The two types of carbohydrates are simple carbohydrates and complex carbohydrates. The difference comes down to chemical structure and how fast your body breaks them down. Simple carbs are short sugar molecules that digest quickly and tend to spike blood sugar. Complex carbs are longer chains that take more time to break down, giving you steadier energy. Current guidelines recommend that 45 to 65 percent of your daily calories come from carbohydrates, with the emphasis on complex sources.
Simple Carbohydrates
Simple carbohydrates are made up of one or two sugar molecules. The single-molecule sugars include glucose (the main fuel your cells use) and fructose (the sugar in fruit). When two of these small sugars bond together, you get things like sucrose (table sugar, which is glucose plus fructose) and lactose (milk sugar, which is glucose plus galactose). Because these molecules are already so small, your digestive system breaks them apart fast, sending glucose into your bloodstream in a rush.
That rapid digestion is why simple carbs are closely tied to blood sugar spikes. A serving of white rice, for example, raises blood sugar almost as much as eating pure table sugar. Foods high in simple carbs include candy, soda, fruit juice, syrups, and many processed snacks. Fruit also contains simple sugars, but its fiber slows digestion enough to blunt the spike, which is why whole fruit and a glass of juice affect your body very differently.
Refined grains sit in an interesting middle ground. Grains like wheat start as complex carbohydrates, but processing strips away the fiber, and sometimes key nutrients along with it. The CDC notes that this processing is done to extend shelf life and reduce cost, but the end result behaves more like a simple carb in your body. White bread and white pasta are common examples.
Added Sugar Limits
The Dietary Guidelines for Americans recommend keeping added sugars below 10 percent of daily calories. For a typical 2,000-calorie diet, that works out to about 50 grams per day, roughly 12 teaspoons. People who eat fewer calories, including younger children and many women, need to stay well under that number. “Added sugars” refers to sweeteners put into food during manufacturing or cooking, not the sugars naturally present in fruit or milk.
Complex Carbohydrates
Complex carbohydrates are long chains of sugar molecules linked together, sometimes hundreds or thousands of units long. Your body has to work through those chains link by link, which is why digestion takes longer and blood sugar rises gradually instead of spiking. There are two major categories: starches and fiber.
Starches
Starch is the form of energy storage plants use. It comes in two shapes. One is a straight chain of roughly 500 to 20,000 glucose units. The other is a massive branched structure containing one to two million glucose units. Your body produces enzymes that can clip apart both forms, eventually turning all that starch into glucose for energy. Because the chains are so long, the process takes time. A serving of lentils, for instance, produces a much slower and smaller blood sugar rise than white rice or table sugar.
Good sources of starch include whole grains (oats, brown rice, quinoa), legumes (lentils, chickpeas, black beans), and starchy vegetables like sweet potatoes and corn.
Fiber
Fiber is the portion of plant cell walls that your body can’t fully digest. Unlike starch, you don’t produce the enzymes needed to break it down into absorbable sugar. That might sound useless, but fiber plays critical roles in your health precisely because it passes through largely intact.
There are two types of fiber, and they do different things. Soluble fiber dissolves in water and forms a gel-like substance that slows digestion. It’s linked to lower cholesterol, better blood sugar control, and a decreased risk of heart disease. You’ll find it in oats, barley, seeds, legumes, and some vegetables. Insoluble fiber doesn’t dissolve. It acts as bulk that helps food and waste move through your gut more easily, keeping things regular. Whole grains, beans, and root vegetables are rich sources.
Most plant foods contain both types in varying amounts, so eating a variety of whole grains, fruits, vegetables, and legumes covers your bases without needing to track each one separately.
How They Affect Blood Sugar Differently
The glycemic index (GI) is a scale that measures how much a food raises blood sugar compared to pure glucose. Pure glucose scores 100. A food with a GI of 28 raises blood sugar only 28 percent as much, while a food scoring 95 acts almost identically to straight glucose. Most simple carbs and refined grains land high on the scale. Most intact complex carbs, especially those rich in fiber, score low.
This matters because high-GI foods create a roller coaster pattern: blood sugar shoots up, your body releases a large burst of insulin to bring it down, and you can end up feeling hungry or tired shortly after eating. Low-GI foods produce a gentler curve, with steadier energy and a more gradual insulin response. Over time, consistently choosing lower-GI carbohydrates is associated with better blood sugar management and more stable energy levels throughout the day.
Choosing Between the Two
The practical takeaway is straightforward. You don’t need to eliminate simple carbohydrates entirely (whole fruit is a simple carb, and it’s packed with vitamins and fiber), but the bulk of your carbohydrate intake should come from complex, minimally processed sources. Whole grains, legumes, vegetables, and fruits give you the glucose your body needs while delivering fiber, vitamins, and minerals along with it.
When reading labels, watch for added sugars under names like sucrose, high-fructose corn syrup, and dextrose. And when choosing grain products, look for “whole grain” as the first ingredient rather than “enriched flour,” which signals that the grain has been refined and had some nutrients added back in after the fact.