Understanding the physiological and metabolic mechanisms underlying the regulation of the different substrates that our body uses as fuel during exercise is something that sport training and nutrition experts should never forget about. In fact, it is a key aspect to know about that will enable us to take rigorous and effective decisions as well as to apply nutritional and training strategies.
This time I want to talk about how the oxidation of fatty acids is regulated during aerobic or endurance exercise. I will go through their transport into the skeletal muscle cell and mitochondria, and their oxidation, focusing on understanding the underlying molecular mechanisms that effectively regulate their oxidation during exercise.
The respiratory exchange ratio (RER) – the ratio between the carbon dioxide produced and the oxygen consumed by our metabolism – is a measure used for estimating the fuel utilization by our body. Although a detailed explanation of this measure is beyond the scope of this post, I just want to make clear that values close to 0.7 reflect a reliance almost exclusively on fat as an energy source, whereas values close to 1.0 or higher reflect a reliance almost exclusively on glucose as the primary energy source.
Prolonged exercise at low-to-moderate intensity results in a gradual decrease in RER, which means an enhanced fatty acid oxidation at the expense of carbohydrates as an energy source. When exercise intensity increases above 55-70% VO2 max (it depends on the fitness level of the individual), the fuel utilization undergoes a shift towards carbohydrate oxidation.
Therefore, it seems obvious that the extent to which fatty acids contribute to the total energy expenditure during exercise through their oxidation principally depends on the intensity and duration of the exercise bout.
But… Why does this shift occur? And how does it occur? What does it limit the oxidation of fatty acids at high intensities during exercise? In this blogpost I will try to break down the mechanisms behind the regulation of the oxidation of fat during exercise, hopefully in a way that is easy for you to understand.
So, without further ado, let’s dive straight into the metabolism of fatty acids!
Where do fatty acids come from?
There are different sources of fatty acids. We first have those that come from the blood, which constitute an important energy source during exercise. The plasma fatty acids pool is made of:
- Albumin-bound fatty acids liberated from lipolysis within the adipose tissue.
- Fatty acids released from very low-density lipoproteins (VLDL) formed in the liver.
Apart from the plasma fatty acids pool, fatty acids are also stored in the form of triglycerides (TG; three fatty acids bound to a glycerol molecule) within the skeletal muscle cells, what we call intramuscular triglycerides (IMTG). IMTG constitute an essential and more available source of energy since they are located closer to the mitochondria, where they are oxidized. As for the breakdown of IMTG for oxidation, although it has long believed that the hormone-sensitive lipase (HSL) is entirely responsible for contraction-induced lipolysis of IMTG, recent studies show that the adipose TG lipase (ATGL) seems to play a crucial role in the process as well.
Transport of fatty acids into the muscle cell
For the oxidation of fatty acids to happen, they must be transported into the muscle cell (if they come from the blood) and into the mitochondria. This is the first step that can potentially limit the rate at which fatty acids are used as an energy source during exercise.
During exercise, several mechanisms allow for an increased blood flow to the exercising muscles, which results in an enhanced delivery of fatty acids (I recommend you to have a look at a previous post about blood flow redistribution during exercise to better understand this process). Increasing the delivery of fatty acids is an effective way to enhance their uptake by the muscle. In fact, up to 75% of them are transported into the skeletal muscle cells.
Fatty acids that reach the muscle, then, must cross the sarcolemma for oxidation. To do so, several mechanisms exist: simple diffusion of the fatty acids within biological membranes and facilitated transport by membrane-associated proteins. Within the proteins that allow for the facilitated transarcolemmal transport, we have the CD36 protein and its “assistant” FABP on one side, and the FATP protein on the other side. Although they have different characteristics, both of those protein systems contribute to the uptake of fatty acids from the blood.
Remember that before entering the mitochondria (a step that I will address soon), fatty acids must be activated by being converted to its fatty acyl-CoA derivatives, which traps the fatty acid within the cell to maintain gradient (similar to the phosphorylation of glucose to glucose-6-phosphate). This is a critical process that is controlled by acyl-CoA synthetases (ACSs), a group of enzymes that convert fatty acids to fatty acyl-CoA derivatives by adding a molecule or Coenzyme A.
While the CD36-FABP transport system does require of an additional enzyme that converts fatty acids into their fatty acyl-CoA derivatives, the FATP transport system contains the acyl-CoA synthetase activity in itself.
Also, the regulation of CD36 resembles that of the GLUT4 glucose transporter during exercise (you can read it here) since, in addition of residing at the plasma membrane, it is also located in cytosolic vesicles. Muscle contractions during exercise, then, induce translocation of CD36 back to the sarcolemma – this has also been shown to be true for FATP and FABP. As you would guess, this mechanism allows for acute exercise-induced changes in fatty acid uptake – in the presence of increasing signals from muscle contraction during physical exercise, the absorption mechanisms increase and allow an enhanced uptake of fatty acids.
Transport of fatty acids into the mitochondria
Before talking about the whole process of fatty acid transport into the mitochondrial matrix, I want to point out that the mitochondria are delimited by two membranes: the outer and the inner mitochondrial membrane. Between them, there is what we call the intermembrane space which allows for a very different environment than the mitochondrial matrix, very important for processes such as the electron transport chain and the synthesis of ATP. Then, fatty acids have to cross both the outer and the inner mitochondrial membrane to reach the mitochondrial matrix, the core of the mitochondria, where the final step for the oxidation of fatty acids occurs.
So once inside the muscle cell and already converted to their fatty acyl-CoA derivatives, fatty acids from plasma or IMTG must enter the mitochondria for final oxidation. This is not an easy step. Indeed, it is more complex than the transarcolemmal transport explained above.
Fatty acyl-CoAs must be converted into their fatty acyl carnitine derivatives, a process that requires carnitine and the enzyme carnitine palmitoyl transferase 1 (CPT1), located at the outer mitochondrial membrane.
After that, fatty acids have to cross the inner mitochondrial membrane. This process is made possible by the carnitine-acyl carnitine translocase (CACT), which allows facilitated diffusion of fatty acyl carnitine through the inner mitochondrial membrane while returning one molecule of carnitine from the mitochondrial matrix to the intermembrane space for every one molecule of fatty acyl carnitine that moves into the matrix.
Finally, after crossing the inner mitochondrial membrane and reaching the matrix of the mitochondria, fatty acyl carnitine derivatives must be converted back to the fatty acyl-CoA they derived from. The carnitine palmitoyl transferase 2 (CPT2) allows for the regeneration of fatty acyl-CoA within the mitochondrial matrix.
As just mention, carnitine plays a key role in delivering fatty acids for final oxidation in the mitochondria. Thus, changes in the free carnitine content in muscle during exercise may contribute to the regulation of fatty acid oxidation.
We’ve now got the fatty acids within the mitochondria ready for their oxidation. Let’s see how this happen!
Oxidation of fatty acids
Whereas it is well known that the rate at which fatty acids are oxidized within the mitochondria is dependent on their transport, the oxidation process itself seems to be less limited. Indeed, 72 to 100% of the fatty acids that are transported within the skeletal muscle cell during exercise are oxidized. Thus, the more fatty acids delivered to the mitochondria, the higher the rate of fatty acid oxidation.
The process by which fatty acids are oxidized within the mitochondrial matrix is called β-oxidation. This process breaks down the fatty acyl-CoAs into chunks of 2 carbons – the famous acetyl-CoA molecules. The rate of β-oxidation is regulated through negative feedback, among other mechanisms of regulation. This means that when the concentration of the product of this process – acetyl-CoA – is high, the pathway will slow down. On the contrary, when the concentration of substrate – fatty acyl-CoA – is high, it will speed up.
Anyway, the end product of β-oxidation is the acetyl-CoA, which is also synthesized from the glycolytic pathway through the action of the pyruvate dehydrogenase (PDH) on pyruvate. Acetyl-CoA is the common intermediate – an interconnexion point – of all oxidative pathways, be it from carbohydrates or fats, which will ultimately enter the tricarboxylic acid (TCA) cycle and the electron transport chain (ETC) for ATP synthesis. If you want to learn more about the latter steps, take a look at the blogpost on the 3 basic energy systems, where you can also see the interconnection between the carbohydrate and fat oxidative pathways.
You have now a pretty good idea of what the whole process of transport and oxidation of fatty acids look like. But we are here to go further and understand the metabolic and physiological mechanisms behind the regulation of their oxidation. Here we go!
How is the oxidation of fatty acids regulated?
To properly understand the regulation of the oxidative flux of fatty acids, we should first be aware of the high complexity and integration of the human body. Everything is interconnected. By this I mean that the regulation of fatty acid metabolism does not work as a simple ON/OFF switch. Different mechanisms are involved in it instead.
In this case, the glycolytic flux regulates the rate of fatty acid oxidation through different mechanisms. But how does it happen? If you reached this section after reading everything above, you are probably very interested in finding an answer to that, and also really close to it. Here it comes the last and most fascinating part of the article.
Let’s start at the beginning. Exercise at high intensity enhances the glycolytic flux, which results in a significant increase in pyruvate and, specially, lactate production in the cytosol of the muscle cell. Both metabolites then enter the mitochondria for their removal through oxidation. There, the enzyme lactate dehydrogenase (LDH) converts lactate back to pyruvate, which will subsequently be transformed to acetyl-CoA by the PDH.
As many other enzymes in our body, the activity of the enzyme responsible for the production of acetyl-CoA from pyruvate – PDH – is regulated by negative feedback. Remember that this means that increasing concentrations the reaction’s product – acetyl-CoA – will inhibit the enzyme, slow down the rate of conversion of pyruvate to acetyl-CoA, and ultimately reduce the glycolytic flux.
At high intensities of exercise, PDH activity increases and the rate of acetyl-CoA generation becomes higher than the rate of its metabolism in the TCA. To prevent product inhibition of PDH and allow high rates of pyruvate oxidation over time, the acetyl-CoA generated by glycolysis during increasing exercise intensity must be buffered. To this end, the carnitine acetyltransferase (CAT) buffers excess acetyl-CoA by converting it to acetyl-carnitine.
This buffering, however, involves the sequestration of carnitine, which is required by CPT1 to transport fatty acids into the mitochondria. Hence, the decline in free carnitine availability when acetyl-CoA production is high will significantly reduce CPT1 activity, in turn decreasing the supply of fatty acyl-CoA for β-oxidation and increasing the reliance on glucose oxidation.
Altogether, during exercise at high intensity, during which the glycolytic rate is high, muscle acetyl-CoA content is increased with a concomitant increase in acetyl-carnitine content, resulting in the carnitine content being decreased. This allows a high rate of pyruvate oxidation while at the same time limiting mitochondrial fatty acid transport and hence fatty acid oxidation.
On the contrary, during low-to-moderate exercise intensities or prolonged exercise, a lower glycolytic flux reduces the rate at which acetyl-CoA is generated, thus raise the free carnitine availability. This will enhance the CPT1 activity and, subsequently, increase the delivery of fatty acids into the mitochondria for β-oxidation. Then, an increased β-oxidation-derived acetyl-CoA in the TCA will consequently increase fatty oxidation at the expense of glucose as a fuel during exercise.
As reviewed in the previous blog about the determinants of substrate utilization, exercise intensity influences the extent to which we use either carbohydrates or fats as an energy source. In this case, exercise intensity regulates the transport and oxidation of fatty acids through the glycolytic flux and the subsequent carnitine availability.
As I already said in the introduction, understanding the underlying mechanisms of fuel utilization is fundamental for understanding the type of food that we should ingest during different training sessions, or even different moments within the same training sessions. In the same way, it is a key aspect to understand what adaptations we want to pursue through training and nutrition, and to effectively design different training and nutritional strategies.
I am conscious of this blog being pretty dense and complex. But I hope you liked it and helped you better understand how substrate utilization is regulated during exercise. I feel satisfied if I achieved that.
Do not hesitate to contact me for any doubt or to discuss anything you found interesting or too complex! We can both learn from each other!
- Lundsgaard, Anne Marie, Andreas Mæchel Fritzen, and Bente Kiens. 2018. “Molecular Regulation of Fatty Acid Oxidation in Skeletal Muscle during Aerobic Exercise.” Trends in Endocrinology and Metabolism 29 (1): 18–30.