As you should be aware, in order to exercise (or just do anything in general) our body needs energy. The big question really is how do we get this energy and what is it produced from? Well if you read my earlier post on A Guide to Nutrients then you would know that we get our energy from carbohydrates, protein and fat, but not in equal measures (carbs and protein produce 4kcal/g where as fat produces 9kcal/g respectively). This is not the full story though, usable energy is created using a molecule called ATP (adenosine triphosphate), but all of these nutrients are involved in different ways to produce this chemical. It is probably worth pointing out that this post is going to be a precursor to one on weight loss through exercise and also another article about pre exercise nutrition! This post is a bit heavy with biology and chemistry but it isn't too complex so don't panic!
So what is ATP?
ATP is thought of as the energy currency of the body, its used in virtually every process in the body. It is made up of adenosine (an adenine ring and a ribose sugar) and 3 phosphate groups. ATP produces energy by hydrolysing (reacting with water) one of the phosphate groups to form ADP (adenosine diphosphate) and a cleaved phosphate group, releasing energy in the process.
ATP + H2O → ADP + Pi
The amount of energy released is about 7.3kcal/mol (1 mol is equal to 6x1023 units of a compound) which is a relatively small amount however as the ADP unit can be regenerated back into ATP by using the nutrients in our food, we can keep repeating this reaction, so long as we have energy with which to do the reverse reaction! An interesting fact about ATP is that we have so little of it in our body, on average we only have about 250g, which when you compare the size of us and the amount of energy we have, is only a very small amount.
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| Adenosine ring (right) with the triphosphate tail (left) |
The Energy systems
- The Phosphagen System
- The Glycogen-Lactic Acid system ( or anaerobic glycolyic system)
- The Aerobic system
All of these systems are used in tandem when ever you exert any effort on a task, they are also all important in their own rights as the exert energy at different rates and at different energy levels!
The Phosphagen system
This system uses phosphocreatine to generate ATP to be used in the body. In every muscle you will automatically have a small amount of ATP, this is used up very quickly and needs to be replaced if any more work is to be done. The Phosphagen system is used to quickly replace this ATP by breaking up phosphocreatine using an enzyme called creatine kinase into creatine and a phosphate group plus a bit of energy. The energy is then used to bind the phosphate group to an ADP molecule thus regenerating the ATP molecule needed for energy transport.
Phosphocreatine → P + Creatine + Energy
ADP + P + Energy → ATP
This process is very rapid and can generate ATP instantly to be used in explosive movements, such as when trying to lift a heavy weight or when sprinting. There is only a small amount of Phosphocreatine available so after 6 seconds or so this system ends and cannot be used again until the creatine is converted back to phosphocreatine which takes time. It is unsurprising then that we can only sprint at full pace for a few seconds as the energy produced cannot match the energy required!
Many of you will have heard of people taking Creatine supplements and this is the reason why. If you can raise the amount of creatine in your body you theoretically should have more explosive energy and should be able to lift heavier weights for prolonged periods, this however is something I plan to discus in a post about supplements in the future! For the sake of this post though its worth saying that creatine is made naturally within the body using amino acids that are obtained from protein in the diet!
The Glycogen-Lactic Acid system
This second system is a longer lasting but a bit less rapid system then the first method and involves the anaerobic breakdown of glucose. As mentioned in the post about nutrients, Carbohydrates are broken down and converted to glycogen for storage. When we need energy they are converted back into glucose and used as a fuel to make ATP. This can be done in 2 different ways, 1 of them using oxygen (aerobic system) and the other without oxygen (anaerobic system). This system focuses on the second one and is important when considering high intensity activity that last for about 1-2 minutes.
When glucose is broken down as a fuel source when there is no oxygen present it gets converted into pyruvic acid and then to Lactic acid, while at the same time producing 2 molecules of ATP. This process is dependent on 2 things, the amount of glucose available, which is obviously limited, and the amount of Lactic acid produced. Lactic acid is a chemical that is linked with fatigue, and causes the burning sensation you feel when you've done some high intensity workouts. It isn't actually the Lactic Acid that causes this burning sensation but the Hydrogen Ions that are produced with it. Either way if the build up is too large then it will cause muscle damage and stop them from contracting. This means that we can only use this system for a short amount of time, which is why it only really dominates in high paced distances such as 400 / 800 meter running or when doing a set of weights in the gym.
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| Aerobic Pathways (left) and Anaerobic Pathways (right) |
If using the anaerobic route only lasts a short amount of time, produces a short amount of energy and also causes harm to our body, they why do we do it? The simple answer is the speed at which it occurs, while not as fast as the Phosphagen system, it is still much quicker then the aerobic system so will help us get energy straight away and get the body moving, this would have been essential to our survival when we first walked this earth as if it took a couple of minutes before we had the energy to run away or fight something we wouldn't have survived as a species!
The Aerobic System
The final system produces by far the most energy but is also by far the slowest method by which ATP is regenerated! It is similar to the anaerobic method in the fact that it converts glycogen to glucose but this time oxygen is present so a different reaction pathway is followed!
The glucose is again converted to pyruvic acid creating 2 units of ATP but this time it doesn't get turned into lactic acid. Instead the pyruvic acid is used in the pathway called the Krebs cycle which is done in the mitochondria within the cell (they are like the batteries of the cell!) and produce another 36 molecules of ATP brining the total up to 38 units of ATP! As you can see this is a massive increase compared to what was available before! On top of this fat can also be used to make ATP using oxygen and this produces between 80 and 200 units of ATP, it isn't surprising that fat makes more as I mentioned before that it is far more energy rich!
This process is clearly far more energy rich, with the only major downside being the rate at which it occurs. The question is; why does it take so long to produce energy? There are 2 reasons for this, the first is that large amounts of oxygen is required, which means that the rate at which you breath needs to be increased, which relatively is a slow process, on top of this the reaction pathway is very long so it takes long for the ATP to actually be made!
As you may have already guessed, this system is more suited for long term 'endurance' events like long distance running or any even that takes an hour or longer. This gives us a clue as to why you will burn off more fat by jogging then by sprinting. Fat can only be burned off through aerobic activities, and as the concentration of glucose in the muscle and blood decreases the amount of fat used for energy increase. It is important to note however that fat needs carbohydrates to be used as fuel, so don't think that you can just cut out all of your carbs and your will just burn through your fat, this just won't happen!
That's your lot on energy systems for now, Sorry for all the text I promise the next post will contain more pictures!! If you need any more detail on anything said above let me know!
Cheers,
Rich
References
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