energy currency and energy systems

Where do we get our energy?

You possibly remember from your school days how plants convert sunlight into chemical energy through a process called photosynthesis. The stored energy found in plants and the animals that eat those plants give us the energy we need to live. The food we eat is full of essential energy in the form of carbohydrate, protein and fat. Energy is needed for muscle contraction when performing physical activity, growth and repair and transport of substances between cells.

All our energy is powered by one essential compound called ATP (adenosine triphosphate). It doesn’t matter if it’s a hop, skip or a 100km bike ride, ATP is the compound that powers it. However, our cells can only store very small amounts, enough to power about 10 seconds of explosive exercise. In order to continue whatever activity you are performing, ATP has to be either replaced or resynthesized.

What is ATP?

A molecule of ATP is made up of adenosine and three inorganic phosphate groups. When a molecule of ATP is combined with water (a process called hydrolysis), the last phosphate group splits away and releases energy. What is left is known as ADP (adenosine diphosphate). A process called phosphorylation resynthesizes ADP back to ATP via a chemical reaction, adding a new phosphate group to the adenosine. When phosphorylation is carried out without oxygen, whilst performing an explosive movement for example, it is called anaerobic metabolism. When it uses oxygen, for example on a long steady jog, it is called aerobic metabolism. Several substances are used in the production and resynthesis of ATP.

Creatine phosphate is stored inside the cell, and produces ATP rapidly. Like ATP, there is only a limited amount stored in the body (about 100-120g). to 100 grams of ATP stored in the body. These two phosphate groups are known as the high-energy phosphogens.

The macronutrients; fat, carbohydrate and protein can also produce ATP.

Fat is our body’s largest energy source. It is mainly stored in adipose tissue, our fat stores are capable of providing in excess of 80,000 calories worth of energy. Fat has to be broken down from triglycerides into its individual components of glycerol and three fatty acids. This process takes time and is the reason fat cannot be used for quick explosive exercise.

Carbohydrate is stored as glycogen in the liver and muscles. Glycogen is broken down into glucose in the liver and transported to muscles to form ATP. Muscle glycogen however, is used only on the specific muscle in which it is stored and cannot be transported. We can store approximately 2000-2500 calories in the muscle and liver. This is generally our body’s preferred fuel source, especially with high intensity training, as it can be processed more rapidly than fat. High intensity training and low carbohydrate intake will inevitably deplete glycogen stores.

Protein is used in times of extreme low carbohydrate intake and during prolonged aerobic activity. Protein needs to be first converted from amino acids to glucose and, as with fat, it is processed slower than carbohydrate. Amino acids are cleaved from muscle tissue, it is thought to produce around 30,000 calories on average, depending how much muscle the individual has.

Energy is measured in calories. A calorie is the amount of heat required to raise 1 gram of water by 1 degree celsius. Our food is measured in kilocalories, so an item of food that contains 100kcals is actually 100,000 calories.

The different types of energy systems

Depending on the sport, duration, intensity or goal your body will use different energy systems. There are several energy systems used to create energy by the body. In the initial stages of energy production, oxygen is not used (anaerobic metabolism) as the duration increases oxygen is required for energy production (aerobic metabolism).

The aerobic and anaerobic energy systems do not work individually, but overlap, although one will be working more than the other. It’s now believed that even when working in the most anaerobic conditions the aerobic system will also be working.

It’s the intensity and duration of a specific activity that determines which energy system will be utilised.

There are three separate energy systems that produce ATP:


The first is known as the ATP-PCr system (adenosine triphosphate - creatine phosphate system), also known as the anaerobic a-lactic (ALA) system. Phosphocreatine is broken down into creatine and a phosphate, releasing energy. The phosphate is then used to rebuild the ADP back to ATP, controlled by an enzyme called creatine kinase. The ATP-PCr energy system is able to function with oxygen (aerobic) or without oxygen (anearobic) but because it does not rely on oxygen it is known to be anaerobic.

Irrespective of the exercise/activity performed and intensity, ATP is relied on almost exclusively. ATP will provide the first 5 seconds of energy, with the phosphocreatine supplementation adding an additional 5-8 seconds. The combined total of energy coming from this system equates to around 3-15 seconds. As these two substrates deplete so too will the power of the activity being performed. This system is used with short duration high intensity activities that last less than ten seconds. Athletes who participate in events/sports such as shot putting, weight lifting, American football, 100m sprint and speed skating will predominately use this system. The waste products from this system are minimal, as the duration is so short. It is in this energy system that strength training takes place.

When an activity lasts longer than this short burst, we move into the next energy system called the glycolytic system.


The next system is called glycolysis, also known as the anaerobic lactic (AL) system. This energy system relies on the breakdown of glucose. This glucose comes from the carbs we eat and can be found either in the blood stream or stored in the liver and muscle. Glycolysis can be identified as either aerobic or anaerobic, depending on the intensity of exercise and subsequently the final products produced. Pyruvic acid is the end product of glycolysis. If it is aerobic glycolysis, the pyruvic acid will be converted to glucose and used in the krebs cycle (next system). In the event of anaerobic glycolysis, it will be converted to lactic acid, leading to fatigue.

Oxygen availability seems to have little to do with which end product is produced, so this particular system can be misleading. Glycolysis can be broken down into fast or slow glycolytic systems. The fast glycolytic system takes over from the ATP-PCr after around 10 seconds, for up to a further 30 seconds. At the 45 second mark, the next energy system takes effect. After 15-20 seconds the pace will inevitably slow. Lactic acid accumulates in the blood and in muscle cells, slows the muscle’s ability to contract, causing a burning sensation, shortness of breath and fatigue. At this point you will either need to stop or lower the intensity and subsequently move into a different energy system.  

Activities like football, rugby, sprinting, American football, middle distance runners (400m-800m) would use this energy system.

I am sure you have heard that training at lower intensities burns a higher amount of fat. Technically this is true. Low intensity exercise will burn a higher percentage of fat than high intensity training. Even when training at low intensities, it takes 15-20 minutes for a majority shift from carb to fat oxidation.

However, training at this intensity burns few calories. You can burn 300 calories from a high intensity session, whilst you would have to walk for hours to burn the same number of calories.

It's okay if you burn a lot of carbs during the workout, this shows you are working at a high intensity, boosting metabolism, increasing fat utilisation for the next 24-36 hours and burning more calories. Any carbohydrate burnt will have to be replaced, by either blood glucose, consumed carbs, or from stored fat converted to glucose (lipolysis). So even if you are oxidising lots of carbs and little fat through the workout, fat still inevitably gets used.

Training within the glycolytic energy system, with explosive short bursts (high intensity interval training) for 30 minutes will burn a lot of calories. At that intensity the main fuel will come from carbohydrate. The high intensity period will release the fatty acids into the bloodstream, but they will not be able to be utilised at that intensity due to the high lactic acid levels in the blood, which block the uptake of fat into the mitochondria (the powerhouse in the cell where fuel is burnt to produce energy). As the intensity drops, so too will the lactic acid levels and the utilisation of fat will increase. This system has a metabolism raising affect. This is called EPOC (excess post-exercise oxygen consumption).

It is believed that training in this way will increase the metabolic rate by 15% over a 24-hour period. In this 24-hour period the body will use fat for energy. When your body trains like this regularly the body becomes more efficient at burning calories and generating energy. This is due to an increase in the number of mitochondria. Exercise creates the ideal conditions for the increased production of mitochondria in muscle. This will in turn maximise the ability of the muscle to burn carbohydrates and fatty acids for ATP. Exercise creates a low-energy signal known as AMP. When there is a rise in AMP, the body will increase ATP production to prevent an energy deficit. Calcium is released due to continued muscle contraction, producing a 300% to 10,000% increase in intracellular calcium increasing contractile ability. The rise in both AMP and calcium are strong signals for the production of new mitochondria, which occurs in the resting state immediately following exercise.

There will also be an increase in testosterone and growth hormone immediately post exercise, which will have a great anabolic (growth) effect. The more muscle you can build the faster your metabolism will become, creating a good fat burning environment while you rest. If there are any ladies who are worried about building muscle, please read the article on why women will struggle to build muscle. Click: Women & Muscle Gain/2.

As each new energy system takes over there is a loss in power.


The next system is called the oxidative or aerobic system. This system uses the heart, lungs and circulatory system to provide the muscles with the necessary oxygen to produce energy. It starts to be engaged after about 30-45 seconds and lasts for hours.

This system consists of 4 steps in the production of ATP.

1. Slow glycolysis (aerobic glycolysis)

2. Krebs cycle

3. Electron transport chain

4. Beta-oxidation

From here on, it’s all aerobic metabolism. Slow glycolysis is the same as fast glycolysis with the end product being pyruvic acid. Instead of it being converted to lactic acid, it is instead converted into a substance called acetyl coenzyme A.

Acetyl coenzyme A enters stage 2, the Krebs cycle. This system is the continuation of the glucose oxidation started with glycolysis. Acetyl coenzyme A enters the krebs cycle and is broken down into carbon dioxide and hydrogen, producing two more ATPs. The hydrogen that is produced causes the cell to become acidic, so it attaches to two enzymes (NAD and FAD) and moves onto stage 3, the electron transport chain.

In the electron transport chain the hydrogen, through a series of chemical reactions combines with water to prevent acidification. This stage yields a further 35 ATPs.

The final stage, stage 4, is called Beta-oxidation. What makes this stage different from all the previous ones is that it can also metabolise fat as well as carb to produce ATP. The breakdown of fat, from triglycerides, to glycerol and free fatty acids is known as lipolysis. The free fatty acids need to be broken down further into acetyl coenzyme A, so they can enter the krebs cycle. This further breakdown is called Beta-oxidation.

Lipolysis produces the most ATP. Every gram of fat is worth 9 calories. The oxidation of fat requires more oxygen than glucose oxidation. Fat produces lots more acetyl coenzyme A than glucose, but because fatty acids contain more carbon than glucose, they require more oxygen to burn them. This system provides energy for low intensity activities, such as long distance running, long distance swimming, crew (rowing) and sea kayaking which rely on the aerobic system.

From the above information you can see why fat can only be used for long duration, low intensity exercise. For the type of training we recommend at YBP, carb will be the preferred fuel source during the training session, leaving fat to be used for the following 24-36 hours, once you have boosted your metabolism.

Only small amounts of protein (no more than 5%) contribute to energy production. However when performing long duration exercise or on when on a low carbohydrate diet, amino acids from protein, taken from muscle, can be converted to glucose (gluconeogenesis). Amino acids can also be converted to acetyl coenzyme A and other intermediates used in the krebs cycle.

This article has
been written by
Terry Fairclough