Understanding Complex Carbohydrates

Baskets full of white potatoes
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Complex carbohydrates are an important source of energy for your body. They provide your body with the sustained fuel needed to exercise, perform activities of daily living, and even to rest. These carbohydrates are found in foods such as grains, vegetables, legumes, nuts, and seeds. Complex carbohydrates may also provide vitamins and minerals.

Different Types of Carbohydrates

There are two main types of carbohydrates: simple carbohydrates and complex carbohydrates. The difference between the two is simply the size of number of molecules.

Simple carbohydrates are made up of only one (monosaccharide) or two (disaccharide) sugar units. Simple sugars include fructose, sucrose, glucose, maltose, galactose, and lactose. These may be found in table sugar, candy, honey, and syrups. Simple sugars provide quick energy.

Complex carbohydrates are many single (monosaccharide) units that are linked together. Oligosaccharides contain between two and 10 simple sugar units. Polysaccharides contain hundreds and even thousands of linked monosaccharides. Complex carbohydrates provide the body with relatively sustained energy. The most important food source of carbohydrates is starches, and they need to be broken down into monosaccharides for absorption.

Complex Carbs
  • Peas, beans, and other legumes

  • Whole grains

  • Starchy vegetables

  • Breads

  • Cereals

  • Whole grain pastas

Simple Carbs
  • Table sugar

  • Honey

  • Maple syrup

  • Candy

  • Soft drinks

  • Milk products

  • Fruit

Types of Complex Carbohydrate


Dietary fiber is the fiber that is naturally found in plant cells. The cell wall contains more than 95% of dietary fiber components, including cellulose, hemicellulose, lignin, pectin, and some nonstarchy polysaccharides. Cellulose forms the structures that give plants their shape and it's the main component of dietary fiber. Vegetables such as green beans, broccoli, and spinach contain starch, but they have more cellulose or fiber. 

Your digestive system can't break cellulose apart. That means that having non-digestible fiber in your digestive tract slows things down. Gastric emptying (the speed at which food leaves your stomach) slows down as does the absorption of sugar into your bloodstream.

Fiber may be soluble (dissolvable in water) or insoluble. Soluble fiber helps slow digestion and soften stools, and may reduce the risk of heart disease by helping to lower cholesterol levels. Insoluble fiber helps bulk up stool and move it through the colon, which can help ease or prevent constipation. Large research studies show that high intake of insoluble fiber or whole grains may reduce insulin resistance and the risk of developing type 2 diabetes by 20% to 30%.

Soluble fiber is found in oats, citrus fruits, apples, barley, psyllium, flax seeds, and beans. Insoluble fiber is found in whole grains, nuts, and vegetables. You need both kinds, but as long as you eat a varied diet with plenty of high-fiber foods, you should get enough of both types without having to track intake. However, most Americans do not get enough fiber overall. Women should aim for 25 grams per day; for men, the goal is 38 grams.


Starch is used by plants to store energy. Starches are found naturally in some foods such as garbanzo beans, lentils, barley, corn, oats, and wheat. Some vegetables also contain starch such as carrots and potatoes. Starches can also be added to foods during processing or in preparation to thicken or stabilize them. 

Starches are digested and absorbed by the human body relatively quickly—although not as quickly as simple sugars. As a result, starchy foods like white bread and pasta can result in a blood sugar spike, like eating something that's high in sugar.

Some people may need to modify their carbohydrate intake. For example, people with type 2 diabetes have difficulty metabolizing carbohydrates and therefore benefit from following a modified carbohydrate diet that is rich in fiber.

Carbohydrate Dense Foods

  • Ramen noodles, any flavor (49g per packet, dry)
  • Russet potatoes (30g per small baked potato)
  • Cooked brown rice (25g of carbohydrate per 1/2 cup)
  • Cooked white rice (22g per 1/2 cup)
  • Wheat crackers (22g per 1/2 cup)
  • Pasta, cooked (21g per 1/2 cup)
  • Pretzels (16g per 1/2 cup)
  • Cream of wheat (16g per 1/2 cup cooked)
  • Instant oats (14g per 1/2 cup cooked)
  • Pancake mix, complete (14g per 4-inch pancake)
  • Corn Flakes cereal (11g per 1/2 cup)
  • Tortilla chips (10g per 1/2 cup)

Source: USDA FoodData Central

How Starch Impacts Digestion

Not all starches are created equal. Some are digested very quickly and cause a more rapid rise in blood sugar. Others are digested more slowly, causing blood glucose to rise less and over a longer period of time. The larger the percentage of rapidly digested starch in a starchy food, the higher the glycemic index of that food.

Some starch, called resistant starch, is not digested in the small intestine at all and causes little or no blood sugar rise.

Starch Structure

Different kinds of starch have different arrangements of molecules, and some are easier for digestive enzymes to get at than others. One kind of starch, called amylose, is broken down quite slowly. The higher the amount of amylose in a starch, the more slowly it is digested.

For example, different types of rice have differing percentages of amylose. Long grain rice is higher in amylose. Shorter grain rice is low in amylose, which means it is digested more quickly (it is more glycemic). Beans contain cellulose, which is a type of fiber. So they are digested very slowly.

Processing Methods

Some of the grains we consume have been processed. For example, many wheat products have been processed by grinding, puffing, flaking, etc.

When grains are processed, the whole grain (which consists of the bran, germ, and edosperm) is taken apart, and a large amount of the fiber is removed. That limits the amount of work that has to be done by the digestive system. Refined starches are the most rapidly digested starches.

If grains or legumes, such as beans, brown rice, or barley, remain whole, the starch is broken down into sugars much more slowly. In fact, some starches are not turned into sugar at all but reach the large intestine intact—these are called resistant starches.

Other Factors

Pasta is often considered to be high in starch, but there are other factors to consider.

Starch molecules in pasta are so tightly packed that only about half is rapidly digested when the pasta is cooked al dente or slightly firm. Cooking time and the thickness of the pasta greatly affects the glycemic index.

Additionally, when some cooked starches, such as certain types of potatoes and rice, are cooked and cooled, a small percentage of the starch takes longer to digest.

How to Measure the Impact

Measuring the exact impact of a complex carb (or any food) on your body can be tricky. It is difficult to know how quickly any one person will digest any individual food or when each part of the process occurs.

Additionally, each person's digestive system is a little different, and factors such as how thoroughly the food is chewed and what other foods are eaten with it also have an effect.

There are various means of testing that are not standardized. The only real way for a person to know how glycemic a food is for their body is to monitor blood glucose.

Factors including structure, processing methods, and the way you cook your food can affect how quickly a starchy food is digested. Individual eating variations (including how much you chew your food) can also have an impact.

Tips for Choosing Foods With Starch

When choosing starchy foods, there are a few guidelines to keep in mind.

  • The starch in whole beans and lentils is either slowly digested starch or resistant starch. However, due to processing methods, canned beans are slightly more quick to digest than beans cooked from the dried state.
  • When choosing grains, chose those that are whole and intact when cooked, such as brown rice, barley, amaranth, or quinoa.
  • Limit baked goods such as cookies and cakes, which tend to be higher in sugar. Choose breads and other types of starches that are made with whole grains, as these will contain more fiber.
  • Choose breakfast cereals that contain at least 3 grams of fiber per serving. Hot cereals, such as oatmeal, are often good choices.

Lastly, consider foods that provide other nutritional benefits. For example, fruits and vegetables provide carbohydrate along with important vitamins. And you'll get a range of minerals in many grains.

16 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Laura Dolson
Laura Dolson is a health and food writer who develops low-carb and gluten-free recipes for home cooks.