fatty acids, particularly one called butyrate, which serves as the primary fuel for the cells lining your colon. Butyrate also has anti-inflammatory properties and may help reinforce the colon’s protective barrier.
Resistant starch shows up in foods in a few different ways. Whole grains and seeds contain starch that’s physically trapped inside the food matrix, making it hard for enzymes to reach. Raw potatoes and green bananas have tightly packed starch granules that resist digestion. Perhaps most interesting is retrograded starch: when you cook and then cool starchy foods like potatoes, pasta, or rice, the starch molecules realign into a crystalline structure that resists enzymatic breakdown. This means a cold potato salad contains more resistant starch than a freshly baked potato.
Regular intake of resistant starch promotes the growth of beneficial gut bacteria, which in turn produce more short-chain fatty acids and help maintain an acidic environment in the colon that discourages harmful microbes.
Best Food Sources of Complex Sugars
The highest-quality sources of complex sugars are whole, minimally processed foods that deliver fiber alongside their starch. These include:
- Legumes and pulses: lentils, black beans, kidney beans, chickpeas, split peas, and pinto beans. These are among the richest sources of both starch and fiber.
- Whole grains: oats, barley, bulgur, quinoa, brown rice, farro, and amaranth. Whole wheat pasta and whole grain cereals also count, as long as they’re genuinely made from intact or minimally refined grains.
- Starchy vegetables: potatoes, sweet potatoes, corn, and winter squash.
- Fruits and vegetables with edible skins or seeds: pears, apples, berries, and similar produce provide both soluble and insoluble fiber.
- Nuts and seeds: almonds, pumpkin seeds, sunflower seeds, and pistachios contribute fiber along with healthy fats.
The WHO’s most recent carbohydrate guidelines emphasize carbohydrate quality over quantity, focusing on how quickly a food’s polysaccharides release glucose, the proportion of sugars versus starches, and the amount of dietary fiber present. In practical terms, this means choosing whole grains over refined ones, eating legumes regularly, and getting fiber from a variety of plant sources rather than relying on supplements.