“I’ve got a ton of energy today!”
We all know what it feels like to utter this statement, and hopefully, you feel like this more often than not. There are high-energy days, low-energy days, and certain times of the day when we feel energy-filled or energy-depleted. Our lives are, quite literally, energy. And though we use the word “energy” in a metaphorical sense to describe how we feel, the literal ways in which our body produces energy determine how we perform.
The chemical energy that we eat as food ultimately gets converted into physical energy — typing on a computer, running, jumping, or lifting a heavy box. But what happens between our big breakfast and our afternoon workout that creates the energy we use to engage in activity?
What happens is metabolism. Metabolism can be a complex discussion, but we’re here to break down the fundamental concepts of how our body produces energy. There are three primary energy-producing systems: the phosphagen system, anaerobic glycolysis, and the oxidative system. This article will dive into how each system gives us the energy we need.
Before we talk about the “how” of energy production, we need to talk about the “what.” What does our body actually use to produce energy?
Though some fad diets demonize them, carbs are, for the most part, our body’s primary source of energy.
All of the carbohydrates that we consume are ultimately metabolized into a simple sugar known as glucose. This glucose then circulates throughout the body (as blood glucose) or gets stored in our muscles and liver as a complex sugar known as glycogen. Glucose and glycogen can be used by all tissues in the body to produce energy in the form of adenosine triphosphate, or ATP.
Glycogen is a fantastic energy source for muscles, and we can store about 2,600 calories (kcals) worth of glycogen in our body. This isn’t much, which is why endurance athletes are advised to consume a high-carbohydrate diet in order to sustain their glycogen levels. One gram of carbohydrate provides us with about 4 calories worth of energy. We’ll talk about how much ATP we get from breaking down glucose later on.
Our body can also use fat (“burn fat”) for energy. We get substantially more energy from a gram of fat (~9 kcals) than we do from a gram of carbohydrate. But, as we will see later, fat metabolism is a complex process and is slower than carbohydrate metabolism.
Our body can store a lot of fat — over 70,000 calories worth. This makes fat a fantastic source of energy, which would have been advantageous for our human ancestors who needed to call on their stored body fat for fuel when food wasn’t always readily available. Having a method for storing energy is a must when you don’t know when your next meal will be.
Protein won’t get much of a discussion here — we rarely use it for energy. Only under extreme circumstances of starvation or energy depletion will we break down protein into amino acids, which can then be used to produce ATP. Like a gram of carbohydrate, a gram of protein provides us with about 4 kcals worth of energy.
While we won’t discuss them here, other fuel sources can also be used for energy. These include acetate, medium-chain triglycerides (MCTs), and ketones like beta-hydroxybutyrate and acetoacetate.
Now that we know the inputs, let’s talk about the pathways in the body that are responsible for the outputs (energy).
Our body uses three primary interacting pathways to regulate the production of ATP at rest and during exercise. As we talk about the three energy systems and how they generate energy for our bodies, we will also talk about when each system is primarily being used, the main fuel sources (inputs) for each system, and how much energy we can derive from each.
The first of the body’s energy systems is the phosphagen system, sometimes called the ATP-PCr system. It’s our body’s “immediate” source of energy. This process doesn’t require oxygen to produce energy, and it’s therefore one of our body’s modes of anaerobic energy production.
In this process, PCr is broken down into creatine and a free phosphate molecule. This phosphate is then used in a reaction with a molecule called adenosine diphosphate (ADP) to produce ATP — our cell’s energy currency. The phosphagen system provides energy rapidly and for a short amount of time during intense exercise. When you take off for a fast sprint or complete a quick, powerful lift, for example, the phosphagen system can provide energy for about 3-15 seconds of near-maximal effort.
Even though it’s rapidly depleted, the system recovers quickly. After about 30 seconds of rest, we’ve replenished about 70% of our phosphagen stores, and after 3-5 minutes, nearly 100%.
After we’ve exhausted the ATP generated through the phosphagen system and our demand for energy increases, we continue to generate energy anaerobically through another process known as glycolysis — the breakdown of glucose. If you’re a physiology or pre-med student, then you’re likely all too familiar with every detailed step of glycolysis. Perhaps you’ve got it memorized (or did at one point).
During glycolysis, glucose (from circulating blood glucose or glycogen stored within our muscles and liver) is broken down in a multi-stage reaction that results in the production of a molecule called pyruvate (or lactate). 2 molecules of ATP are also produced in the process. If we break down glycogen, 3 molecules of ATP are produced instead.
Glycolysis is also an anaerobic process — it doesn’t require oxygen — and it doesn’t produce that much ATP. One advantage of anaerobic glycolysis is that it can produce ATP at a fast rate, which is why it’s the energy system that predominates during “all-out” exercise. Unfortunately, we only get enough ATP from anaerobic glycolysis for around 1-3 minutes of high-intensity exercise. Anaerobic glycolysis also produces energy during the initial stages of prolonged endurance exercise.
Another “drawback” of anaerobic glycolysis is that one of its products, pyruvate, is converted to lactate when oxygen isn’t present. As lactate levels increase, our muscles become more acidic due to the release of hydrogen ions from lactate, and this acidity impairs the ability of the muscle to produce energy and undergo contraction. This is the reason why lactate is often (falsely) accused of being the culprit for fatigue during exercise.
The third energy-producing system in the body is known as the oxidative system or aerobic metabolism. Aerobic metabolism is a slow process, so it’s what we use to produce energy at rest or during long-duration exercise at a lower intensity — when ATP demand isn’t as high.
As we increase the duration of exercise from several minutes to hours, our energy production becomes mostly aerobic — meaning that the process utilizes oxygen. Aerobic energy production occurs in the mitochondria (our cellular powerhouses), during which ATP is produced from the breakdown of carbohydrates and fat. The two primary aerobic energy pathways are the Krebs cycle — sometimes called the citric acid cycle — and the mitochondrial electron transport chain.
Aerobic metabolism uses carbohydrates and fat for energy, so let’s discuss each of these substrates separately.
The process of glycolysis discussed above also occurs during aerobic metabolism. The only difference is that the end product of glycolysis — pyruvate — isn’t converted to lactate when oxygen is present. Instead, pyruvate is converted into another molecule known as acetyl coenzyme A (acetyl CoA). Acetyl CoA then enters into an energy-producing pathway known as the Krebs/citric acid cycle. Like glycolysis, we also derive about 2 molecules of ATP for every acetyl CoA that passes through the Krebs cycle.
During glycolysis and the Krebs cycle, hydrogen ions (electrons) are produced. These electrons can enter into a process known as the mitochondrial electron transport chain. The electron transport chain involves a series of steps during which electrons pass through specialized protein complexes in the mitochondria, ultimately resulting in the formation of about 3 molecules of ATP.
In total, the breakdown of one glucose molecule through the oxidative system can provide us with 32 or 33 molecules of ATP.
Our body can also use fat for energy — commonly called “fat burning.” To use fat for energy, we first have to liberate triglycerides (our stored form of fat) from our body’s fat stores and break them down into free fatty acids. Free fatty acids are also used to produce acetyl CoA and enter the Krebs cycle. The only difference between carbohydrate and fat metabolism is that acetyl CoA is produced from fat metabolism through a process called beta-oxidation. Beta-oxidation happens in the mitochondria. After beta-oxidation occurs and acetyl CoA is produced, it enters the Krebs cycle, and the process of energy production is the same as it is during carbohydrate oxidation.
One of the main differences between breaking down fat and carbohydrate (glucose) is the amount of energy they provide. The breakdown of a gram of fat can produce as much as 129 molecules of ATP, compared to the 32 or 33 that we get from the breakdown of a gram of glucose. Fat is a much more energy-dense substrate compared to carbohydrates.
But breaking down a gram of fat requires more oxygen than breaking down a molecule of glucose, and we actually get less ATP from fat for every molecule of oxygen (about 5.6 ATP) compared to the ATP we get from carbohydrates (about 6.3 ATP).
Second, energy production from fat metabolism is much slower than that of carbohydrate metabolism. This is why during high-intensity exercise, most of our energy is derived from the breakdown of carbohydrates — which provide energy at a much faster rate to meet the increased demand for ATP.
It may seem redundant to have all of these systems producing our energy. But under different scenarios — our state of activity or our state of being fed or fasted — different energy systems will dominate. And, as we’ve seen, not only will different energy systems dominate under different conditions, but the energy substrates we use will also change.
For instance, the PCr system, as mentioned earlier, provides all of our energy at an intensity above our maximal oxygen consumption (VO2 max), but can only last for about 10-15 seconds. After that amount of time, our PCr stores run out, and we need time to replenish them (this usually takes around 30-180 seconds and is why you need a long rest interval between lifting heavy weights or doing all-out sprints).
Anaerobic glycolysis can provide energy for around 1-3 minutes at an intensity right around our VO2 max. After this time, the ability to produce energy is limited by our blood becoming too acidic at high exercise intensities, which impairs the function of glycolytic enzymes and the muscle’s ability to contract.
A key feature of our anaerobic energy systems is that energy can be provided quickly. When we exercise at a high intensity, we need rapid ATP production because we are breaking down ATP so quickly. These energy systems can activate within seconds and are therefore ideal for providing energy during high- to maximal-intensity exercise.
Our aerobic energy systems take a bit longer to warm up. But after a few minutes of exercise, aerobic metabolism becomes the main provider of ATP. However, it is important to note that aerobic energy systems provide energy at a low to moderate exercise intensity when we are using ATP less rapidly and these slower-acting pathways are sufficient to supply energy at the body’s rate of demand.
Now that we’ve talked about how we use carbs and fat for fuel, it’s important to discuss how our metabolic state (or metabolic rate) determines which fuel we’re using.
At rest, when energetic demands are low, our body is mainly utilizing fat as a fuel, with some mixture of carbohydrates as well. If we’ve just eaten a meal that contains carbohydrates, however, we will switch over to carbohydrate oxidation for a short while — about 3 hours or so until we return to the “fasted” state. The supply of energy will determine what we’re using.
Our body continues to utilize fat as a primary fuel source up until an exercise intensity of around 60-70% of our max capacity, after which carbohydrates become the primary energy source. As the intensity of exercise increases even further, the relative contribution of free fatty acids to ATP production decreases, and the contribution of plasma glucose and muscle glycogen increases.
It’s easy to understand why carbohydrates have been the mainstay for high-level endurance performance. Aerobic exercise at a moderate to high intensity requires a high rate of energy production which can only be met by the oxidation of carbohydrates (glucose). Compared to that of fat, the production of ATP from carbohydrate oxidation is faster and more efficient. As exercise intensity increases, our ability to use fat as a fuel is reduced due to reasons that might include fewer fatty acids being released from stored fat tissue and the downregulation of enzymes involved in the release, transport, and breakdown of fatty acids.
As such, carbohydrates are ideal for providing energy for higher-intensity activity.
There are a few drawbacks to relying on carbohydrates to fuel performance. As we discussed earlier, our carbohydrate stores (i.e. glycogen) are fairly limited — we have enough stored glycogen to provide energy for about 90-120 minutes of sustained exercise. This is in contrast to the nearly limitless pool (~70,000+ calories worth) of stored triglycerides (in adipose tissue) on even the leanest of humans. Once glycogen stores start to run low as exercise duration extends (and/or intensity increases), our body starts to fatigue. It’s been hypothesized that muscle and brain glycogen depletion may play a role in fatigue development — our mind and body may both suffer the negative effects of running low on energy. Fatigue onset may occur and muscle contraction may be impaired at around 40-50% glycogen depletion.
Since our carbohydrate stores are limited, traditional advice for athletes taking part in endurance activities is to supplement with additional carbohydrates before or during exercise to maintain adequate energy levels. Recently, however, some athletes are taking a low-carb approach to performance in an attempt to enhance their endurance, reduce their reliance on external energy sources, and derive more of their energy from fat.
No matter your diet or lifestyle, we all produce energy through the same conserved metabolic pathways. Knowing how your body produces energy can help you find ways to optimize your energy levels throughout the day — whether it’s through food, light exposure, caffeine, sleep, or exercise. Since our body’s energy systems are also regulated by circadian rhythms, the timing of our lifestyle factors also makes a huge impact on our energy levels throughout the day. The “whats” and the “hows” are important, but so is the “when”.
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