The Shift from Sugar to Fat
Under normal circumstances, your body's cells primarily rely on glucose, a simple sugar derived from carbohydrates, as their main energy source. This glucose is either used immediately or stored in the liver and muscles as glycogen for later use. However, when you drastically cut down on carbohydrates, the body is forced to find an alternative fuel source. This is the starting point for ketosis.
After a few days of consuming a very low-carb diet (typically less than 50 grams per day), the body's stored glycogen reserves become depleted. With glucose no longer readily available, the body's metabolic machinery shifts gears, signaling the liver to begin breaking down fat for energy. This process is known as ketogenesis, and it is here that the body reveals precisely what it burns during ketosis: fat.
The Role of Fat and Ketone Bodies
During ketosis, the fat burned for energy comes from two main sources: the fat you consume in your diet and your body's stored fat. As the liver breaks down fatty acids, it produces a new type of fuel molecule called ketone bodies. These molecules are released into the bloodstream and are eagerly used by the heart, muscles, and especially the brain for energy.
There are three main types of ketone bodies produced in the liver:
- Beta-hydroxybutyrate (BHB): This is the most abundant and most efficiently utilized ketone body, often comprising over 80% of the circulating ketones. Most tissues can readily convert BHB back into acetyl-CoA, which enters the Krebs cycle to produce ATP.
- Acetoacetate (AcAc): This ketone is the initial product of ketogenesis. It can be used directly for energy by cells or can be converted into BHB or acetone.
- Acetone: This is a less abundant and less important energy source, produced spontaneously from acetoacetate. It is often exhaled, leading to the characteristic fruity odor known as "keto breath".
Fuelling the Brain with Ketones
One of the most remarkable aspects of ketosis is the brain's ability to adapt its energy source. While the brain typically relies on glucose, it cannot directly use fatty acids for fuel. However, ketones can readily cross the blood-brain barrier and serve as a vital energy source when glucose is scarce. This provides a stable and efficient energy supply for the brain, with studies showing that after prolonged adaptation, ketones can provide up to 60% of the brain's energy needs. This switch to a more efficient, cleaner-burning fuel source may explain the improved mental clarity and cognitive function many people report during ketosis.
The Function of Gluconeogenesis in Ketosis
During ketosis, the body still requires a minimal amount of glucose to power specific cells and functions that cannot use ketones, such as red blood cells and parts of the kidney and brain. The body cleverly produces this necessary glucose through a process called gluconeogenesis (GNG). The liver and kidneys can manufacture new glucose from non-carbohydrate sources like lactate, glycerol (from fat breakdown), and certain amino acids.
Contrary to a common misconception, the body does not simply burn muscle tissue for this purpose, especially with an adequate protein intake. The body's innate wisdom prioritizes fat as the primary fuel in ketosis, and the rate of gluconeogenesis is carefully regulated to prevent muscle wasting. In fact, the production of ketones reduces the overall need for GNG, thereby sparing protein.
Ketosis vs. Standard Metabolism: A Comparison
| Feature | Standard (Glucose-Based) Metabolism | Ketosis (Fat-Based) Metabolism |
|---|---|---|
| Primary Fuel Source | Carbohydrates (broken down into glucose) | Fat (broken down into fatty acids and ketones) |
| Brain Fuel | Primarily glucose | Primarily ketones (up to 60%) |
| Hormonal State | Higher insulin levels to regulate blood sugar | Lower insulin and higher glucagon levels to signal fat release |
| Storage Method | Excess glucose stored as glycogen or converted to fat | Excess fat stored in adipose tissue, excess GNG glucose stored as glycogen |
| Fuel Efficiency | High speed, less efficient energy production (higher glucose fluctuations) | Slower, more stable, and more efficient energy production from fat |
| Source of Energy | Dietary carbohydrates and stored glycogen | Dietary fat and stored body fat |
The Power of Fat Adaptation
Ultimately, when asking what does the body burn during ketosis?, the answer is fat. This metabolic state signifies a profound physiological adaptation where the body and brain transition to using fat as their primary and most efficient energy source. The process is a testament to the body's evolutionary resilience, providing a backup fuel system that sustains life and performance even in the absence of carbohydrates. While the keto diet is the most popular way to induce this state, ketosis also occurs naturally during fasting or prolonged exercise.
For further reading on the biochemistry and therapeutic potential of ketone bodies, the National Institutes of Health provides extensive resources and reviews, such as this overview on ketone metabolism(https://www.ncbi.nlm.nih.gov/books/NBK554523/).
Conclusion
In conclusion, the metabolic state of ketosis is a powerful fat-burning process. It is characterized by the liver's production of ketone bodies from dietary and stored fat, which then circulate to fuel the body's tissues. While a minimal amount of glucose is still produced via gluconeogenesis for specific cellular needs, fat is the dominant energy source. This metabolic shift offers a steady and efficient alternative to glucose, with ketones providing a reliable fuel source for the brain and muscles. Understanding this process demystifies how a low-carbohydrate diet works and provides clarity on how the body's physiology adapts to burn fat for fuel.
