Question

when someone enters into ketosis what changes happen in the body as far as switching its primary energy source from carbohydrates to lipids? what happens on a cellular level? how do the molecules produce energy?

Answer 1

The body has an ability to store excess glucose in the form of glycogen. In this way, energy can be stored for later use. Glycogen consists of long chains of glucose molecules and is primarily found in the liver and skeletal muscle. Liver glycogen stores are used to maintain normal levels of glucose in the blood while muscle glycogen stores are mainly used to fuel muscle activity.

Carbohydrates are the body’s primary source of glucose and other sugar molecules such as fructose and galactose. During carbohydrate restriction, both proteins and fats can be used for energy. In fact, most cells can use fatty acids for energy, but brain cells and developing red cells are more dependent on glucose for energy supply. However, brain cells can adapt and use ketones from fat breakdown for its energy needs.

When no carbohydrate is available, the liver will not break down fat completely. Instead, it produces ketone bodies that are used by most cells to provide energy. When ketone bodies are produced more quickly than the body needs, ketone levels build up in the blood, resulting in a condition known as ketosis. Ketosis is most commonly caused by very low carbohydrate consumption or prolonged fasting.

When the body’s glycogen stores become depleted, the breakdown of body fat (mainly triglycerides) results in increased availability of fatty acids. Most cells can use fatty acids for energy production. However, many fatty acids can not pass the blood-brain barrier. Therefore, the brain becomes dependent on ketone bodies produced by the liver.

The breakdown of fatty acids results in the production of an important substance called acetyl CoA. When fat and carbohydrate metabolism is in the balance, most acetyl CoA enters the so-called citric acid cycle (Krebs cycle) where it is used for energy production. When acetyl CoA cannot enter the citric acid cycle, it is shunted to form ketone bodies. This process is called ketogenesis.

Ketone bodies include three compounds: acetone, acetoacetate, and beta-hydroxybutyrate. Acetone can sometimes be smelled from the breaths of people with high levels of ketone bodies in the blood. You may be acquainted with the smell because some nail polish removers contain acetone.

Ketone bodies are not only produced when the glycogen stores become depleted. In fact, ketone bodies are produced by the liver all the time. Research indicates that the heart and kidneys prefer to use ketone bodies rather than glucose as a fuel resource. To dispose of excess ketone bodies, the body uses the kidneys to excrete them in urine, and they are exhaled from the lungs. During ketosis, ketones can easily be detected in the urine.