Dr. Loren Bertocci

by Loren Bertocci, Ph.D

The Endurance Adaptation – Part 3

In my last two articles (Part 1, Part 2) , I wrote about the adaptations that occur, as a result of training, within the myocardium and outwards towards the spaces outside of skeletal muscle. So, as a result of endurance training, what happens within muscle cells? The simple answer is that a lot of adaptations occur in skeletal muscle. For this article, we are going to consider only one of the many adaptations that occur – the training induced increase in the capacity to use fat as a fuel. This phenomenon is known as the glycogen sparing effect of training.

First of all, let’s take a short step back. The goal of endurance training is to increase the capacity for sustained physical activity. In that muscles only contract when there is an adequate supply of ATP, this means that the goal of endurance training is to increase the capacity for sustained ATP synthesis. In order to make ATP, you must deliver two things to your muscles: oxygen and a fuel that can be oxidized (aka “burned”). The “central” endurance adaptations I described in the first of these two previous articles increases the capacity to deliver oxygen to working muscles. The “peripheral” endurance adaptations I described in the second of these two previous articles increases the capacity to deliver fuel to working muscles. Put together, the effect of all this endurance training increases the capacity to deliver the ingredients that are required to make the ATP required by working muscles. So, are we done yet? Nope. Why not? Because, quite simply, the metabolic pathways in muscle cells treat carbohydrates quite differently from how they treat fats.

In order to make sense of this, we need to consider a small bit of chemistry. When a molecule of fat is used as a fuel, it is first converted to a molecule of acetyl-CoA. These molecules of acetyl-CoA enter the citric acid cycle, where, via a series of reactions, they produce the molecules required for mitochondria to use the oxygen that was delivered to make ATP. When a molecule of glycogen (the form of carbohydrate stored in muscle cells) is used as a fuel, not only can it lead to the production of acetyl-CoA, it can also lead to the production of another molecule (oxaloacetate) that dramatically increases the activity of the citric acid cycle.

So, what does this have to do with endurance?

In order to produce ATP at a maximum rate, there must be an adequate supply of both acetyl-CoA and oxaloacetate. Because fat is stored all over the body, and because (relatively speaking) we have a near limitless supply of fat, we have a near limitless capacity to make acetyl-CoA. In contrast, because oxaloacetate can only be produced from glycogen, muscle cells must have a supply of glycogen, and muscle cells only have a limited amount of glycogen. Once the glycogen supply is depleted, the ability of muscles to make ATP is cut to roughly 50%.
Chronic endurance training increases the content of muscle mitochondria. The greater the mitochondrial content, the greater is the ability to use acetyl-CoA to make ATP. Thus, in endurance trained muscle, fat can be used to make acetyl-CoA, sparing any requirement to use glycogen to make acetyl-CoA. Thus, in endurance-trained muscle, fat is preferentially used a fuel source, functionally “sparing” the use of glycogen.

Taken together, endurance-trained muscle is better at using the relatively limitless supply of fat while preserving the relatively limited amount of intracellular glycogen. This is chemistry underlying the glycogen sparing effect of training.

So, from a practical perspective, what does this all mean to the athlete or coach? In order to maximize the ability to use fats as a fuel, and spare the use of glycogen only to make oxaloacetate, the training that is done must be designed to stimulate the greatest possible increase in the content of skeletal muscle mitochondria. With such a training adaptation, the athlete has a dramatically increased capacity to perform extended bouts of exercise. This allows both a far greater capacity for training as well as for any sort of lengthy (several hours) competition. It also means that it is much easier to recover from extended bouts of training and/or competition because there would have been less of a depletion of the glycogen in the working muscles. It also means that dietary supplements known to contain molecules that stimulate an increase of mitochondrial content will augment any endurance-training program. It is for this reason that several of the molecules found in Origins by Pangea were included in the mixture.

If any of you have any specific questions, including any greater details about any of this, please do not hesitate to ask me directly on Facebook.