A complex sugar is a carbohydrate made of many sugar units linked together in long chains, as opposed to simple sugars like glucose or table sugar, which contain only one or two units. Starch, glycogen, and cellulose are all complex sugars. The distinction matters because chain length determines how quickly your body can break the carbohydrate down and use it for energy.
How Complex Sugars Differ From Simple Ones
All carbohydrates are built from the same basic building blocks: single sugar molecules like glucose and fructose. Simple sugars contain just one or two of these units. Table sugar, for example, is two units bonded together. Complex sugars are macromolecules formed from many sugar units connected by chemical linkages, sometimes hundreds or thousands in a single chain. Some of these chains are straight, others branch out like a tree.
This structural difference has a direct effect on digestion. Your body can absorb simple sugars almost immediately, which is why a spoonful of honey hits your bloodstream fast. Complex sugars require enzymes to snip apart each link in the chain before the individual sugar units can be absorbed. That process takes time, which is why complex carbohydrates generally produce a slower, more gradual rise in blood sugar.
The Three Main Types
Most complex sugars you encounter fall into three categories, each with a different job in nature.
Starch is the storage form of energy in plants. It’s packed into seeds, grains, and tubers like potatoes. Starch accounts for more than 50% of the carbohydrates in the average human diet. It’s actually a mixture of two polymers: one that coils like a spring (with six glucose units per turn) and a branched version that your body can break apart even faster because enzymes can attack multiple branch points at once.
Glycogen is the animal equivalent of starch. Your liver stores 4% to 8% of its weight as glycogen, and your skeletal muscles store a smaller percentage. When you need quick energy between meals or during exercise, your body breaks glycogen back down into glucose. Structurally, glycogen is more heavily branched than starch, with branches every 8 to 12 glucose units. All that branching means your body can mobilize it rapidly.
Cellulose is the structural fiber in every plant cell wall. It’s made of the same glucose units as starch, but the chemical bonds linking them together face a different direction. That small difference makes cellulose completely indigestible by human enzymes. The glucose units lock into rigid, straight chains that pack tightly together through hydrogen bonding, forming the tough fibers you feel when you bite into raw celery. Cellulose passes through your digestive tract intact, which is exactly what makes it useful as dietary fiber.
How Your Body Breaks Them Down
Digestion of complex sugars starts in your mouth. Saliva contains an enzyme that begins splitting starch chains into shorter fragments. The process continues in the small intestine, where additional enzymes chop these fragments into individual glucose molecules that can pass through the intestinal wall into your bloodstream.
Fiber-type complex sugars like cellulose take a different path. Since human enzymes can’t break them down, they travel to the large intestine largely intact. There, trillions of gut bacteria possess specialized enzymes that hydrolyze complex carbohydrates into simpler sugars through a two-stage process: first breaking the long chains into individual sugar units, then fermenting those sugars. The fermentation produces short-chain fatty acids (primarily acetate, propionate, and butyrate) that serve as the primary energy source for the cells lining your colon.
Soluble vs. Insoluble Fiber
Not all dietary fiber behaves the same way. Soluble fiber, found in oats, beans, and many fruits, attracts water and turns into a gel during digestion. This gel slows the digestive process, which helps moderate blood sugar spikes after eating. Some types of soluble fiber also help lower the risk of heart disease.
Insoluble fiber, found in whole grains, nuts, and vegetables, doesn’t dissolve. Instead, it adds bulk to stool and helps food move more quickly through the stomach and intestines. Most whole plant foods contain both types in varying proportions.
Complex Sugars and Blood Sugar
The glycemic index (GI) measures how much a food raises blood sugar compared to pure glucose. A food with a GI of 28 raises blood sugar only 28% as much as pure glucose, while a food with a GI of 95 acts essentially like glucose itself.
Foods rich in intact complex carbohydrates tend to fall in the low range (GI of 55 or less): most fruits and vegetables, beans, minimally processed grains, pasta, and nuts. Moderate-GI foods (56 to 69) include potatoes, corn, white rice, and some breakfast cereals. Highly processed complex carbohydrates like white bread, bagels, and rice cakes can score 70 or above, because processing has already broken the long chains into shorter fragments that your body absorbs almost as quickly as simple sugar.
This is an important nuance. “Complex” doesn’t automatically mean “slow to digest.” A baked potato (complex carbohydrate) can spike your blood sugar faster than a handful of strawberries (which contain simple sugars but also fiber and water that slow absorption). The physical structure of the food, how much it’s been processed, and what you eat it with all matter as much as the sugar chain length.
How Much Fiber You Need
The U.S. Dietary Guidelines recommend 14 grams of fiber per 1,000 calories consumed. In practice, that works out to daily targets that vary by age and sex:
- Women ages 19 to 30: 28 grams
- Women ages 31 to 50: 25 grams
- Women 51 and older: 22 grams
- Men ages 19 to 50: 34 grams
- Men 51 and older: 28 grams
Dietary fiber is considered a nutrient of public health concern in the U.S. because most people fall well short of these targets. The richest sources are legumes, whole grains, vegetables, fruits, nuts, and seeds. Increasing your intake gradually (rather than all at once) gives your gut bacteria time to adjust and reduces the bloating and gas that can come with a sudden jump in fiber.