Which are the two main classes of complex carbohydrates? Starch and Fiber Explained

January 4, 2026 •

3 min read

According to nutrition experts, a healthy diet should derive a significant portion of its energy from complex carbohydrates. Which are the two main classes of complex carbohydrates? They are starches and dietary fiber, two types of polysaccharides that, while similar in their complex structure, play distinctly different roles in the human body.

Quick Summary

This article explores the two main classes of complex carbohydrates—starch and fiber—detailing their functions, digestion processes, and the food sources where they can be found.

Key Points

Starch and Fiber: The two main dietary classes of complex carbohydrates are starch and dietary fiber.
Function of Starch: Starches are broken down into glucose by the body, serving as the primary fuel for sustained energy.
Role of Fiber: Fiber is an indigestible complex carbohydrate that promotes digestive health, regulates blood sugar, and creates a feeling of fullness.
Digestion Speed: Complex carbohydrates are digested more slowly than simple carbs, which helps prevent rapid blood sugar spikes.
Nutrient-Dense Sources: Healthy sources include whole grains, legumes, and most vegetables, which contain both starch and fiber.
Processing Matters: Refined grains have been processed to remove fiber and nutrients, making them less healthy than their whole grain counterparts.
Glycogen: While a complex carbohydrate, glycogen is the storage form of glucose in animals, not a primary dietary class.

Understanding Complex Carbohydrates

Complex carbohydrates, also known as polysaccharides, are long chains of sugar molecules that take the body longer to break down than simple sugars. This slower digestion process results in a more gradual release of glucose into the bloodstream, providing sustained energy rather than the rapid spike and crash associated with simple carbs. Complex carbohydrates are an essential part of a balanced diet and are found abundantly in whole, unprocessed plant foods.

The Two Main Classes: Starch and Fiber

While all complex carbohydrates are polysaccharides, they are functionally categorized into two primary dietary classes: starches and dietary fiber. Though both are made from sugar molecules, the specific way these molecules are linked determines how the body interacts with them.

Starches: The Body's Primary Fuel Source

Starches are the storage form of carbohydrates in plants, and they represent the most common type of complex carbohydrate consumed by humans. When you eat starches, your body's enzymes can break them down into glucose, the primary fuel for your cells and brain. Digestion of starches begins in the mouth with salivary amylase and continues in the small intestine, providing a steady, long-lasting energy supply.

Food sources rich in starch include:

Whole Grains: Brown rice, oats, quinoa, whole-wheat bread, and pasta.
Legumes: Beans, lentils, and peas.
Starchy Vegetables: Potatoes, corn, and sweet potatoes.

Dietary Fiber: The Indigestible Powerhouse

Unlike starch, dietary fiber is a complex carbohydrate that the human body cannot break down into glucose for energy. It passes through the digestive system relatively intact, offering a variety of crucial health benefits. Fiber is divided into two main types:

Soluble Fiber: Dissolves in water to form a gel-like substance. It can help lower blood cholesterol and glucose levels. Sources include oats, beans, apples, and citrus fruits.
Insoluble Fiber: Does not dissolve in water and adds bulk to stool. It promotes regular bowel movements and helps prevent constipation. Sources include whole grains, nuts, and many vegetables.

Digestion and Health Benefits

The fundamental difference in how starch and fiber are processed by the body is key to understanding their health impacts. The slow digestion of starchy foods helps to stabilize blood sugar levels and promotes satiety, which can assist with weight management. Fiber, while not providing direct energy, is a critical component of gut health. It promotes the growth of beneficial gut bacteria and helps regulate digestion. Together, the two main classes of complex carbohydrates contribute significantly to overall wellness, reducing the risk of chronic diseases such as heart disease and type 2 diabetes.

Glycogen: The Animal-Based Counterpart

It is also worth noting that animals, including humans, store carbohydrates in the form of glycogen. Glycogen is a complex carbohydrate with a structure similar to starch but is not a dietary class consumed from plant-based foods. The body stores excess glucose as glycogen in the liver and muscles for quick energy reserves.

Comparison Table: Starch vs. Fiber


Feature	Starch	Dietary Fiber
Digestibility	Digestible by human enzymes	Indigestible by human enzymes
Function	Primary source of energy for the body	Promotes digestive health, gut bacteria, and satiety
Impact on Blood Sugar	Gradual and sustained rise	No direct impact, helps regulate overall levels
Caloric Value	Approximately 4 kcal per gram	Technically provides 0 kcal from absorption
Main Sources	Grains, legumes, root vegetables	Fruits, vegetables, legumes, whole grains

Making Healthier Carbohydrate Choices

Choosing whole food sources of complex carbohydrates is crucial for maximizing their nutritional benefits. Processed grains, such as white bread and white rice, have been stripped of much of their fiber and nutrients, causing them to behave more like simple carbohydrates in the body. Opting for whole grains, beans, and vegetables ensures a higher intake of both beneficial starches and fiber, along with essential vitamins and minerals.

For more guidance on healthy carbohydrate consumption, consult reliable resources like the CDC's Choosing Healthy Carbs guide.

Conclusion

In summary, the two main classes of complex carbohydrates found in our diet are starches and dietary fiber. Starches are digestible and provide the body with a slow, sustained release of energy, while dietary fiber is indigestible and supports essential digestive functions. A healthy, balanced diet should prioritize whole-food sources of these two vital components, harnessing their combined power for stable energy levels, improved digestion, and long-term health.

Frequently Asked Questions

Primary food sources for starches include whole grains (such as brown rice, oats, and whole-wheat bread), legumes (like beans, lentils, and peas), and starchy vegetables (including potatoes, corn, and sweet potatoes).

Dietary fiber is abundant in fruits, vegetables, legumes, nuts, seeds, and whole-grain products. For example, apples, berries, broccoli, beans, and oats are excellent sources.

The key difference is that starch is digestible by human enzymes, which break it down into glucose for energy. Fiber, however, is indigestible and passes through the digestive system largely intact, providing no calories but aiding health.

Complex carbohydrates are digested slowly, leading to a gradual and sustained release of glucose into the bloodstream. This helps prevent the sharp spikes in blood sugar that are typical with simple carbohydrates.

Fiber is a complex carbohydrate, but since the human body cannot break it down and absorb it for energy, it is considered to provide zero calories.

Yes, glycogen is a complex carbohydrate. However, it is the form in which animals (including humans) store glucose for future energy use, and it is not one of the two main dietary classes like starch or fiber.

Unrefined complex carbohydrates, such as whole grains, retain their bran and germ layers, which contain fiber, vitamins, and minerals. Refined versions are stripped of these beneficial nutrients during processing.