Metabolism is the breaking down of fuels, for energy. This happens in every cell in the body, and is often referred to as cellular respiration. It is essentially a series of chemical reactions, which take place within the cell, where fuels are broken down, and energy for function is one of the byproducts of the reactions.
There are two pathways present in metabolism. Basically, they are the same process, but in opposite directions.
Anabolism is the process of building up. This is usually done by combining energy and fuels to cause the building process.
Catabolism is the reverse process. This is breaking down into smaller parts.
When energy expended, and fuel taken in are in balance, homeostasis is maintained. However, if energy expenditure increases, this means the breaking down of fuels for that energy also increases. This means that catabolism increases. For anabolism to increase, fuel intake needs to meet, or even exceed, the increased demand for energy.
In order to build muscle cells (anabolism) there needs to be sufficient fuel taken in. In order to decrease stored fat, energy expenditure must exceed energy intake. However, if the balance is skewed too far, the body will go into shock and begin breaking down proteins of the body for fuel, instead of the stored fat reserves.
ENERGY SYSTEMS
What gives us energy for muscle contractions?
As mentioned earlier, muscular contraction uses energy at the base of the myosin heads, to cause them to twist and collapse. This movement results in shortening of the muscle fibre. The chemical used for this energy is adenosine triphosphate (ATP). As the name suggests, it is one adenosine molecule, bound to three phosphate molecules. When broken down, this is converted into adenosine diphosphate (ADP) and a phosphate molecule, and a fairly large amount of energy is released. However, there is a very small amount of ATP available in the muscle cells. So, it is understandable that this would be depleted rather shortly. As a result, muscles need a way of generating more energy, or more ATP, in order for them to continue contracting and relaxing, to perform movements.
ATP itself isn’t completely remade. Rather, ADP and the free P are bound back together. This replenishes the ATP stores, and allows the muscles to keep working. This is done using 3 pathways.
ATP-PC System
The first of the three systems, this energy system produces a large amount of ATP in a very short space of time. This is used to meet the demand of very high intensity, short duration, activities. It uses the muscles’ stored creatine phosphate (also referred to as phosphocreatine) to regenerate the ATP stores. But, this speed of generation comes at a price. The PCr stores are only enough to provide energy for approximately 10 seconds, on average. Fortunately, as with all the energy systems, these stores can be replenished. And, in the case of PCr, this takes in the region of 5 minutes, as long as all the necessary fuels and chemicals are available.
Lactate System
The next system from which we get energy is known by many names. Anaerobic glycolysis, lactic glycolytic, lactate system, and even lactic acid system are all among the names given. And, depending on how academic the person you may be speaking to, these will often receive differing reactions. The reasons for this are that, while oxygen is not used it is still present. And, while the lactate molecule itself is technically an acid, blood is basic. This means that an acid cannot really exist, in the basic medium. So, with these points in mind, there are a few very important aspects to be taken from this. This system breaks down (or “burns”) muscle glycogen (carbohydrate stored in the muscle) without the use of oxygen. And, a byproduct of this process is lactate.
This energy production is not quite as fast as the earlier discussed system. But, it is still fairly rapid. This means the system is still used for high intensity activity, but for activities that last slightly longer. And, as we have seen a few times already, speed leads to exhaustion. This system can provide energy for an average of 90 seconds, before a combination of built up lactate, and muscle glycogen depletion will force the participant to either slow down, or stop the activity. Continuing on, at a lowered intensity, will allow for shuttling of the lactate build-up out of the working muscles. Circulation will remove all excess lactate, over time. So, as long as the intensity is low enough to result in less lactate being produced than can be removed, it won’t build up any further. As for the glycogen stores, these will take significantly longer to replenish, once they have been completely depleted. In order to do this, food needs to be taken in, and converted into muscle glycogen, which can run into a few hours.
Aerobic System
The final system is also the one with the slowest rate of energy production. Similar to the lactate system, this also uses glycogen. But, it doesn’t only use glycogen from the muscles. It also uses the glycogen stored in the liver, along with fats available to the body. These fuels are broken down using oxygen. The process is significantly longer than the previous system. However, the amount of ATP yielded, for the same amount of fuel, is vastly greater. It can be up to 10 times the amount. Understandably, a combination of these attributes means this system is more dominant in lower intensity activities. And, as for duration, as long as the fuel is maintained and byproducts removed, the system can continue almost indefinitely.
And, considering exhaustion of this system is dependent on factors other than the energy production; its recovery rate is also dependent on those factors, instead of on fuel replenishment, or waste management
Here is a brief summary of the Energy Systems:
| Name | Fuel | Duration |
| ATP-PC | Phosphocreatine | Approx 10secs |
| Lactate | Carbohydrate, without oxygen | Approx 90secs |
| Aerobic | Carbohydrate and fat, with oxygen | Potentially indefinite |
The so-called “Fat-Burning Zone”
What has been termed the ‘fat-burning zone” is a level of exercise intensity where fat is the predominant fuel. It has been used for many years as a target intensity for fat loss.
This is actually a misunderstanding, for the most part. Is there a level of exercise intensity that uses mainly fat for energy? Yes, there clearly is. But, if a person trains once, at this intensity, they don’t suddenly lose fat. The next time that person eats, homeostasis will be restored. And, this will include the amount of fat storage. Similarly, someone who never trains at this intensity, but rather trains regularly, at higher intensity, will increase their lean muscle mass (provided they eat appropriately). This person will then burn more calories at rest, and therefore slowly decrease fat stores.
So, it can be seen that decreasing fat stores is not done by purely training at a particular intensity. It is a result of regular training, combined with eating habits appropriate for the training being done.
Another thing with mentioning is the label of “myth” given to the fat-burning zone. As we said, the zone is a misunderstanding. However, to call it a myth is also a misunderstanding. There is, in fact, a level of intensity that will use more fat than other fuels. So, calling it a myth is a misnomer. It is very likely that that name attached itself to the idea that training at this intensity isn’t all that it involved in decreasing fat stores, and has now become the popular perception.