In previous articles I discussed the fueling of two of our energy systems – our ATP-PC system as well as glycolysis. In the final installment of this series, we are going to dive into fueling the oxidative system.

This is our primary source of ATP at rest and during longer duration physical activity. Understanding this energy system and how to fuel it can help increase performance in endurance events.

Enough Calories for the Oxidative System

The oxidative system is also known as the Krebs cycle and the citric acid cycle. In this system, carbohydrates and fats are the primary energy sources converted into ATP and this process takes place in the mitochondria of the cell.

Protein is typically not utilized during this energy system except during bouts of exercise greater than ninety minutes and during starvation.¹ This means it is critical to be taking in enough calories of carbohydrates and fats to fuel endurance activity.

Low calorie eating and long, slow distance running are common amongst individuals attempting to lose weight. During these bouts of starvation or prolonged exercise we will use our protein to fuel activity.

Our greatest source of protein in the human body is our muscle tissue. If you do not eat enough or work out too much, you run the risk of burning up muscle tissue for energy. This process is known as gluconeogenesis.

Too few calories from under eating or from over exercising can also lead to weight gain. Going too low in calories can decrease our thyroid hormone T3 by as much as 66%.²

This puts our body into an energy conservation mode and can make weight loss extremely difficult. Having adequate fats and carbohydrates in the diet can help avoid these negative situations.

Carbohydrates and Fats

At rest, fats contribute 70% to energy needs and carbohydrates about 30%. As we learned from previous articles, as intensity increases we shift to using more carbohydrates for energy.

As the activity becomes longer in duration (more than three minutes), we shift to using fats as the primary source of energy. The key to this transition is the amount of oxygen present in the blood.

If we have enough oxygen present in the blood, then pyruvate, the end product of glycolysis, is shuttled to the mitochondria and we enter the oxidative energy system.

In this process we get six molecules of NAHD and two molecules of FADH2. These substrates are then brought through the electron transport chain where they are used to convert ADP into ATP.

This process is known as oxidative phosphorylation. This yields us approximately 38 ATP from one molecule of glucose. This is a much higher energy yield than the other two energy systems.

Our stored fat can also be utilized in the oxidative system. Free fatty acids can be broken down into acetyl-CoA and hydrogen. The acetyl-CoA enters the Krebs cycle and the hydrogen atoms are brought through the electron transport chain and ATP is produced. A limiting factor of all this is oxygen uptake.

The Importance of Oxygen

Oxygen uptake is literally a person’s ability to take in and use oxygen. The beginning of all activity is anaerobic, or without oxygen. This is roughly the first three minutes of activity.

After this three minute period we are left with what is known as an oxygen deficit. This is why we continue to breathe heavily once we stop our activity. We need to replenish the oxygen debt.

Remember that enough oxygen being present is what allows us to utilize our long duration energy system. Once the oxygen deficit becomes too high, we will continue to utilize anaerobic mechanisms to fuel activity and blood lactate concentrations will raise and cause fatigue.

This is why it is important to train in all energy systems. Training long, slow distance can help us build an aerobic base and help strengthen this oxidative system by increasing your VO2 max, which is our ability to utilize the oxygen we take in.

Interval training can help us recover by increasing our body’s ability to decrease blood lactate levels as well as making us more proficient at replenishing our oxygen debt.

Ketone Bodies

Another form of usable fats for energy are ketone bodies. Ketone bodies can be found in medium chain triglyceride fats. These are unique because they do not require bile salts for digestion. Instead they are shuttled to the liver, converted to ketones, and immediately used by cells. Research has been done on the use of ketone bodies in endurance training.

Depletion of muscle glycogen, our stored sugar reserves, leads to fatigue. Some research suggests supplementation with medium chain triglycerides can stave off fatigue by sparing our stored glycogen.³

This is most likely due to the easy nature in which medium chain triglycerides are converted to usable energy. Medium chain triglycerides can be supplemented in the diet by using MCT oil or by cooking more with coconut oil.

Half of the fats in coconut oil are medium chain triglycerides. The evidence in the literature is contradictory on the use of medium chain triglycerides, but I have seen it work for a number of clients.

In conclusion, if we are working out for short term, high intensity bouts we need to make sure we ingest enough carbohydrates to fuel activity and to replenish our stored glycogen for recovery. As we exercise longer there is a shift to utilizing fats as a primary source of energy.

Making sure we are getting enough fats in our diet to fuel longer duration activity can help improve performance. Also, adding medium chain triglycerides to the diet may help spare stored glycogen due to the easy conversion to usable energy in the form of ketones.

It is important to keep in mind that not everyone is the same. Some people do well higher carb and others do well higher fat. Planning the best diet for you and your performance will take some tinkering around, but at least now with an understanding of how our energy systems work you can have a good starting point.

References:

1. Thomas Baechle and Roger Earle. Essentials of Strength Training and Conditioning. Human Kinetics (2008).

2. Wadden, TA et al., Effects of very low calorie diet on weight, thyroid hormones, and mood. International Journal of Obesity (1990). Accessed on October 11, 2013.

3. Van Zyl, CG et al., Effects of medium-chain triglyceride ingestion on fuel metabolism and cycling performance. Journal of Applied Physiology (1996). Accessed on October 11, 2013.

Krebs cycle graphic by RegisFrey (Own work) [CC-BY-SA-3.0 or GFDL], via Wikimedia Commons.

Energy pathways chart property of Breaking Muscle.

Other photos courtesy of Shutterstock.

The post Understanding the Oxidative Energy System and How to Properly Feed It appeared first on Breaking Muscle.

In theprevious article Bioenergetics And Nutrition: Creatine, Carbs, And Protein, I explained the first of three energy systems, our ATP-PC system. In this article I would like to dive into glycolysis.

Glycolysis takes over as the main energy system in activities that are slightly longer in duration and have a smaller energy demand than our ATP-PC system. Many of us train in this pathway and many sports require a high demand of the glycolytic pathway for fuel. Understanding the system and substrates involved can help increase you performance in these areas.

Glycolysis is the breakdown of carbohydrates. It lasts from roughly ten seconds into physical activity up to about two to three minutes. The energy for glycolysis comes from glucose, or our stored form of glucose – glycogen.

Glycogen is stored in muscle tissue and the liver, and the average person holds about 1,500-2,000 calories of stored glycogen. Broken down there are about 100g of glycogen in the liver and upwards of 400g of stored glycogen in muscle tissue.

Glycogen in the Liver and Muscles

Storing glycogen in the liver and muscles serves an important function in human metabolism. Our liver is the organ responsible for controlling blood sugar between meals.

When insulin levels fall, the opposing hormone, glucagon, is released. Glucagon stimulates the liver to release some of its stored glycogen into the blood to maintain blood sugar levels.

Glycogen stored in the muscle tissue serves an important role as well. Our muscles main function is to move bones. This allows us to do all the locomotive tasks associated with daily living.

What better place to store energy then within the tissues that require this energy to move us around? After the first seven to ten seconds of moving we utilize this glycolytic pathway for energy.

The first ten seconds of activity utilizes the ATP readily available in the cytosol of our cells. After that timeframe our body needs to resynthesize ATP from glucose and our stored glycogen.

This process requires quite a few chemical reactions. Due to the increase in reactions, this energy system takes longer to kick in then the ATP-PC system, but it will be able to supply a higher amount of total energy.

Fast Glycolysis and Slow Glycolysis

Glycolysis can be broken up into two different parts – fast glycolysis and slow glycolysis. The determining factor is the direction in which the end product, pyruvate, goes. Within fast glycolysis the pyruvate is converted into lactate. With lactate our body can resynthesize ATP at a much faster rate. This would occur when the activity requires a higher energy demand.

Pyruvate on the left, lactate on the right.

In slow glycolysis the pyruvate is shuttled to our mitochondria and we enter the citric acid cycle, or the oxidative system.

In the oxidative system the resynthesis of ATP happens at a much slower rate, but we can maximize the number of ATPs produced, yielding us with the highest amount of energy.

Lactate sometimes gets an undeserving bad wrap. Many people mistakenly associate an increase in lactate with an increase in lactic acid. However, lactic acid cannot exist when the body’s pH is around seven.

Instead, exercise decreases the body’s pH and this is known as metabolic acidosis. In fact, lactate may actually be a buffer to this metabolic acidosis.¹ Lactate is actually utilized as energy by type 1 muscle fibers and cardiac muscle fibers.

With that said the body’s lactate levels are relatively low at rest and increase with an increase in physical activity. Byproducts of these reactions may be responsible for metabolic acidosis.

A clearing of the lactate from the blood is therefore a return to homeostasis. This is one aspect we attempt to train during high intensity interval training.

With enough oxygen present in the mitochondria, the powerhouse of our cell, the pyruvate is converted is shuttled into the mitochondria with NADH, a byproduct of glycolysis, and then converted into acetyl CoA. This is the start of the oxidative metabolism, which we will cover, in my next article.

Glycolysis and Proper Nutrition

Glycolysis is an anaerobic metabolic pathway. The only macronutrient that can be synthesized into usable ATP under anaerobic conditions is carbohydrates.

We need to make sure we take in enough carbohydrates to fuel glycolysis during activity. We also need to make sure we take in enough carbohydrates to keep our glycogen stores full. A reduction in muscle glycogen is associated with fatigue.²

This is where the importance of post-workout nutrition comes into play. Studies have shown an increase in glucose uptake by muscle tissue post workout.³

This makes carbohydrates an important piece of our post-workout recovery meal.

Simple starch may be the best source of carbohydrates post workout because of their ability to raise blood sugar levels quickly. This can allow for faster uptake by muscle cells to recover.

Fruit may even be a better option. Fructose gets immediately shoveled to our liver when ingested. Upon reaching the liver it is converted into glycogen to refill liver stores (it will not refill muscle stores because muscle cells do not contain a receptor for the GLUT5 transporter required to carry fructose).

Under conditions of decreased liver glycogen, such as exercise, fruit may have the ability to resupply the liver faster. Sweet potatoes, white potatoes, yams, and even white rice are good additions to the post-workout meal.

Other vitamins such as vitamin A, B2, niacin, and pantothenic acid are important for energy metabolism.

This puts a high emphasis on quality foods such as fruits and vegetables for high-level athletes. Foods such as these can be obtained throughout the day. In some cases supplementation may be necessary.

Some athletes require such a large amount of nutrients that it becomes difficult to obtain from food alone. In these cases, supplementation would be warranted. Make sure to work with a healthcare practitioner to determine if supplementation is right for you.

In conclusion, if you participate in sports or gym activities that require high energy outputs for two to three minutes, you need to make sure you are ingesting plenty of carbohydrates.

This is to ensure our muscle glycogen stores stay full to keep fatigue away as well as to supply our body with the necessary fuel to perform. The carbohydrates can be ingested from fruit, which may replenish liver glycogen stores more rapidly, or other safe starches such as potatoes.

In my next article we will look at the oxidative system and see how the energy requirements and substrates differ.

References:

1. Robergs, RA, et al., Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology (2004). Accessed on September 30, 2013.

2. Ortenblad, N et al., Muscle Glycogen Stores and Fatigue. Journal of Physiology (2013). Accessed on September 30, 2013.

3. Poehlman, Eric et al., Effects of resistance training and endurance training on insulin sensitivity in nonobese, young women: A controlled randomized trial. The Journal of Clinical Endocrinology and Metabolism (2000). Accessed on September 30, 2013.

4. Thomas Baechle and Roger Earle. Essentials of Strength Training and Conditioning. Human Kinetics (2008).

Energy pathways chart property of Breaking Muscle.

Glucose metabolism by Mikael Häggström [Public domain], via Wikimedia Commons.

Pyruvate/Lactate by Yikrazuul (Own work) [Public domain], via Wikimedia Commons.

Photos courtesy of Shutterstock.

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