Difficulty: Intermediate | Prerequisites: Parts 1 and 2 of these notes (energy systems, crossover concept, hormonal regulation, lipolysis). Basic understanding of mitochondrial function is helpful.
This final set of notes brings together the structural and dietary factors that shape how the body fuels exercise. VO2 max sets the ceiling on aerobic performance. The body's fuel storage pools (glycogen, fat) determine how long exercise can continue and at what intensity. Training-induced muscle adaptations change which fuels the body can access and how efficiently it uses them. Finally, the ketogenic diet and ketone body metabolism represent an alternative fuelling strategy with specific implications for exercise performance. This material sits at the intersection of physiology, training science, and nutrition.
VO2 max reflects the upper limit of your aerobic engine, shaped by genetics, training, and metabolic efficiency. The body stores carbohydrates in limited quantities (~420 g total) but carries enormous fat reserves (~107,800 kcal in a 70 kg male). Training increases mitochondrial density, enzyme activity, and transporter expression, shifting the body toward more efficient fuel use. Ketogenic diets force reliance on fat and ketone bodies, which can sustain low-to-moderate exercise but may limit high-intensity performance.
VO2 max (maximal oxygen consumption)
The highest rate at which the body can take up, transport, and utilise oxygen during intense exercise. It reflects the combined capacity of the cardiovascular, respiratory, and muscular systems. In simple terms, VO2 max is the ceiling of your aerobic fitness, the point where your body simply cannot process any more oxygen no matter how hard you push.
FATmax
The exercise intensity at which absolute fat oxidation (grams per minute) is highest, typically occurring just below the lactate threshold. In simple terms, FATmax is the sweet spot where you burn the most total fat per minute, even though a higher percentage of energy comes from fat at lower intensities.
Mitochondrial density
The number and volume of mitochondria per unit of muscle tissue. Higher mitochondrial density means greater capacity for oxidative phosphorylation. Think of it as having more power plants in each muscle fibre.
Citrate synthase
A key enzyme of the TCA (Krebs) cycle, often used as a marker of mitochondrial content and oxidative capacity. Higher citrate synthase activity indicates better-trained aerobic machinery.
Beta-oxidation
The metabolic pathway that breaks fatty acid chains into acetyl-CoA units for entry into the TCA cycle. Each cycle removes a two-carbon fragment. In simple terms, beta-oxidation is the process of chopping up fat molecules so they can be fed into the aerobic energy system.
Ketone bodies
Small water-soluble molecules (acetoacetate, 3-beta-hydroxybutyrate, and acetone) produced in the liver from fatty acids when carbohydrate availability is very low. They serve as alternative fuels for muscle and brain tissue.
Acetoacetate (AcAc)
One of the three ketone bodies, produced in the liver during fatty acid metabolism. It can be used directly for energy or converted to 3-beta-hydroxybutyrate.
3-beta-hydroxybutyrate (3HB)
The most abundant ketone body in circulation during ketosis. It is the primary ketone used by muscles and the brain for energy. Sometimes written as beta-hydroxybutyrate (BHB).
Ketogenic diet
A dietary pattern characterised by very low carbohydrate intake (typically below 50 g/day), moderate protein, and high fat. It forces the body into a state of ketosis, where ketone bodies become a significant fuel source.
Ketosis
A metabolic state in which blood ketone levels are elevated (roughly 0.5–10 mM), indicating that the liver is actively converting fatty acids into ketone bodies due to low carbohydrate availability.
Lactate threshold
The exercise intensity at which blood lactate begins to accumulate above resting levels, indicating that glycolytic ATP production is outpacing lactate clearance. Closely related to FATmax, as fat oxidation typically peaks just below this point.
VO2 max is expressed in absolute terms (L/min) or relative to body mass (mL/kg/min). The relative value is more useful for comparing individuals of different sizes.
Factors that determine VO2 max:
Genetics: a substantial portion of VO2 max is heritable. Some individuals have a naturally higher ceiling than others regardless of training.
Training adaptations: endurance training increases cardiac output (stroke volume), capillary density in muscle, mitochondrial density, and the activity of oxidative enzymes. These collectively raise the rate at which oxygen can be delivered and consumed.
Metabolic efficiency: trained individuals extract more energy per litre of oxygen, partly due to better substrate selection (more fat oxidation at a given intensity, sparing glycogen) and partly due to improved mitochondrial coupling.
Higher VO2 max correlates with better endurance performance because it allows a greater absolute workload to be sustained aerobically before crossing the lactate threshold.
VO2 max can improve substantially with training (typically 15–25% over several months in untrained individuals), but the ultimate ceiling is genetically constrained.
Carbohydrate stores are limited and deplete-able:
Liver glycogen: ~60 g. This pool maintains blood glucose between meals and during exercise. It can be substantially depleted after an overnight fast.
Muscle glycogen: ~350 g. This is the primary carbohydrate fuel for exercising muscles. Unlike liver glycogen, muscle glycogen cannot be exported as free glucose; it is used locally.
Blood glucose: ~10 g circulating at any given time. A small but tightly regulated pool.
Total carbohydrate energy: roughly 1,600–2,000 kcal, enough for perhaps 90–120 minutes of moderate-to-high intensity exercise.
Fat stores are vast:
A 70 kg male carries approximately 14 kg of fat in adipocytes, equivalent to roughly 107,800 kcal.
Fat is stored as triglycerides and mobilised via lipolysis (covered in Part 2).
Even lean athletes have sufficient fat stores to fuel many hours of low-to-moderate intensity exercise.
Diet composition shifts these storage amounts:
A high-carbohydrate diet (such as pre-race carbohydrate loading) increases muscle and liver glycogen stores, extending high-intensity exercise capacity.
A low-carbohydrate diet reduces glycogen stores but upregulates fat oxidation pathways, increasing reliance on fat and (if carbs are very low) ketone bodies.
Low-intensity exercise (~20% VO2 max):
Fat supplies roughly 66% of energy.
Total energy expenditure is low, around 3 kcal/min.
Most of the fat comes from adipocyte stores (plasma FFAs) and intramuscular triglycerides.
Higher-intensity exercise (~60% VO2 max):
Carbohydrate dominates, providing roughly 83% of energy.
Total energy expenditure is about 9 kcal/min.
Despite the lower percentage from fat, the absolute rate of fat oxidation can be similar or even higher (~3 kcal/min) because total expenditure is tripled.
FATmax: the intensity at which absolute fat oxidation peaks. This occurs just below the lactate threshold, typically around 45–65% of VO2 max (varies with training status). Above FATmax, fat oxidation declines because rising lactate and catecholamines shift metabolism toward carbohydrate.
This is why "exercising in the fat-burning zone" (very low intensity) is a misleading concept for total fat loss: although a higher percentage of calories come from fat, the total calories burned per minute are low. Moderate intensity often burns more total fat per session.
Regular endurance training induces several muscle-level adaptations:
Increased mitochondrial density: more mitochondria per fibre means a greater capacity for aerobic ATP production. This is arguably the most important adaptation for endurance performance.
Enhanced oxidative enzyme activity: enzymes such as citrate synthase (TCA cycle) and beta-oxidation enzymes increase in activity, allowing faster processing of both carbohydrate and fat through aerobic pathways.
Improved substrate transport: GLUT4 expression and translocation increase with training, meaning trained muscles are better at pulling glucose from the blood. Similarly, fatty acid translocase (FAT/CD36) expression rises, improving fatty acid uptake.
Greater intramuscular triglyceride stores: trained muscle stores more fat locally, providing a readily accessible fuel source that does not depend on adipose tissue lipolysis and blood transport.
Collective effect: these adaptations delay fatigue by improving the efficiency of fuel use, sparing glycogen, and increasing the proportion of energy derived from fat at any given intensity. A trained individual oxidises more fat and less carbohydrate at the same absolute workload compared to an untrained person.
A ketogenic diet restricts carbohydrate to very low levels (typically below 50 g/day), with high fat and moderate protein.
When carbohydrate availability drops sufficiently, the liver increases production of ketone bodies from fatty acids:
Acetoacetate (AcAc): the first ketone body produced.
3-beta-hydroxybutyrate (3HB): the most abundant circulating ketone, converted from AcAc.
Acetone: a volatile byproduct, exhaled through the lungs (responsible for the characteristic "fruity" breath in ketosis).
Circulating ketone levels during fasting or a ketogenic diet range from roughly 0.5 to 10 mM, compared with less than 0.3 mM on a normal mixed diet.
Ketone bodies can be used by skeletal muscle and the brain (which normally depends almost exclusively on glucose). This sparing of glucose is the survival advantage of ketosis during prolonged fasting.
Exercise implications:
Fat-adapted athletes on a ketogenic diet show very high rates of fat oxidation during low-to-moderate intensity exercise.
High-intensity performance may be compromised because glycolytic flux is limited by depleted glycogen stores and reduced pyruvate availability.
Some endurance athletes report sustained performance on ketogenic diets, though research results are mixed and individual responses vary.
Ketone bodies may also have signalling roles (e.g., reducing inflammation, sparing muscle protein), which are areas of active research.
Approximate body fuel stores (70 kg male, mixed diet):
Fuel source | Storage amount | Energy equivalent |
|---|---|---|
Liver glycogen | ~60 g | ~240 kcal |
Muscle glycogen | ~350 g | ~1,400 kcal |
Blood glucose | ~10 g | ~40 kcal |
Adipose triglyceride | ~14 kg | ~107,800 kcal |
Fuel contribution by intensity (approximate):
Intensity | % VO2 max | % energy from fat | % energy from CHO | Total kcal/min |
|---|---|---|---|---|
Low | ~20% | ~66% | ~34% | ~3 |
Moderate-high | ~60% | ~17% | ~83% | ~9 |
Carbohydrate loading (eating a high-carbohydrate diet in the days before a long race) works by maximising muscle glycogen stores, extending the time before glycogen depletion forces a slow-down, the phenomenon marathon runners call "hitting the wall." Conversely, some ultra-endurance athletes adopt ketogenic or high-fat diets to become more fat-adapted, reducing their dependence on limited glycogen stores during events lasting many hours. The training adaptations described here are also why sedentary individuals become progressively less fatigued doing the same workout over weeks: their muscles are building more mitochondria and oxidative enzymes, making fuel delivery more efficient.
Students frequently assume that low-intensity exercise is the best way to lose body fat because a higher percentage of energy comes from fat. In absolute terms, moderate intensity often burns more total fat per minute because total energy expenditure is much higher. The "fat-burning zone" on cardio machines is misleading.
A common error is believing that muscle glycogen can be released into the bloodstream to raise blood sugar. Muscle lacks glucose-6-phosphatase, so muscle glycogen is trapped for local use. Only liver glycogen can contribute to blood glucose.
Students sometimes confuse ketosis (a normal metabolic state with ketone levels of 0.5–10 mM) with diabetic ketoacidosis (a dangerous condition where ketone levels far exceed 10 mM, paired with very high blood glucose and acidosis). They are distinct conditions.
Another misconception: "training makes you burn more fat because you have less glycogen." Training does shift fuel use toward fat, but this is because of enhanced mitochondrial and enzymatic capacity, not because glycogen is depleted. Trained individuals often have higher glycogen stores than untrained individuals.
⚠️ Be able to list the three sites of carbohydrate storage and their approximate quantities. Know which one can export glucose to the blood (liver) and which cannot (muscle).
⚠️ Understand why absolute fat oxidation and percentage fat oxidation tell different stories at different exercise intensities. The FATmax concept ties these together.
⚠️ Know at least three training-induced muscle adaptations and how each one improves exercise performance or fuel efficiency.
⚠️ Be able to name the three ketone bodies and explain when and why their production increases.
⚠️ If asked to compare fuel utilisation at low versus high intensity, make sure to address both percentage contribution and absolute energy expenditure.
True or false: Muscle glycogen can be exported as free glucose to maintain blood sugar during exercise.
Fill in the blank: Total fat stores in a 70 kg male provide approximately ________ kcal of energy.
True or false: FATmax occurs at the highest possible exercise intensity.
Fill in the blank: The three ketone bodies are acetoacetate, ________, and acetone.
True or false: Endurance training increases mitochondrial density in skeletal muscle.
Answers: 1. False (muscle lacks glucose-6-phosphatase; only liver glycogen can contribute to blood glucose). 2. 107,800. 3. False (FATmax occurs just below the lactate threshold, at moderate intensity). 4. 3-beta-hydroxybutyrate (3HB). 5. True.
Q: Why does a trained endurance athlete burn more fat at the same absolute workload compared to an untrained individual?
A: Training increases mitochondrial density and the activity of oxidative enzymes (citrate synthase, beta-oxidation enzymes), which allows the muscle to process fatty acids more efficiently. Greater GLUT4 and FAT/CD36 transporter expression improves substrate delivery. Increased intramuscular triglyceride stores provide a local fat source. Together, these adaptations shift the fuel mix toward fat at any given submaximal workload, sparing glycogen.
Q: A 70 kg runner is planning a marathon. Explain why carbohydrate loading is used and what limits it is trying to address.
A: Total carbohydrate stores (liver glycogen ~60 g, muscle glycogen ~350 g, blood glucose ~10 g) provide roughly 1,600–2,000 kcal. A marathon at moderate-to-high intensity may require 2,500+ kcal, with a large proportion from carbohydrate. Carbohydrate loading (eating a high-carb diet for 2–3 days before the race) maximises glycogen stores, delaying the point at which glycogen depletion forces a dramatic reduction in pace ("hitting the wall"). It does not remove the limit entirely, which is why in-race carbohydrate intake (gels, drinks) is also important.
Q: Name the three ketone bodies and explain which tissues can use them for fuel.
A: The three ketone bodies are acetoacetate (AcAc), 3-beta-hydroxybutyrate (3HB), and acetone. Skeletal muscle and the brain are the primary consumers. The brain normally relies on glucose, but during prolonged fasting or very low carbohydrate intake, it can derive a significant portion of its energy from ketone bodies (primarily 3HB). Acetone is mostly exhaled and contributes little to energy production.
Q: Explain the concept of FATmax and why exercising well above this intensity reduces total fat oxidation.
A: FATmax is the exercise intensity at which the absolute rate of fat oxidation (grams per minute) is highest, typically just below the lactate threshold. Above this intensity, rising catecholamine levels strongly stimulate glycogenolysis, increasing glycolytic flux. Simultaneously, increasing lactate inhibits fatty acid transport into mitochondria (via malonyl-CoA and CPT-1 regulation). The result is a shift away from fat oxidation and toward carbohydrate, even though total energy expenditure continues to rise.
VO2 max and training adaptations connect back to the crossover concept (Part 1): a trained individual's crossover point shifts to a higher intensity, meaning they burn a greater proportion of fat at workloads that would push an untrained person into carb-dominant metabolism. Glycogen storage and depletion link to the EPOC discussion (Part 1), since glycogen resynthesis is one of the drivers of elevated post-exercise oxygen consumption. The ketogenic diet material ties directly to the lipolysis and hormonal regulation content in Part 2, since sustained low insulin and high catecholamines are the hormonal conditions that drive ketone body production.
VO2 max, maximal oxygen consumption, aerobic capacity, FATmax, fat oxidation zone, fat burning zone, mitochondrial density, mitochondrial biogenesis, citrate synthase, beta-oxidation, fatty acid oxidation, GLUT4, FAT/CD36, glycogen storage, liver glycogen, muscle glycogen, carbohydrate loading, glycogen supercompensation, hitting the wall, bonking, ketogenic diet, keto diet, ketosis, ketone bodies, acetoacetate, AcAc, 3-beta-hydroxybutyrate, BHB, 3HB, acetone, fat adaptation, low-carb high-fat, LCHF, lactate threshold, endurance training adaptations, exercise physiology, EXS 101