The Origins of Energy

Welcome back! In this blog, I am going to talk about the exciting topic of energy metabolism, the process of using nutrients and other energy sources to produce energy. First, it's important to know that all energy sources must be broken down and converted to adenosine triphosphate (ATP), a compound that provides energy needed for a variety of processes in the cells in our body. Do you know how we get our energy for sports, exercise, and daily activities? The main sources are phosphocreatine, glycogen, and fat, and I will go into more detail about each of these. At the end, you will have an understanding of what makes each source unique, and you should be able to identify which source you use in your sports or exercise!

Phosphocreatine

Let's start with phosphocreatine, a phosphate compound that is abundant in energy. Where is phosphocreatine stored in our body? Skeletal muscle. (My previous blog is all about skeletal muscle. Check it out if you haven't already!) Phosphocreatine is an immediate source of energy that doesn't require oxygen, but because its stores are relatively small, it can only support exercise for up to 30 seconds or so. Remember the phosphagen system that I discussed in my first blog? Phosphocreatine is what supplies the immediate energy in that system. It's the ideal energy source for short, high-intensity exercises like powerlifting and sprinting, which are anaerobic exercises that do not require oxygen (more on this in a future blog involving macronutrients!).

Glycogen

Do you know what glycogen is? Glycogen is a long chain of glucose units located in skeletal muscle as well as the liver. Essentially, it's how our body stores the carbohydrates that we consume in our diet. There is a notable difference between the glycogen in the muscle and the glycogen in the liver: muscle glycogen can only be used locally in the muscle itself, whereas liver glycogen can release glucose into the blood for use by other tissues. Ultimately, the glycogen from the muscle or the glycogen released from the liver are both very important sources of energy that support our sports and exercise.

Glycogen can be used to generate energy via two different pathways: anaerobic glycolysis and aerobic breakdown. Let's explore how these two processes work.

Anaerobic Glycolysis

During anaerobic glycolysis, glucose is broken down into a compound called pyruvate, which is then converted to lactate in the absence of oxygen. The production of lactate results in acidification, or the feeling of soreness in our muscles, which is certainly a downside of this pathway. The advantage is that anaerobic glycolysis can provide energy relatively quickly. As a result, this is how energy is mostly generated for exercises that generally last up to about 2 minutes. Longer sprints and weightlifting are examples of exercises that use anaerobic glycolysis for energy. In general, high-intensity movements that last from around 30 seconds to 2 minutes are powered by energy that is produced via anaerobic glycolysis.

Aerobic Breakdown

Aerobic breakdown uses both glucose and oxygen to produce energy. Like in anaerobic glycolysis, glucose is converted to pyruvate, but pyruvate is then converted to a molecule called Acetyl-CoA, which goes through a process called the Krebs cycle (the specific details of this cycle are not very important for this basic discussion of energy metabolism). After the Krebs cycle is a process called oxidative phosphorylation, which is responsible for generating a majority of the ATP in aerobic breakdown. The disadvantage of this pathway is that it takes a longer time to generate energy compared to phosphocreatine and anaerobic glycolysis. However, its advantage is that it has an enormous capacity: it can supply energy for up to 90 minutes of endurance exercise!

Fat

Finally, let's discuss fat. Out of all the sources that we've explored in this blog, fat is the most energy-dense and has massive stores in our body (in adipose tissue and some muscle tissue). After fatty acids are released from these stores, they undergo beta oxidation, which is similar to the aerobic breakdown of glucose. The advantage of fat as an energy source is that it generates about 4 times more ATP than glucose does. On the other hand, fat produces ATP at a slower speed compared to that of glucose since fatty acid molecules require more oxygen during beta oxidation. Therefore, fat is a more suitable energy source for long, low-intensity exercise that does not have high, immediate energy demands.

Final Message

An important difference to understand between glucose and fat in terms of energy metabolism is that glucose is the main energy source during high-intensity endurance exercise, whereas fat is the preferred energy source during low-intensity exercise. Like I mentioned earlier, though, we should also consider the timing of the exercise to determine where our energy is coming from. The bottom line is that the energy source that we rely on in a certain situation depends on both the intensity and duration of our exercise or sport. As the intensity decreases and the duration increases, the main energy source shifts from phosphocreatine to glucose to fat.

Hopefully, you've just learned many interesting facts about the different energy sources that we use to power our exercise and sports. Think about what sports you play or what types of exercise you like to do. Can you determine where your energy is coming from during these activities? Feel free to respond in the comments! Thanks for reading this blog!