Sports metabolism: energy systems, metabolic flexibility, and what the evidence supports
How the body produces energy during exercise, why lactate is fuel and not a villain, and where metabolic optimization has a basis and where it becomes a fad.
The energy bill is paid by three systems
Phosphocreatine, glycolysis, and oxidation do not compete with each other; they take turns according to the intensity and duration of the effort.
The cell's energy currency is ATP, and muscle stores little of it: it must resynthesize it continuously during exercise. Three systems make this payment. The phosphocreatine system, or ATP-PCr, is immediate, lasts seconds, and does not depend on oxygen; the glycolytic system is fast, uses glucose and glycogen, sustains efforts from seconds to a few minutes, and generates lactate; the oxidative, mitochondrial system is slower, has high capacity, burns fat and carbohydrate, and sustains prolonged effort (Egan and Zierath, 2013).
All three operate at the same time; what changes is which one predominates. A ten-second sprint is paid for mainly by phosphocreatine and glycolysis; a marathon, by oxidation. It is a continuum regulated by intensity and duration, not a switch that turns one system on and another off.
The trained body remodels this machinery: more mitochondria, greater enzymatic density, and more capillaries, which shifts the same intensity toward a more efficient and sustainable pathway (Hawley et al., 2014).
| System | Fuel | Dominant duration | Depends on oxygen |
|---|---|---|---|
| ATP-PCr | Phosphocreatine | Seconds | No |
| Glycolytic | Glucose and glycogen | Seconds to about 2 minutes | No |
| Oxidative | Fat and carbohydrate | Minutes to hours | Yes |
Fat or carbohydrate: the crossover concept
Intensity decides the fuel mix, and there is a point at which carbohydrate overtakes fat.
At low intensity, fat dominates the energy supply. As intensity rises, the mix crosses over and carbohydrate takes over, a predictable shift known as the crossover concept (Brooks and Mercier, 1994). The respiratory quotient tracks this exchange.
There is an intensity of maximal fat oxidation, FatMax, generally moderate, individual, and trainable (Achten and Jeukendrup, 2003). It is a useful concept for training, but not a magic fat-loss zone: to lose fat, total energy balance and adherence decide more than the chosen intensity.
The trained endurance athlete oxidizes more fat at the same intensity, sparing glycogen. This is a physiological adaptation to training, not the effect of a supplement.
| Intensity | Predominant fuel | Marker |
|---|---|---|
| Light (below about 50% of VO2max) | Fat | Low RER, FatMax range |
| Moderate | Mix of fat and carbohydrate | Crossover point |
| High (above threshold) | Carbohydrate | Blood lactate rises |
Lactate is fuel, not poison
The idea of lactic acid as the cause of fatigue and next-day soreness is outdated; lactate is substrate and signal.
The image of lactic acid poisoning the muscle, causing acute fatigue and delayed muscle soreness, is outdated. Lactate is produced all the time, at rest and during exercise, and works as a preferred fuel that circulates between muscle fibers, the heart, the brain, and the liver, the so-called lactate shuttle theory (Brooks, 2018).
Lactate is not the simple villain of acidosis: it is a substrate for glucose production in the liver through the Cori cycle, it is a signaling molecule, and it is oxidized by the heart and by slow-twitch fibers. The rise of lactate in the blood marks that intensity has crossed a threshold, not that poisoning has occurred.
In practice, lactate thresholds are tools for training and prescription. Meanwhile, lactate or exogenous ketone supplements sold as performance enhancers lack robust outcome evidence and fall into fad territory.
Metabolic flexibility: switching fuel efficiently
Metabolic health is the capacity to alternate between fat and carbohydrate according to demand; disease is the rigidity of that switch.
Metabolic flexibility is the capacity to switch substrate according to availability and demand: oxidizing fat in the fasted state and at low intensity and switching to carbohydrate when fed and at high intensity (Goodpaster and Sparks, 2017).
Metabolic rigidity, the opposite, accompanies insulin resistance, obesity, and type 2 diabetes: the muscle loses the fluidity to switch fuel. Physical training restores this flexibility, and it is here that sports metabolism meets the cardiometabolic medicine of the rest of this library.
The caveat against the fad: metabolic flexibility is real physiology, but diets and supplements that promise to unlock fat burning overstate it. What does the work is training and diet quality, not the product.
| Situation | Flexible (healthy or trained) | Rigid (insulin resistance) |
|---|---|---|
| Fasted or low intensity | Oxidizes fat efficiently | Oxidizes less fat |
| Post-meal or high intensity | Switches to carbohydrate quickly | Slow and incomplete switch |
| Marker | RER responsive to the condition | RER barely variable |
Measuring the engine: VO2max, thresholds, and performance
VO2max and thresholds organize training and, in the clinic, VO2max is a vital sign of risk.
VO2max, the maximal oxygen consumption, is the ceiling of aerobic power. The lactate and ventilatory thresholds divide the intensity domains, and concepts such as FatMax and critical power guide training prescription.
Beyond sport, cardiorespiratory fitness, measured by VO2max, is one of the strongest predictors of mortality, to the point that the American Heart Association argues it should be treated as a vital sign in clinical practice (Ross et al., 2016). Low fitness rivals classic risk factors.
In practice, a laboratory is not mandatory: field tests and wearable devices approximate these markers. The point that matters is that VO2max is trainable and relevant to health, not just to competition.
| Marker | What it measures | Use |
|---|---|---|
| VO2max | Maximal aerobic power | Performance ceiling and health prognosis |
| Lactate threshold | Sustainable intensity | Prescription of training zones |
| FatMax | Peak fat oxidation | Aerobic base work |
| Critical power or MLSS | Maximal lactate steady state | Race pace |
Fueling the work: periodization and fad
Fuel for the work required: not always more carbohydrate, not always less, it depends on the session and the goal.
The principle of fueling for the work required says to adjust carbohydrate to the demand of the session, rather than keeping it always high or always low (Impey et al., 2018). Training with low glycogen in some sessions can amplify the adaptive signal, but compromises the quality of high intensity: you periodize it, you do not turn it into dogma.
The basis is molecular: exercise activates pathways such as AMPK and PGC-1 alpha and triggers mitochondrial biogenesis, and nutrient availability modulates this signal (Egan and Zierath, 2013; Hawley et al., 2014). This is why context decides.
Against the fad: the ketogenic diet increases fat oxidation, but usually impairs high intensity and is not a universal upgrade; exogenous ketones and lactate have no robust performance evidence; and fasted cardio burns more fat in the moment without beating total energy balance for fat loss. It is individualized, and total training and adherence decide.
Why this matters for your care
For the athlete and for the metabolic patient, exercise metabolism is the bridge between performance and cardiometabolic health: training metabolic flexibility and VO2max improves both the race and the prognosis at the same time. Fuel and intensity decisions must be individualized by a professional; this text is educational and does not replace medical evaluation.
References
- Egan B, Zierath JR. Exercise metabolism and the molecular regulation of skeletal muscle adaptation. Cell Metabolism. 2013. doi:10.1016/j.cmet.2012.12.012
- Hawley JA, Hargreaves M, Joyner MJ, Zierath JR. Integrative biology of exercise. Cell. 2014. doi:10.1016/j.cell.2014.11.029
- Brooks GA. The science and translation of lactate shuttle theory. Cell Metabolism. 2018. doi:10.1016/j.cmet.2018.03.008
- Goodpaster BH, Sparks LM. Metabolic flexibility in health and disease. Cell Metabolism. 2017. doi:10.1016/j.cmet.2017.04.015
- Brooks GA, Mercier J. Balance of carbohydrate and lipid utilization during exercise: the crossover concept. Journal of Applied Physiology. 1994. doi:10.1152/jappl.1994.76.6.2253
- Achten J, Jeukendrup AE. Maximal fat oxidation during exercise in trained men. International Journal of Sports Medicine. 2003. doi:10.1055/s-2003-43265
- Impey SG, Hearris MA, Hammond KM, et al. Fuel for the work required: a theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Sports Medicine. 2018. doi:10.1007/s40279-018-0867-7
- Ross R, Blair SN, Arena R, et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign. Circulation. 2016. doi:10.1161/CIR.0000000000000461
Educational and scientific content. It does not constitute diagnosis, prescription or individual clinical guidance, and does not replace a medical consultation. Management decisions must be individualized by a physician.