Biologically speaking, carbohydrates are molecules that contain carbon, hydrogen and oxygen atoms in specific ratios.
But in the nutrition world, they’re one of the most controversial topics.
Some believe eating fewer carbohydrates is the way to optimal health, while others prefer higher-carb diets. Still, others insist moderation is the way to go.
No matter where you fall in this debate, it’s hard to deny that carbohydrates play an important role in the human body. This article highlights their key functions.

Complex Carbohydrates
One of the primary functions of carbohydrates is to provide your body with energy.
Most of the carbohydrates in the foods you eat are digested and broken down into glucose before entering the bloodstream.
Glucose in the blood is taken up into your body’s cells and used to produce a fuel molecule called adenosine triphosphate (ATP) through a series of complex processes known as cellular respiration. Cells can then use ATP to power a variety of metabolic tasks.
Most cells in the body can produce ATP from several sources, including dietary carbohydrates and fats. But if you are consuming a diet with a mix of these nutrients, most of your body’s cells will prefer to use carbs as their primary energy source (1).
SUMMARY:One of the primary functions of carbohydrates is to provide your body with energy. Your cells convert carbohydrates into the fuel molecule ATP through a process called cellular respiration.

If your body has enough glucose to fulfill its current needs, excess glucose can be stored for later use.
This stored form of glucose is called glycogen and is primarily found in the liver and muscle.
The liver contains approximately 100 grams of glycogen. These stored glucose molecules can be released into the blood to provide energy throughout the body and help maintain normal blood sugar levels between meals.
Unlike liver glycogen, the glycogen in your muscles can only be used by muscle cells. It is vital for use during long periods of high-intensity exercise. Muscle glycogen content varies from person to person, but it’s approximately 500 grams (2).
In circumstances in which you have all of the glucose your body needs and your glycogen stores are full, your body can convert excess carbohydrates into triglyceride molecules and store them as fat.
SUMMARY:Your body can transform extra carbohydrates into stored energy in the form of glycogen. Several hundred grams can be stored in your liver and muscles.

Glycogen storage is just one of several ways your body makes sure it has enough glucose for all of its functions.
When glucose from carbohydrates is lacking, muscle can also be broken down into amino acids and converted into glucose or other compounds to generate energy.
Obviously, this isn’t an ideal scenario, since muscle cells are crucial for body movement. Severe losses of muscle mass have been associated with poor health and a higher risk of death (3).
However, this is one way the body provides adequate energy to the brain, which requires some glucose for energy even during periods of prolonged starvation.
Consuming at least some carbohydrates in the diet is one way to prevent this starvation-related loss of muscle mass. These carbs will reduce muscle breakdown and provide glucose as energy for the brain (4).
Other ways the body can preserve muscle mass without carbohydrates will be discussed later in this article.
SUMMARY:During periods of starvation when carbohydrates aren’t available, the body can convert amino acids from muscle into glucose to provide the brain with energy. Consuming at least some carbs can prevent muscle breakdown in this scenario.

Unlike sugars and starches, dietary fiber is not broken down into glucose.
Instead, this type of carbohydrate passes through the body undigested. It can be categorized into two main types of fiber: soluble and insoluble.
Soluble fiber is found in oats, legumes and the inner part of fruits and some vegetables. While passing through the body, it draws in water and forms a gel-like substance. This increases the bulk of your stool and softens it to help make bowel movements easier.
In a review of four controlled studies, soluble fiber was found to improve stool consistency and increase the frequency of bowel movements in those with constipation. Furthermore, it reduced straining and pain associated with bowel movements (5).
On the other hand, insoluble fiber helps alleviate constipation by adding bulk to your stools and making things move a little quicker through the digestive tract. This type of fiber is found in whole grains and the skins and seeds of fruits and vegetables.
Getting enough insoluble fiber may also protect against digestive tract diseases.
One observational study including over 40,000 men found that a higher intake of insoluble fiber was associated with a 37% lower risk of diverticular disease, a disease in which pouches develop in the intestine as a result of straining during bowel movements (6).
SUMMARY:Fiber is a type of carbohydrate that promotes good digestive health by reducing constipation and lowering the risk of digestive tract diseases.

Certainly, eating excessive amounts of refined carbs is detrimental to your heart and may increase your risk of diabetes.
However, eating plenty of dietary fiber can benefit your heart and blood sugar levels (7, 8, 9).
As soluble fiber passes through the small intestine, it binds to bile acids and prevents them from being reabsorbed. To make more bile acids, the liver uses cholesterol that would otherwise be in the blood.
Controlled studies show that taking 10.2 grams of a soluble fiber supplement called psyllium daily can lower “bad” LDL cholesterol by 7% (10).
Furthermore, a review of 22 observational studies calculated that the risk of heart disease was 9% lower for each additional 7 grams of dietary fiber people consumed per day (11).
Additionally, fiber does not raise blood sugar like other carbohydrates do. In fact, soluble fiber helps delay the absorption of carbs in your digestive tract. This can lead to lower blood sugar levels following meals (12).
A review of 35 studies showed significant reductions in fasting blood sugar when participants took soluble fiber supplements daily. It also lowered their levels of A1c, a molecule that indicates average blood sugar levels over the past three months (13).
Although fiber reduced blood sugar levels in people with prediabetes, it was most powerful in people with type 2 diabetes (13).
SUMMARY:Excess refined carbohydrates can increase the risk of heart disease and diabetes. Fiber is a type of carbohydrate that is associated with reduced “bad” LDL cholesterol levels, a lower risk of heart disease and increased glycemic control.

As you can see, carbohydrates play a role in several important processes. However, your body has alternative ways to carry out many of these tasks without carbs.
Nearly every cell in your body can generate the fuel molecule ATP from fat. In fact, the body’s largest form of stored energy is not glycogen — it’s triglyceride molecules stored in fat tissue.
Most of the time, the brain uses almost exclusively glucose for fuel. However, during times of prolonged starvation or very low-carb diets, the brain shifts its main fuel source from glucose to ketone bodies, also known simply as ketones.
Ketones are molecules formed from the breakdown of fatty acids. Your body creates them when carbs are not available to provide your body with the energy it needs to function.
Ketosis happens when the body produces large amounts of ketones to use for energy. This condition is not necessarily harmful and is much different from the complication of uncontrolled diabetes known as ketoacidosis.
By using glucose instead of ketones, the brain markedly reduces the amount of muscle that needs to be broken down and converted to glucose for energy. This shift is a vital survival method that allows humans to live without food for several weeks.
However, even though ketones are the primary fuel source for the brain during times of starvation, the brain still requires around one-third of its energy to come from glucose via muscle breakdown and other sources within the body (14).
SUMMARY:The body has alternative ways to provide energy and preserve muscle during starvation or very low-carb diets.

Carbohydrates serve several key functions in your body.
They provide you with energy for daily tasks and are the primary fuel source for your brain’s high energy demands.
Fiber is a special type of carb that helps promote good digestive health and may lower your risk of heart disease and diabetes.
In general, carbs perform these functions in most people. However, if you are following a low-carb diet or food is scarce, your body will use alternative methods to produce energy and fuel your brain.