I keep mentioning these things called metabolic pathways, without really explaining what the hell they are. To be fair, there are whole books on the study of metabolism. This means, that a short article will not do justice in explaining the science behind these energy systems, but will serve to give you an overview of why they are important to you in training, and training specific nutrition.

First, it is important to note that understanding metabolic pathways is a means to achieving your goals effectively, not a magical key to your training. As with all things fitness and life, you must put in the hard work to get the desired results. However, an understanding of these pathways can catalyze your success, minimize wasted energy on unnecessary training that stymies your goals, and help optimize your fitness.

The basics of metabolism, are that there are three pathways used to create ATP (adenosine tri-phosphate). For simplification sake, you can think of ATP as cellular currency, in that most cellular processes must have ATP to occur and it is spent in order for those functions to happen. Our body is an amazing machine, and loves to run an efficient system (the FED could learn a thing or two) that does not run out or store an excess of ATP. For this reason, there are pathways that create energy in different ways, with different fuels. It is important to note, that ATP never runs out in any system and all systems are working at all times; However, our body will always let the most reliable pathway dominate for the activity at hand.

The first pathway, that is the most efficient is called the ATP-PC pathway. This stands for Adenosine triphosphate - phosphocreatine pathway (phosphocreatine is the fuel). This is an alactic(no lactic acid) and anaerobic (without oxygen) pathway, which is subsequently our most powerful. ATP-PC is responsible for explosive, or maximal effort movements and is designed in such a way that it can produce energy in a fast manner. Adenosine triphosphate is broken down into adenosine diphosphate (energy used) and is recombined with phosphocreatine to create more ATP (energy). Simple and effective. The problem here, is that ATP is heavy, and our body can’t rely on this pathway at all times because it would be extremely inefficient to store so much ATP. For this reason, this pathway dominates from 0-12* seconds, and isn’t a long term solution to our energy needs. An Example of an ATP-PC dominated exercise would be a 1rm clean and jerk, or 100m sprint.

The second pathway, which is lactic(produces lactic acid) and anaerobic, is the glycolytic pathway - also known as glycolysis. The first thing to take away from the title of this pathway, is that “glyco” is in it’s name. Any time the prefix ‘glyco’ is seen, I want you to think carbohydrate. In fact, lysis actually means disintegration or decomposition, making glycolysis roughly stand for glucose decomposition. This is important to understand, in order to know that the fuel for this pathway is carbohydrates. These carbohydrates specifically, are in the form of glycogen, which is stored in the blood, muscle, or liver. Unlike our first pathway, glycolysis takes about 10 steps, making it much less efficient for us to get energy from it quickly, which is why this pathway dominates between :30 to 1:30* and is extremely useful for moderate duration exercises. To make this more tangible, an easy example of a glycolysis dominated movement is a 400m sprint. An important side note, is that through the process of glycolysis breaking down glucose, we are left with pyruvic acid. Pyruvic acid becomes useful in our next pathway, oxidative phosphorylation. However, in the absence of enough oxygen excess pyruvic acid becomes fermented and turns into lactic acid. Effectively, as acid builds up around the muscle cells, muscles lose their ability to contract fully and efficiently (damn you acid!). It is important to acknowledge that lactic acid is actually cleared and returned to the liver to create more glucose to further the bodies ability to create ATP (part of metabolism), but the PH decrease (increase in acidity) is what causes extreme muscular fatigue. In essence, lactic acid is our friend, PH dropping hydrogen ions can kick rocks.

Tangent aside, our third pathway is called oxidative phosphorylation and is dominant at rest and during long duration physical activity (120+ seconds*). This pathway takes place in our cell's mitochondria and is aerobic (with oxygen). Oxidative phosphorylation can use remnants of glycolysis(carbs) or fats to produce energy. Remember, after glycolysis we are left with pyruvic acid. This pyruvic acid is put through something called the citric acid cycle (Krebs cycle), and is then modified to produce energy precursors for oxidative phosphorylation ( 2x FADH2 and 6x NADH). In the next step, chemical reactions take place in the electron transport chain, hydrogen combines with oxygen, water is produced, and acidity is prevented. This is an extreme oversimplification of the electron transport chain, but it is key is that you notice this is much more complicated and the more complicated, the longer it takes to yield ATP. Furthermore, this pathway needs oxygen to create ATP, which is limited by an individuals VO2 max. Subsequently, this is why the oxidative energy takes longer to use and the intensity of effort always declines in this pathway. A good example of exercise dominated by this pathway would be 5k run.

So now you ask, why does all of this matter? For one, it is extremely important to train for your goals and knowing the duration and dominance of these pathways will help in that regard. For instance, if your goal is to be a powerlifter, your main focus should be on training the pathway that dominates your sport, ATP-PC. If you’re a sprinter, both the ATP-PC and glycolytic pathways may be the best place to focus your energy in order to get the most out of your training. If you’re training for an endurance event, working in the OXPHOS system is extremely necessary to be prepared for the rigors of a long trial. Furthermore, muscle fiber type is affected by different pathways used. Training high endurance work will produce more low output, type 1 fibers(slow twitch) instead of type II fibers(fast twitch). These fiber types are optimal for oxidative phosphorylation because they are more dense with mitochondria, raising their ability to buffer PH(use pyruvate) and resist fatigue. However, these type 1 fibers are smaller and generally weaker than type IIa/b/x fibers. Being that absolute muscle strength is a function of total muscle fiber cross sectional area, dominantly training this pathway would be the wrong focus for a strength athlete who needs bigger cross sections. Furthermore, understanding the basics of these pathways gives us knowledge on our dietary needs to fuel our exercise properly. The basis for nutrition should be performance and these pathways can shape nutrition through understanding what our sport of choice requires for energy. For instance, if we know that the fuel for maximal effort strength is creatine, supplementation of creatine becomes relevant to a strength athlete, in that increasing creatine levels may help recovery efficiency in the ATP-PC pathway. On the other hand, a strength athlete likely won’t need high levels of carbohydrates a triathlete or CrossFit enthusiast will for glycolysis or oxidative phosphorylation. This also shows that more fat will be required for the endurance athlete who doesn’t have readily available carbohydrates during a race. Simply put, knowledge of these pathways allow us to train more effectively, get better results faster and save us tons of trial and error in the long run. #science

*NOTE THAT TIMING OF PATHWAYS VARY IN DIFFERENT STUDIES. THESE FEW SECONDS ARE NOT EXTREMELY IMPORTANT AS ALL SYSTEMS ARE WORKING ALL THE TIME AND THERE ISN’T A GUARANTEED “SWITCH” POINT FOR ANY SYSTEM.