The Fundamental Shift in Fuel Sourcing
In a standard, carbohydrate-rich diet, the body's cells rely primarily on glucose for energy. Glucose is readily available from the breakdown of ingested carbohydrates and is stored in the liver and muscles as glycogen for future use. However, when following a ketogenic diet or during a period of fasting, carbohydrate intake is drastically reduced. This signals the body to adapt its metabolism, transitioning from a glucose-dependent state to one that relies predominantly on fat and ketone bodies for fuel.
This process is initiated by a drop in insulin levels and an increase in glucagon, which triggers the release of fatty acids from the body's fat stores. These fatty acids travel to the liver, where they are oxidized and converted into ketone bodies, including acetoacetate and beta-hydroxybutyrate (BHB). The liver itself cannot use these ketones for energy, so it releases them into the bloodstream to be used by extrahepatic tissues like the heart, brain, and, importantly, the skeletal muscles.
Ketones as an Alternative Fuel for Muscles
Once ketones reach the muscle cells, they are converted back into acetyl-CoA, which can then enter the Krebs cycle to produce ATP, the cell's main energy currency. Skeletal muscles, particularly the slow-twitch fibers that power endurance activities, are highly efficient at utilizing ketones. This is due to a higher concentration of the enzyme succinyl-CoA:3-oxoacid-CoA transferase (SCOT), which is crucial for the metabolic pathway that allows muscles to consume ketones.
The Process of Keto-Adaptation
For muscles to effectively use ketones, the body must first undergo a period of keto-adaptation. This can take several weeks and involves a series of physiological changes:
- Enzyme Upregulation: The muscles increase the expression of enzymes necessary for both fatty acid and ketone oxidation, enhancing their capacity to burn these fuels efficiently.
- Mitochondrial Biogenesis: The number and size of mitochondria—the cellular powerhouses—can increase in muscle cells, improving overall oxidative capacity.
- Shifting Fuel Preference: The body learns to prioritize fat and ketones, conserving the limited glucose reserves for essential functions.
During the initial phase of adaptation, known as the 'keto flu,' some individuals may experience fatigue and decreased athletic performance. However, once fully adapted, many report stable energy levels and enhanced endurance, especially for lower-intensity activities.
Muscle Fuel Utilization and Exercise Intensity
The type and intensity of exercise dictate which fuel source a muscle prefers, even in a state of ketosis. This is a critical distinction for athletes considering a ketogenic lifestyle.
Aerobic Exercise (Low to Moderate Intensity)
For low-intensity, steady-state exercises like jogging, cycling, or walking, the body has ample time to deliver oxygen to the muscles. In this aerobic state, keto-adapted muscles are highly efficient at using both fat and ketones for fuel. The body's vast fat stores provide a nearly limitless energy supply, which is why endurance athletes on a keto diet can perform for extended periods without 'bonking'—the sudden depletion of glycogen stores that affects those on a high-carb diet.
Anaerobic Exercise (High Intensity)
High-intensity, anaerobic activities like sprinting, heavy weightlifting, and HIIT require a rapid burst of energy that relies on the quick breakdown of glucose through glycolysis. Since glucose production in ketosis is limited, performance during these activities can be compromised, especially during the adaptation period. While the body can still produce some glucose via gluconeogenesis, it is not enough to sustain high-intensity efforts for long. For athletes who need bursts of power, targeted ketogenic diets (TKD) or cyclical ketogenic diets (CKD) are sometimes used to strategically reintroduce carbohydrates around workouts.
The Role of Glycogen and Gluconeogenesis
Even in deep ketosis, the body requires a small amount of glucose for certain cells and organs, such as red blood cells and parts of the brain. This is where gluconeogenesis comes in. This process, which primarily occurs in the liver, creates new glucose from non-carbohydrate sources, such as amino acids derived from protein and glycerol from triglycerides. This production ensures a minimal, but vital, supply of glucose is always available. The ketogenic diet's ability to provide a potent, alternative fuel source (ketones) means less reliance on this more energetically costly process.
Comparison of Energy Sources in Ketosis
| Feature | Glucose (Pre-Ketosis) | Ketones (In Ketosis) | Fat (In Ketosis) |
|---|---|---|---|
| Primary Source | Carbohydrates | Liver production from fatty acids | Body fat stores |
| Delivery Speed | Very fast | Fast | Relatively slow |
| Intensity Level | High-intensity (anaerobic) | Low-to-moderate intensity (aerobic) | Low-to-moderate intensity (aerobic) |
| Energy Reserves | Limited (glycogen stores) | Moderate (from fat oxidation) | Abundant (body fat stores) |
| Energy Yield | High ATP yield, less efficient | High ATP yield, more efficient than glucose | High ATP yield, very efficient |
| Muscle Fiber Type | Fast-twitch | Slow-twitch | All muscle fiber types |
| Limitation | Depletes quickly, causing 'bonking' | Requires adaptation, can affect high intensity | Slower delivery for bursts of power |
Maintaining Muscle Mass on a Ketogenic Diet
A common concern is that the body will break down muscle tissue for gluconeogenesis in the absence of carbohydrates. While this can happen, especially with insufficient protein intake, it is largely preventable. During keto-adaptation, the body prioritizes using fat and ketones, and adequate protein intake ensures amino acids are available for muscle repair and growth, rather than being diverted to produce glucose. Research suggests that combining a ketogenic diet with resistance training and sufficient protein can preserve or even increase muscle mass. The key is to consume enough protein to stimulate muscle protein synthesis while keeping carbohydrate intake low enough to remain in ketosis. For more detailed information on keto-adaptation and endurance exercise, see this review on the impact of keto-adaptation.
Conclusion: Fueling Muscles Flexibly
In ketosis, muscles power themselves by burning fat and, more uniquely, ketone bodies produced by the liver. This metabolic state provides a consistent and abundant energy source, making it highly effective for endurance and low-intensity exercise. While high-intensity performance may be temporarily impacted during adaptation, the body's improved metabolic flexibility is a significant benefit. Ultimately, the way muscles get energy in ketosis is an elegant biological adaptation that shifts reliance from a limited glucose reserve to a vast supply of fat and ketones, paving the way for enhanced metabolic health and endurance.
