Carbohydrates fall into three main types based on their chemical structure: monosaccharides (simple sugars), disaccharides (double sugars), and polysaccharides (complex carbohydrates). The difference comes down to how many sugar molecules are linked together, which directly affects how quickly your body can break them down and use them for energy.
Monosaccharides: The Simplest Sugars
Monosaccharides are single sugar molecules, the smallest carbohydrate units. Your body can absorb them directly into the bloodstream without breaking them down further. The three most common ones are glucose, fructose, and galactose.
Glucose is the most abundant sugar in nature and the primary fuel your cells run on. Your blood sugar level is literally a measure of how much glucose is circulating in your blood at any given moment. Fructose, sometimes called fruit sugar, is the sugar that makes fruits and honey taste sweet. Galactose rarely shows up on its own in food but plays a key role as half of the sugar found in milk.
Every other type of carbohydrate, from table sugar to a bowl of rice, ultimately gets broken down into these monosaccharides before your body can use it.
Disaccharides: Two Sugars Linked Together
Disaccharides form when two monosaccharides bond together. Your body needs specific enzymes to split them apart during digestion. The three you encounter most often are sucrose, lactose, and maltose.
Sucrose is ordinary table sugar, made of one glucose molecule bonded to one fructose molecule. It’s found naturally in sugarcane, sugar beets, and many fruits. Lactose is the sugar in milk and dairy products, made of glucose plus galactose. People who are lactose intolerant don’t produce enough of the enzyme (lactase) needed to split lactose apart, so it passes undigested into the lower gut and causes bloating and discomfort. Maltose, or malt sugar, is two glucose molecules joined together. It forms naturally when starch begins to break down, which is why you can taste a slight sweetness if you chew plain bread long enough.
Polysaccharides: Long Chains of Sugar
Polysaccharides are large molecules built from hundreds of individual sugar units linked in long chains. They take significantly longer to digest than simple sugars, which means they release energy more gradually. The three most important polysaccharides are starch, glycogen, and cellulose, and all three are built entirely from glucose, just arranged differently.
Starch is how plants store energy. It’s especially concentrated in seeds, cereal grains, potatoes, and tubers. When you eat starchy foods, enzymes in your saliva and small intestine break the chains back down into individual glucose molecules. Digestion actually starts in your mouth: salivary amylase begins splitting starch apart the moment you start chewing.
Glycogen is the animal equivalent of starch. Your body assembles glucose into glycogen and stores it primarily in the liver (where it makes up 4% to 8% of the tissue’s weight) and in skeletal muscle. When you skip a meal or need a burst of energy, your body draws on these glycogen reserves first. During fasting, glycogen stores typically sustain you through roughly the first day before the body shifts to burning fat.
Cellulose is the structural material in plant cell walls. It’s made of the same glucose units as starch and glycogen, but the bonds linking them are arranged differently, and humans lack the enzyme to break those bonds. That means cellulose passes through your digestive tract intact. This is what we call dietary fiber.
Fiber: The Carbohydrate You Can’t Digest
Even though your body can’t extract calories from fiber, it plays a critical role in health. Fiber comes in two forms, and each does something different in your gut.
Soluble fiber dissolves in water and forms a gel-like material in your stomach that slows digestion. This slowing effect helps moderate blood sugar spikes after meals. Soluble fiber also helps lower LDL (“bad”) cholesterol by interfering with cholesterol absorption in the intestine. Good sources include oats, beans, flaxseed, and oat bran.
Insoluble fiber doesn’t dissolve in water. Instead, it adds bulk to stool and helps material move through the digestive tract more efficiently, which reduces constipation. You’ll find it in whole wheat, vegetables, and the skins of fruits.
Current U.S. dietary guidelines recommend 14 grams of fiber per 1,000 calories you eat. For someone on a 2,000-calorie diet, that works out to about 28 grams per day. Most people fall well short of this target.
Sugar Alcohols: A Category in Between
Sugar alcohols are a modified form of carbohydrate found in many sugar-free products. Common examples include xylitol, sorbitol, maltitol, and erythritol. They taste sweet but contain fewer calories than regular sugar because your body only partially absorbs them.
Regular table sugar provides 4 calories per gram. Most sugar alcohols provide roughly 2 to 2.7 calories per gram, about half as much. Erythritol is the outlier: it contains essentially zero calories because nearly all of it is absorbed and then excreted unchanged. The trade-off is that sugar alcohols, particularly in larger amounts, can cause digestive discomfort because the unabsorbed portion ferments in the lower gut.
How Carbohydrate Type Affects Blood Sugar
The type of carbohydrate you eat determines how fast and how high your blood sugar rises after a meal. This is what the glycemic index measures: how quickly a carbohydrate-rich food breaks down and enters the bloodstream compared to pure glucose.
White bread, for example, breaks down rapidly because its starch has been heavily processed and the fiber stripped away. The glucose hits your bloodstream fast. A bowl of lentils, by contrast, contains starch wrapped in fiber and protein, so digestion takes longer and blood sugar rises more gently. The glycemic index captures this speed, but it doesn’t account for portion size. That’s where glycemic load comes in: it combines the glycemic index with the actual amount of carbohydrate in a typical serving, giving a more realistic picture of a food’s impact.
In practical terms, choosing carbohydrates that are less processed, higher in fiber, and closer to their whole-food form tends to produce steadier energy and smaller blood sugar swings. This means favoring whole grains over refined grains, whole fruit over fruit juice, and legumes or starchy vegetables over sugary snacks.
Added Sugars and Current Guidelines
The most recent Dietary Guidelines for Americans (2025-2030) take a notably strict position on added sugars, stating that no amount of added sugar is considered part of a healthy diet. In practical terms, the guidelines recommend that no single meal contain more than 10 grams of added sugar. This is a significant reduction from the previous guideline, which set the ceiling at 10% of total daily calories, or about 50 grams per day on a 2,000-calorie diet.
Added sugars are chemically identical to the sugars found naturally in fruit or milk. The difference is context: naturally occurring sugars come packaged with fiber, vitamins, and minerals that slow absorption and provide nutritional value. Added sugars deliver calories with nothing else alongside them. Sweetened drinks, desserts, and processed snacks are the primary sources in most people’s diets.