When discussing metabolism, or converting food into energy, it is important to understand what mitochondria does. Mitochondria is an organelle that lives in our cells and its major function is production of energy. Whether you are sleep, sitting, exercising, reading book, and so on, mitochondria are consistently at work providing us with energy that we are required to complete the tasks.
Before moving on, it is important for me to share terminology so following discussion makes more sense. The terminology to know is ATP, which stands for adenosine triphosphate. You can consider ATP as the unit for energy that is produced and used in our body.
Lets talk about mitochondrial respiration.
Physiology of Mitochondrial Respiration
Mitochondrial respiration refers to how mitochondria produce energy from fat and glucose for our daily performance. We have two energy sources that can be converted into energy, fatty acid, and glucose. Glucose will convert into two molecules called pyruvate and lactate.
Ideal primary energy source is fat. Stored fat or triglyceride can break up into 3 fatty acids and glycerol by the enzyme called hormone sensitive lipase (HSL). Fatty acid, then, with a help of the amino acid called carnitine, it enters mitochondria through the CPT1 transporter. Once the fatty acid enters the mitochondria, the process called Beta-Oxidation starts. This is the process where mitochondria further break down into acetyl-CoA. After the breakdown into acetyl-CoA then enters the last phase called Kreb cycle then to electron transport. Through this process mitochondria can generate 129 ATP from fatty acid.
Glucose has 2 pathways. First pathway is glucose breakdown to pyruvate through a process called glycolysis, then enter mitochondria convert into acetyl-CoA then enter Kreb cycle and electron transport like fatty acids. This process creates 32 ATP.
As we exercise, regardless of how fit you are, energy demand will increase. We do normally produce some amount of lactate, however, as energy demand increases we create more lactate through glucose. Therefore, the second pathway for glucose is after broken down to pyruvate with decrease in cellular oxygen, then convert into lactate. If there’s enough MCT1 transporters in mitochondria, the lactate enters mitochondria then convert back to pyruvate. Pyruvate converts into acetyl-CoA and enters Kreb cycle and electron transport. In this process 32 ATP is created. You may see 2 ATP is created, however, studies have shown that even with lactate pathways can generate 32 ATP.
Mitochondrial Respiration in Exercise
How does this mechanism work in exercise? Energy usage can shift from fatty-acid, pyruvate, and lactate according to physical exertion. At rest in healthy individual, fat utilization is the dominant. The body still use glucose and lactate but in terms of ratio, it is very low. This may change if you have sedentary lifestyle or have high stress. This can promote lactate level to increase. Problem is when you are not trained to use lactate as energy source, the cell in our body cannot hold it and spit it into the blood stream. This may lead to fatigue due to change in blood pH, which also can lead to dehydration due to change in tonicity in the blood even when you are drinking water during the day.
Back to energy usage in exercise. As the physical exertion increases the ratio of energy usage starts to shift. At certain level of physical exertion, primary energy use will shift from fat dominant to glucose dominant. This threshold is also known as lactic threshold. Eventually, the body will not be able to utilize fat as energy source not because lack of fat, but body needs faster fuel. This rate of change can vary depending on how the individual been trained or quality of their recovery from their training.
Comparing fat and glucose, although fat can create more ATP than glucose, it is slower process to create energy though can be long lasting. Glucose on the other hand, despite the lower ATP creation, it is faster production of energy. What this means is, if your goal is to use fat as primary energy source, you want to keep the intensity level low such as brisk walk. If your goal is to lift heavy weight or perform in high intensity training, you will be utilizing glucose and lactate as energy source. Reason why heavy lifting or high intensity training prefers glucose and lactate as energy source is because the muscle can work more efficiently and produce stronger contraction with glucose.
Which Energy System is Important?
This may bring you to the question of which energy system (fat and glucose) is more important. When you are thinking of longevity it is important to be training in both energy system.
In ideal world, you want the fat to be the primary energy source. Reason is it is long lasting, and our body has abundant of them. Even if you weigh 130lb and have bodyfat percentage of 10%, you have 13lb of bodyfat. Which means this person has over 53,000 calories stored in the body. Our body can only store limited amount of glucose. Let’s say you are consuming 300g of carbohydrate. That’s 1200 calories. No where near the number of calories through your body fat. Even if you were able to replenish immediately after or in between your physical activity, frequent intake of glucose may lead to insulin resistance.
Another reason is using glucose as energy produces CO2 in our body. So, frequent production of CO2, which is toxic, will cause the oxidation process in the body faster. It is very important fuel source for generating power, however, it is taxing to the body. In well trained individual, they are trained to exhale CO2 more efficiently than those who are not trained.
Training energy system should be designed towards what you are working for. My primary focus for my training is for soccer performance. Which means, I will need to sprint, run, kick the ball, withstand tackles and shoving for 90 minutes. Therefore, I will need to train in both fat energy system so I can save my glycogen as much as I can and glucose energy system so when I need the most strength, the body can use it. I will also need to train my body to be able to utilize lactate as energy source especially towards the end of the game. This is also requiring my energy system to be able to switch from one to another, which means I need to train my recovery.
If the primary goal for me was heavy lifting, then my primary focus would be training more in glucose energy system because I need more explosive movement which would be more efficient with glucose.
If the goal is fat loss, then it is necessary to train accordingly. For unfit individual, rather than focusing on calories, think of what your body need to work on so that fat can be utilize more efficiently. I admire people training hard but may not leading to what they are wanting to accomplish.
Kota Shimada
Commentaires