You wouldn’t get very far if you tried to drive your car with an empty fuel tank, the same applies to your body. Muscle stores of the main fuel for exercise, carbohydrate, can only sustain moderate intensity exercise for around 90 minutes1, so how can you keep yourself revving all the way to the finish line?
- ATP-CP: adenosine triphosphate is the compound the body breaks down to release energy. There are limited stores of ATP readily available in the muscles, enough to provide around 3 seconds of energy, after which creatine phosphate (CP) is used to produce more ATP, providing another 6-8 seconds. This is the energy system used for activities such as the 100m sprint, a high jump or Olympic weight lifting.
- Anaerobic Glycolysis: this energy system uses sugar stored in the muscles in the form of glycogen to provide a few minutes of energy. Glycogen can be partially broken down without the need for oxygen, but when sugar is only partially metabolised a by-product is lactic acid, which causes muscle fatigue. Anaerobic glycolysis (also known as the lactic acid system) can only provide enough energy for exercise such as the 400m sprint, but is also called upon during a sprint in the final stages of a race.
- Aerobic Metabolism: the aerobic energy system can provide energy for much longer activities, kicking in at around 5 minutes of exercise, and keeping us going for hours or even days. This system draws on stores of carbohydrate, fat and, in lesser amounts, protein to keep us fuelled. In low intensity activities (60-70% of maximum heart rate, or 6-7 out of 10 effort) fat is the main fuel metabolised, and provided there is enough oxygen being taken in, this can sustain exercise for days. However, fat is not adequate for moderate to high intensity exercise (70-90% max HR, or 7-9 out of 10) and this is where the metabolism of carbohydrate comes in. With oxygen present, stored glycogen can be broken down without producing lactic acid, and can provide enough energy for around 90 minutes of moderate exercise.
What happens when stores get low?
90 minutes or more
What to take on
Most sports products have recommendations written on them about how much and how often to consume, but as a general rule about 60-80g of carbohydrate per hour should be plenty, which is the equivalent of an energy gel every 20 minutes or so, or a sports drink sipped over the course of an hour.
Here are some pros and cons of the different products available:
Look out for the next post, where we’ll focus on pre- and post- exercise nutrition.
- Hermansen, L., Hultman, E., Saltin, B., 1967. Muscle glycogen during prolonged severe exercise, Acta Physiologica Scandinavica, [online] Available at: http://homepages.wmich.edu/~ccheatha/hper6760/files/Topic02-Articles-Combined.pdf[Accessed 8th October 2012].
- Coyle, E.F., Coggan, A.R., Hemmert, M.K., Ivy, J.L., 1986. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate, Journal of Applied Physiology, [online] Available at: http://www.edb.utexas.edu/coyle/pdf%20library/(29)%20Coyle,%20Muscle%20glycogen%20utilization%20during%20prolonged%20strenuous%20exerc%20when%20fed%20CHO,%20JAP%2061(1)%20165-72,%201986.pdf[Accessed 8th October 2012].
- Hargreaves, M., Costill, D.L., Coggan, A., Fink, W.J., Nishibata, I., 1984. Effect of carbohydrate feedings on muscle glycogen utilization and exercise performance, Medicine and Science in Sports and Exercise, [online] Available at: http://www.ncbi.nlm.nih.gov/pubmed/6748917[Accessed 8th October 2012].