In the nutrition world, they’re one of the most controversial topics.

Most of the time, the brain uses almost exclusively glucose for fuel.
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.
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. [...] 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
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.
[...] By using ketones instead of glucose, 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.
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.
As viscous 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% (10Trusted Source).

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 (11Trusted Source).

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 (12Trusted Source).
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 (2Trusted Source).

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.