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The Metabolic Basis of Endurance

How is it that endurance athletes get faster and go farther?

Many of us are familiar with the primary building blocks of an endurance training regimen; the easy days, the speed and threshold workouts, the endurance (long) efforts, typically structured with approximately 80% of total weekly volume at low intensity and 20% at moderate to high intensity.

There’s a solid scientific foundation underlying the practice. But what is it? Why are these conventions essentially universal for endurance athletes? The science of endurance adaptation is a deep topic, but I think it benefits all of us to have a clear understanding of what we’re really pursuing with ritualistic training regimens.

There are other aspects to endurance of course, but the foundation which can’t be denied are metabolic. Fuel delivery is essential and arguably the most stubborn to improvement.

The Energy Sources of The Body

The human body relies largely on carbohydrates, fats, and proteins to stay alive. Of these, protein acts more like a building block for repair (more on that later) than as a true fuel source, so first we’ll be focusing on the major fuel sources that we rely on to keep us going for our daily workouts. These fuels are converted to create our raw energy source, known as Adenosine Triphosphate (ATP), in our muscles. ATP is what it’s all about in terms of our endurance capabilities. The faster we can make it, the faster we can use it, and the faster (generally speaking) we can go. So, how can we ramp up production of this magic stuff?

Avenues for Carbohydrate Metabolism

Carbohydrates are the macronutrient that gets all the attention from endurance athletes, and there is some merit to this. They naturally dominate as the single greatest fuel source to humans, and they’re used by working muscles through two distinct processes. These are anaerobic glycolysis and what we will just call “aerobic respiration” (which in truth is a complex series of processes beyond our need). They are both happening all of the time in the body, but what’s critical to endurance is the rates of each, and how those rates correspond to one another. We will come back to this very important point later.


Before carbohydrates become ATP, but after they move from the primary steps of digestion, they reach working muscle through the blood as a molecule called glycogen. Once glycogen arrives at the working muscle, it will first undergo glycolysis, every time. This is the required first step of glycogen metabolism. The reaction of one glycogen in glycolysis produces two molecules of the chemical pyruvate.

The path of a pyruvate molecule is uncertain at this point. If it remains in the cytoplasm of the cell for too long, pyruvate undergoes anaerobic glycolysis, resulting in the production of two ATP molecules and the formation of lactic acid. For all of its negative reputation, lactic acid does not stick around long. The unstable molecule immediately dissociates to produce lactate and hydrogen ions that will acidify the cellular fluid. This is an unfavorable result for endurance athletes, and is what leads to the burning painful feeling of exhaustion our muscles experience in a hard and extended sprint. Aside from the discomfort, increasing acidity of the cells reduces their capacity for ATP production through our one energy alternative-- the aerobic process-- and effectively chokes the engine athletes rely on for power production.

The other potential fate of pyruvate is far more beneficial. If the molecule is taken up by the right parts inside of the cell to be metabolized further, through the process of aerobic respiration, then 30-32 additional ATP molecules are created.

It is also worth noting that while the hydrogen ions resulting from lactic acid dissociation are a downside, the corresponding lactate molecule can be utilized in aiding the glycolysis step of glucose to form more pyruvate. Thus, lactate can still provide some benefit.

Flowchart of Anaerobic Glycolysis and Aerobic Respiration
A fork in the road: how will our muscles use a pyruvate molecule?

Influencing Metabolism of Glycogen

Glycolysis is fast and easy for cells to perform. It’s also, as already mentioned, a prerequisite for aerobic metabolism of pyruvate. When the demand for energy by working muscle cells is high, glycolysis is how that energy is acquired because it is the fast and easy option. But rapid glycolysis will result in rapid production of pyruvate molecules as well, which, if not aerobically metabolized, will give break down into lactic acid and damage the cell. That means if we’re going to maintain a high level of intensity level without doing muscle damage, we have to burn those pyruvate through aerobic processes fast enough to stave off lactic acid accumulation.

Fat Metabolism

Fat is metabolized by the same pathway as aerobic carbohydrate metabolism, but is naturally predisposed to a slower rate of utilization. In an untrained athlete the body will metabolize little or no fat at high intensities, turning instead to anaerobic glycolysis of glycogen energy stores. However, research has shown that with diligent training, fat metabolism can be strengthened considerably so that it adds very large contributions to aerobic base and also can provide substantial contributions of energy at the highest intensity levels as long as the aerobic respiration pathway is robust.

Fat provides a nearly infinite energy source to our bodies-- more than 100,000 calories in the leanest athletes-- compared to the 2,000 calories of glycogen that can be stored by the liver. It also can only be used in the efficient aerobic process of the body, meaning it doesn't pose a risk for lactic acid production. These characteristics make fat a preferred fuel for some athletes.

The Obstacles to a Record-Setting You

Human endurance (that is, not just running) is fueled by carbohydrates and fats. Our bodies can use carbohydrates anaerobically or aerobically, and fats can only be used aerobically. So how do we teach our bodies to utilize these fuels faster and more efficiently so that we as athletes can go farther or faster than ever before?

Muscles make the magic happen. There are, roughly speaking, three types of muscle fibers to be found in the body (in reality most muscle fibers fall along a continuum). These are typically referred to as Type I (slow twitch), Type IIA (semi-fast twitch), and Type IIB (very fast twitch). All humans have a combination of all of these fiber types, and the ratio proves to be heavily influenced by genetics, training history, nutrition, and other factors. All three fiber types demonstrate capability in producing ATP by glycolysis, but Type IIA and Type IIB fibers are limited by a key piece of gear for initiating the aerobic process. This is the organelle (organ of a cell) known as the mitochondrion. It’s primary function is to be the energy production factory of the cell, creating ATP in large numbers through aerobic respiration.

The problem is that not all muscle fibers are well endowed with a large quantity of mitochondria. Very fast Type IIB fibers have a sparse number, and Type IIA fibers only have a marginally greater number. Thankfully, slow-twitch Type I muscle fibers have an abundance of mitochondria making them efficient aerobic energy producers.

When we’re operating at high intensity, all three types of muscle fiber may be active because the demand on the body is high, and all of them are capable of anaerobic glycolysis metabolism of carbohydrates. However, since only Type I fibers have high capabilities for aerobic metabolism of pyruvate, the burden of metabolizing the pyruvate made in all three cell types falls almost exclusively on this one subset of muscle fibers. The consequence of not meeting that need is acid accumulation in the muscle.

In practice, two obstacles stand between You today and the Record-Setting You of tomorrow. First, your muscles must become skilled in shuttling pyruvate molecules out of fast-twitch fibers and into slow-twitch fibers before they break down into lactate and hydrogen ions. Second, the quantity and capacity of our slow-twitch muscle fibers must be ready to process all those pyruvate molecules when they arrive.

Training Endurance

The obstacles of (1) shuttling pyruvate into slow-twitch fibers and then, (2) effectively utilizing pyruvate through aerobic respiration to produce large quantities of ATP, are what define endurance metabolism. They’re drastically different goals to the muscle and thus are realized through drastically different training.

High intensity speed work recruits a large percentage of muscle fibers, leading to rapid production and accumulation of pyruvate and lactate. This buildup is essential to training the body’s ability to shuttle the byproducts into cells with high quantities of mitochondria that can proceed with aerobic respiration.

Low intensity activity requires much less ATP per second. We don’t have the chaotic buildup and shuttling of fuels from cell to cell here. There’s not much lactic acid formation, which is why athletes don’t encounter the same muscle burning sensation. The benefit is that this level of activity is sustainable for a much longer time, and our mitochondria are still using aerobic respiration at an elevated level compared with resting periods. Our muscle cells need to work just hard enough and for long enough that they are stimulated to adapt by producing more mitochondria and more Type I muscle cells so that the capacity for efficient ATP production has increased.

The Takeaway

The most fundamental aspect of endurance is to teach the body to turn fuel into energy. In simplest terms, that’s what our muscles are getting better at with every stride. These concepts of metabolism allow athletes to do everything from run 100 miles to throw down a new fastest mile time. Understanding the fuel sources of the body is a powerful tool in adapting your training to meet the demands of a race or goal.

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