The Basics: What’s Needed to Generate Energy?

Every movement your body makes, from lifting a finger to running a marathon, requires ATP (adenosine triphosphate) – the body’s energy currency.

• ATP is like a charged battery. When your muscles need to contract, ATP is broken down into ADP + energy.

• Muscles store only a tiny amount of ATP, so the body must constantly resynthesise ATP from fuels.

  
  

Where the Fuel Comes From

The body derives energy from three primary macronutrients: carbohydrates, fats, and proteins. Their contribution depends on the intensity and duration of exercise, as well as oxygen availability.

1.  Carbohydrates

• Stored as muscle glycogen or circulating blood glucose.

• Can be metabolised anaerobically (glycolysis in the cytoplasm) or aerobically (via the citric acid/Krebs cycle in mitochondria).

• Anaerobic glycolysis provides rapid ATP but produces lactate, limiting duration.

• Aerobic carbohydrate metabolism is slower but more sustainable.

2. Fats

• Stored as triglycerides in muscle or adipose tissue.

• Fatty acids undergo beta-oxidation in mitochondria, producing acetyl-CoA, which enters the citric acid cycle.

• Provides a vast energy reserve, especially important during prolonged, lower-intensity exercise.

  
  

3. Proteins

• Used minimally under normal conditions but can contribute more during prolonged exercise or inadequate carbohydrate availability.

• Proteins are hydrolysed to amino acids, which are then converted into intermediates of the citric acid cycle or acetyl-CoA for oxidation.

ATP as the Final Common Pathway

Regardless of the fuel source—carbohydrate, fat, or protein—the ultimate goal is the production of ATP.

• Anaerobic metabolism (glycolysis): Rapid ATP generation without oxygen.

• Aerobic metabolism (citric acid cycle + oxidative phosphorylation): Efficient ATP generation with oxygen.

Exercise Intensity and VO₂ Relevance

The respiratory oxygen uptake (VO₂) reflects the body’s capacity to use oxygen for aerobic metabolism.

VO₂max—the maximal oxygen uptake—is a key indicator of aerobic fitness and endurance capacity.

• At low intensities, fat is the main fuel source.

• At moderate intensities, a mix of carbohydrate and fat is used.

• At high intensities, carbohydrate predominates, and anaerobic pathways are engaged.

Muscle Fibre Types and Fuel Choices

• Type I (slow-twitch): endurance fibres, packed with mitochondria, specialise in fat burning. Perfect for long, steady activity.

• Type II (fast-twitch): powerful but fatigue quickly, rely heavily on glycogen and anaerobic glycolysis. Critical for sprints, lifting, HIIT.

Your workout determines which fibres are recruited – and therefore which fuels dominate.

Duration Matters Too

• First 20–30 minutes: mostly carbs (muscle glycogen is the quickest source).

• After ~30–60 minutes: fat contribution rises as glycogen drops and hormones stimulate fat release.

• Beyond 90 minutes: fat becomes the major fuel, though performance may drop if carbs aren’t replenished.

Exercise Intensity, VO₂, and Fuel Use

Takeaways

1. Fat is burned at all intensities, but its contribution is highest at lower to moderate VO₂ levels.

2. Carbs dominate as exercise intensity rises, because they can be mobilised and burned faster.

3. Muscle type matters – slow-twitch = fat burners, fast-twitch = sugar burners.

4. Duration shifts fuel use toward fat as glycogen stores run low.

5. Training and diet influence flexibility – trained athletes and low-carb diets enhance fat use; high-carb diets maximise glycogen availability for performance.

For more precise, personalised advice Choose a Consultation.

References:

• Callahan, P.A., Leonard, H. and Powell, T. (2020) Fuel Sources for Exercise. Nutrition Science. Open Oregon Pressbooks. Available at: https://openoregon.pressbooks.pub/nutritionscience/chapter/10b-fuel-sources-exercise.

• Boutcher, S.H. (2011) ‘High-Intensity Intermittent Exercise and Fat Loss’, Journal of Obesity, 2011, p. 868305. Available at: https://doi.org/10.1155/2011/868305.

• Muscella, A., Stefàno, E., Lunetti, P., Capobianco, L. and Marsigliante, S. (2020) ‘The Regulation of Fat Metabolism during Aerobic Exercise’, Biomolecules, 10(12), p. 1699. Available at: https://doi.org/10.3390/biom10121699.

• Hall, J.E. and Guyton, A.C. (2020) Guyton and Hall Textbook of Medical Physiology. 14th edn. Philadelphia: Elsevier. Chapter 72: Dietary Balances; Regulation of Feeding; Obesity and Starvation; Vitamins and Minerals.