If marathon runners relied solely on carbohydrate as an energy source, they would be
exhausted after about 90 minutes of running. As the world record for the men’s marathon is close to
125 minutes, this shows the importance of using a variety of fuel sources during prolonged exercise (Maughan, 2000).
Fat provides the largest source of nutrients that can be used to power physical activity. During low intensity exercise, such as walking, most energy requirements can be met through fatty acid oxidation (use of fat as fuel), with small contributions from carbohydrate. During the long slow run, fat is used as the fuel source for approximately 50% of energy needs (Burke & Deakin, 2000).
This process occurs by fat (or glycogen) being converted into ATP (adenosine triphosphate, a molecule that provides the energy for muscle contraction).
Research shows that regular training results in skeletal muscle adapting to include an increased number of mitochondria (the powerhouse of the cell), and thus respiratory capacity of muscle fibres. This results in slower use of glycogen and blood glucose, and increased reliance on fat as the energy source. Furthermore, less lactate is produced. This results in improved ability to perform prolonged strenuous exercise as a response to endurance training (Holloszy & Coyle, 1984).
So, in a nutshell, the better trained you are, the better you will be at using fat as a fuel source in endurance events. Would love to hear any questions or comments!
Chloe McLeod is a dietitian at BJC Health.
This blog focuses on diet & nutrition generally and diet & nutrition in relation to the treatment of arthritis and arthritis-related diseases. Contact us if you'd like our help in managing diet-related health issues.