Ketogenic diets have gained significant attention for their potential to enhance athletic performance, improve metabolic health, and support brain function. With this attention has come numerous misconceptions about ketones and their role as an energy substrate. One of the most common myths is that ketones act as a universal replacement for other fuel sources across the body. Many people frequently reference the “clean, stable energy” from ketones as a primary reason for adopting a ketogenic diet.
Ketones are often recognized as a more efficient fuel source than glucose, not only in terms of ATP yield per unit of oxygen consumed but also due to their higher free energy output during ATP hydrolysis (Murray et al.,2016)(Cox et al., 2014). This increased free energy output enhances overall energy efficiency, particularly in high-demand tissues such as the brain and heart.
This is an important aspect of my research into the effect of the ketogenic diet on sports performance. As I’ve been digging into fuel substrate efficiency, I’ve started evaluating which substrate provides the most energy for the least metabolic overhead. Initially, I focused on comparing the ATP yield of different pathways:
- Anaerobic Glycolysis = 2 ATP
- Aerobic Glycolysis = 32 ATP
- Fatty Acid Oxidation = 129 ATP
- Ketolysis = 22 ATP
While fatty acids produce the most ATP, we know that as work intensity increases, fatty acid utilization decreases, making them an inconsistent energy source at higher intensities. Additionally, the oxygen consumption required for aerobic glycolysis, fatty acid oxidation, and ketolysis differs, as each pathway has a unique respiratory quotient. I also discovered variations in substrate affinity across different muscle fiber types and organs, further complicating the analysis.
I then learned that the free energy derived from ATP hydrolysis can vary depending on the substrate from which the ATP was produced. Meaning that even an ATP isn’t an ATP! This realization completely changed my perspective. This deeper understanding shifted my perspective from focusing solely on ATP or energy yield to considering the complex, system-level interactions of fuel substrates in different tissues under varying conditions.
Initially, identifying the “best” fuel seemed straightforward, but I soon realized this approach was overly reductionist. While one substrate may produce more ATP than another, does it matter if a specific organ or muscle tissue doesn’t use that substrate? In reality, evaluating the best fuel is like focusing on a single puzzle piece without considering how it fits into the entire system.
Research shows ketones influence enzymatic function, protein coupling, endocrine signaling, and various other processes (Newman & Verdin,2017). While ketones are indeed an efficient and vital energy source in certain contexts, their role is far more nuanced. Let’s explore the specific mechanisms of ketone metabolism and clarify where ketones replace glucose and where they don’t.
Ketones as an Energy Substrate: The Contextual Role
Ketones, primarily beta-hydroxybutyrate (BHB) and acetoacetate (AcAc), are produced by the liver during periods of low carbohydrate availability. Their primary function is to serve as an alternative energy source when glucose is scarce, but they don’t act as a blanket replacement for glucose in all situations.
Where Ketones Replace Glucose
The brain primarily consumes glucose in carbohydrate-dominant environments. However, in carbohydrate-restricted environments where a state of ketosis is maintained, ketones can supply up to 75% of the brain’s energy needs, significantly reducing its glucose demand (Cahill, 2006). This glucose-sparing effect is crucial during prolonged fasting or carbohydrate restriction, ensuring that the limited glucose supply is reserved for tissues incapable of efficiently utilizing ketones, such as red blood cells.
The heart preferentially oxidizes fatty acids under normal conditions but readily switches to ketones when abundant. Ketones provide an efficient energy source for the heart, reducing reliance on glucose and enhancing overall metabolic flexibility.
Notice that skeletal muscle isn’t a place where ketones play a major role as an alternative fuel.
Yes, skeletal muscle uses ketones, but after adaptation, the current information shows it’s only about 5% of the total contribution to muscle energy during exercise (Evans et al., 2017). How can ketones benefit sports performance if the body isn’t leveraging the fuel efficiency of ketones for energy?
Where Ketones Act as a Potentiator
During the initial stages of ketosis, skeletal muscle significantly increases ketone oxidation. However, as adaptation progresses, the muscle begins to rely more on fatty acids for energy, sparing ketones for the brain and other essential organs. This is evidenced by a documented reduction in ketone utilization markers over time and increased crossover point for fatty acid utilization at high percentages of VO2Max effort (Noakes et al., 2023). This shift illustrates that ketones do not permanently replace fatty acids or glucose in muscle but act as a transitional fuel during the adaptation phase.
Ketones enhance the rate of fatty acid oxidation in the liver by providing an additional outlet for acetyl-CoA, a byproduct of beta-oxidation (Cantrell & Mohiuddin, 2023). This process helps maintain energy production while preventing the accumulation of excess acetyl-CoA, which would otherwise slow down fat metabolism. Additionally, ketones help decrease reactive oxygen species (ROS) production, lowering oxidative stress. They also reduce lactate buildup during high-intensity exercise, which may improve endurance and recovery (Ma & Suzuki, 2019).
The benefits of ketones may lie less in their direct energy contribution and more in how they support various systemic functions. This distinction is crucial when evaluating the role of ketones in performance and metabolism, as these other areas directly impact recovery, training volume, muscle protein synthesis, lactate threshold, time to exhaustion, and overall endurance capacity.
References
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Cantrell CB, Mohiuddin SS. Biochemistry, Ketone Metabolism. [Updated 2023 Apr 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554523/
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