There has been a burst of social media hype around the idea of using very low carbohydrate (ketogenic or keto) diets for endurance events. The keto diet usually consists of 5-10% of total kcal (~50g or less) from carbohydrates per day, 75% from fat and 10-20% from protein, although there is no set standard of carbohydrate level. In this article I want to talk about how the body uses energy during exercise before we get into the research.
If we look at the stores of energy in the body, we know that our ability to store carbohydrates as glycogen is limited. We can store carbohydrates in our muscles and liver and the more trained an individual is the more they can store, however it is still limited and can only supply about 1500-2000kcal and muscle glycogen is depleted within 1hour of intense exercise. Fat on the other hand is very calorically dense and we can store 65 000 kcals (could fuel about 20 marathons!) in our adipose (fat) tissue and within the muscle. Fat can supply significantly more energy than carbohydrates without supplementation. This diet is seen as attractive because it promotes the idea that we can max out our capacity to burn fat as a fuel while running, reducing the amount of food or energy we need to take on the run. Therefore, it seems logical to adopt a keto diet, max out our ability to burn fat as a fuel, right? Well let’s look at this topic a little deeper.
Understanding Energy Production During Activity
Our body uses two processed to produce energy in the form of adenoside triphosphate (ATP). The first method is anaerobic (without oxygen) and the second is aerobic (with oxygen). There are 3 energy systems that the body uses depending on a few factors, such as intensity, length of event, and availability of oxygen.
(1) Phosphocreatine System
This system is the quickest way to produce energy and is the first system to turn on to crank out ATP. Let’s say as you are reading this article your fire alarm stated to ring, you would immediately jump up and run out the door. In this fight or flight response you are using mostly the phosphocreatine system. It works by taking the phosphocreatine that is stored in the muscles, and through an enzymatic reaction the phosphate is split off and added to adenosine diphosphate to make ATP. This is a very simple reaction, and the body can use this system for about 8-10 seconds before it’s tapped out. This system can recover; it takes about 4 minutes before the system is ready for another intense bout.
(2) Anaerobic glycolysis
This is the second system to turn on to support energy production. To continue with out analogy from above, after our initial jumping and sprinting for the door, after the 8ish seconds our body tends to rely more heavily on the anaerobic glycolysis system. This system uses carbohydrates (glucose) and through a series of steps breaks down 1 glucose molecule into 2 pyruvate molecules. Without oxygen, this pyruvate can be further broken down into lactate and a hydrogen ion and ATP. This system will last about 1-2 minutes. In repeated sprints, such as hockey shifts, research shows that subsequent sprints rely on anaerobic glycolysis to provide 50% of the energy.
(3) Oxidative phosphorylation (Aerobic system)
This system takes a while to get warmed up and to get going once you start exercising. In untrained individuals, it can take a few minutes for this engine to get running fully, but in elite athletes it can fully turn on within about 30-60seconds. In our analogy above, lets say we lived in the country and we needed to go run to the fire station down the road, after our initial sprint and quick getaway, our aerobic system turns on and produces a majority of the energy needed. This system can last forever; at rest we are aerobically oxidizing mostly fats to provide energy for the body at rest.
The aerobic system can burn carbohydrates, fat and protein depending on availability and intensity. Protein used in very small amounts to provide energy for exercise and we wont discuss it further here. I want to discuss the differences between fat and carbohydrate oxidation.
The classic study from Romijn et al. showed that the fuel sources changed depending on the intensity of exercise. They looked at 25% VO2Max, 65% VO2Max and 85% VO2Max. The found that as intensity increased, so did caloric expenditure and the more intense exercise relied more heavily on carbohydrate use. This is because at higher intensity of exercise the fat breakdown and transport into the mitochondria and oxidation rates are too slow to keep up with the energy demands.
Carbohydrate is also the preferred fuel during intense activity as it provides 5.5% more kcal/L of oxygen compared to fat oxidation, meaning it is a more efficient fuel source. Research shows that elite level marathoners fuel use is 85% carbohydrate and 15% fat oxidation.
One thing to note is, you do not only use one system at a time, your aerobic system is always running in the back ground. Think of these systems like dimmer switches, they can be turned up or turned down depending on the situation. For example, if we take a cyclist who is riding on a flat surface, they have their aerobic system pumping out most of the ATP to cover the cost of their cycling. When they hit a hill, there is an increased demand for ATP. The aerobic system take a little while to adjust, and therefore the anaerobic system has to kick in to provide some quick energy, and that means the phosphocreatine system turns on, and the anaerobic glycolytic system turns on to meet the ATP needs of the body.
- Fat is slower to provide energy, and therefore intensity is lower when using fat as a fuel source
- Fat is less efficient; it uses more oxygen to produce less ATP
- High burst of exercise- ie the energy change when cycling or running up a hill is usually covered by anaerobic glycolysis which uses glucose (carbohydrate)
- Most elite athletes run at high %VO2 and burn a large amount of carbohydrates
Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, Wolfe RR. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am J Physiol. 1993;265:E380-E391.
O’Brien MJ, Viguie CA, Mazzeo RS, et al. Carbohydrate dependence during marathon running. Med Sci Sports Exerc. 1993;25:1009–1017. doi: 10.1249/00005768-199309000-00007
Bosch, AN, Goslin BR, Noakes TD, Dennis SC. Physiological differences between black and white runners during a treadmill marathon. Eur J Appl Physiol. 1990;61:68-72.