When participating in low-intensity activities over a long period of time the body primarily is used for energy?

Running is simple, we put one foot in front of the other, and allow it to happen naturally.

That is how we start, but once we get into running a little more, we want to learn more about how to improve our speed by increasing our step frequency and step length, we want to know which foods will give us the most energy on our runs, and we want to understand which energy systems we use in a 400m sprint.

There are so many questions, and there is so much to learn about running. If you want to be the best runner you can be, these are areas you probably want to start paying attention to.

If you want to increase your running speed, you probably already know that it is actually not your speed holding you back, but your aerobic endurance, and while knowing what to eat before, during, and after each type of training run is very important, I should firstly introduce the physiology of energy metabolism during different levels of exercise.

If you need the aerobic energy system explained, you are in the right place. Today we we break the three energy systems down, so you can learn how you have the energy to sprint as fast as you can, how the anaerobic energy system works, and what the aerobic energy system is.

Each of these play a role in us being able to run faster, so let’s learn more about them:

Why do I need to know about energy metabolism?

Knowing the predominant energy system you are using during your workouts will help you determine your recovery needs for nutrition and rest.

Energy is stored in the body in various forms of carbohydrates, fats, and proteins as well as in the molecule creatine phosphate.

Carbohydrate and fat are the primary sources of energy, with protein contributing a minimal amount under normal conditions.

Adenosine triphospate (ATP) is the body’s usable form of energy. The body uses 3 different systems of metabolism to transfer stored energy to form ATP.

What are the 3 Energy Systems?

The Phosphagen System

The phosphagen system of energy transfer does not require oxygen (anaerobic) and is called upon when there is a sudden increase in energy demand such as starting a workout, starting explosive hill sprints, or throwing a discus.

It is the most direct and quickest form of energy production but can only supply enough energy for a short burst intense activity like a maximum weight lift or a 5 second sprint.

This system relies on the availability of creatine phosphate, which is in limited supply and is depleted quickly.

When creatine phosphate is used up, the body must call on other systems of energy transfer to sustain continued activity.

Glycolysis (anaerobic) System

Another system that doesn’t require oxygen is glycolysis, also known as the lactate system.

This system provides enough ATP to fuel 1 to 3 minutes of intense activity when adequate oxygen isn’t available for aerobic metabolism.

Lactate or lactic acid is something that most runners have heard of and may even fear because of its connection with sore muscles and fatigue.

Hopefully the following explanation of glycolysis will help you picture what is going on.

Glucose is the only fuel that can be used during glycolysis, which literally means the breakdown of glucose.

This breakdown creates ATP as glucose is converted into 2 molecules of pyruvate.

Now:

Hydrogen is also produced during this process and if oxygen is present, the aerobic system (explained next) can use hydrogen and pyruvate to produce more ATP.

However, often times the aerobic system cannot keep up with the excess hydrogen being produced so instead the hydrogen combines with pyruvate to form lactic acid.

Lactic acid then enters the bloodstream and is cleared by the liver.

The point at which the production of lactate is faster than lactate clearance is called the lactate threshold, also referred to as the anaerobic threshold, when lactic acid begins to accumulate in the blood.

The increased acidity of the blood inhibits the use of fatty acids for energy production through aerobic metabolism and thus increases the body’s reliance on carbohydrate and glycolysis.

As blood lactate levels continue to rise and carbohydrate stores become depleted, the muscles begin to fatigue and performance is diminished.

An athlete can increase their lactate threshold through adaptations made during proper endurance training.

This is where my knowledge of the lactate threshold ends and I leave it to the expert coaches to figure out the best way to do that!

I will say though that one of those adaptations is the increasing the efficiency of the aerobic system.

The Aerobic System

The aerobic system can use carbohydrates, fats, or proteins to produce energy.

Energy production is slower, but more efficient than the other two systems.

As you can tell by the name, the aerobic system requires that there be adequate oxygen available to the working muscles.

Therefore this system is used more heavily during low-intensity activity, but actually, most of our races, even a 5k mostly use the aerobic system.

One key highlight of aerobic metabolism is the ability to burn fat as fuel.

Our bodies have a seemingly unlimited capacity for storing fat and fat provides over twice as much energy per gram than protein or carbohydrate, making it a very attractive choice for energy production.

In prolonged activities where intensity is low, the body will use fat as a main energy source and spare the use of muscle glycogen and blood glucose so that it is available for use if exercise intensity increases and oxygen availability is decreased.

Keep in mind that aerobic metabolism doesn’t use one substrate exclusively.

Although you may be burning mostly fat, a steady supply of carbohydrate is still necessary for the breakdown of fat into an energy source.

What’s the bottom line?

Just like the aerobic system isn’t exclusive to one substrate, energy metabolism isn’t exclusive to one system.

All 3 systems are working simultaneously to fuel the body during exercise.

Remember this:

However, certain characteristics such as exercise duration and intensity will determine the predominate system and thus how long the activity can be performed at that level.

Other factors that influence what substrates and systems are being used include the fuels that are available, the fitness level of the athlete, and the nutritional status of the athlete.

These factors may change over time and through training so just like overall nutrition, energy metabolism is very individualized and dynamic.

What Should I Eat Before Running Workouts?

If you are still wondering about eating before a run or workout, make sure you go back and read more about when you should and should not eat before.

One big reason for having adequate fuel before a workout, as well as on a daily basis, is to prevent the use of protein as a fuel source.

Protein is usually spared from being used as an energy source and is used predominately by the body for tissue maintenance, growth, and repair.

However, when glycogen stores are depleted, amino acids from muscle protein can be used to produce glucose.

As we learned before, glycogen stores can be depleted through intense and prolonged exercise, a chronic low carbohydrate diet, or an overall low-energy diet that cannot keep up with the body’s demands.

This is important:

If the body consistently relies on protein for fuel, muscle protein stores will begin to decrease along with lean body mass, which can be detrimental to performance.

This highlights the importance of fully replenishing glycogen stores after intense workouts, as well as on a daily basis.

What you eat really does have an impact on how effectively and efficiently you can provide energy to your working muscles. The body converts food into adenosine triphosphate (ATP) for fuel through several different energy pathways. Understanding these systems can help you train and eat more effectively, and boost your overall sports performance.

Because the body cannot easily store ATP (and what is stored gets used up within a few seconds), it is necessary to continually create ATP during exercise. In general, the two major ways the body converts nutrients to energy are:

• Aerobic metabolism (with oxygen)
• Anaerobic metabolism (without oxygen)

These two pathways can be further divided into three main energy systems (listed below). Most often it's a combination of energy systems that supply the fuel needed for exercise. The intensity and duration of the exercise determine which method gets used when.

The ATP-CP energy pathway (sometimes called the phosphagen system) is an anaerobic pathway because it doesn't require any oxygen to create ATP. The "CP" stands for creatine phosphate, a naturally occurring compound that enables short bursts of energy.

The ATP-CP pathway supplies about 10 seconds worth of energy and is used for short bursts of exercise, such as a 100-meter sprint.

This pathway first uses up any ATP stored in the muscle (about 2 to 3 seconds worth). Then it uses creatine phosphate (CP) to recycle ATP until the CP runs out (another 6 to 8 seconds). After the ATP and CP are used, the body will move on to either aerobic or anaerobic metabolism (glycolysis) to continue to create ATP to fuel exercise.

Glycolysis is both an anaerobic and anaerobic system which creates ATP exclusively from carbohydrates, with lactic acid being a byproduct. Anaerobic glycolysis provides energy by the (partial) breakdown of glucose without the need for oxygen.

Glycolosis is considered both an aerobic and anaerobic pathway. This process produces energy for short, high-intensity bursts of activity lasting no more than several minutes.

After several minutes, the lactic acid build-up reaches a threshold known as the lactate threshold (LT). When you reach this threshold, you experience muscle pain, burning, and fatigue, making it difficult to keep exercising at this intensity. However, training can increase the threshold.

Aerobic metabolism fuels most of the energy needed for long duration activity. It uses oxygen to convert macronutrients (carbohydrates, fats, and protein) to ATP. This system is a bit slower than the anaerobic systems because it relies on the circulatory system to transport oxygen to the working muscles before it creates ATP.

Aerobic metabolism is used primarily during endurance exercise, which is generally less intense and can continue for long periods of time.

During exercise, an athlete will move through these metabolic pathways. As exercise begins, ATP is produced via anaerobic metabolism. With an increase in breathing and heart rate, there is more oxygen available and aerobic metabolism begins and continues until the lactate threshold is reached and anaerobic metabolism kicks in again.

Sports nutrition is built upon an understanding of how macronutrients, such as carbohydrates, fat, and protein, contribute to the fuel supply needed by the body to perform. Macronutrients contribute to the process in different ways.

Each macronutrient has unique properties that determine how it gets converted to ATP.

• Carbohydrate is the main nutrient that fuels moderate to high intensity exercise.
• Fat can fuel low-intensity exercise for long periods of time.
• Protein is generally used to maintain and repair body tissues and is not normally used to power muscle activity.

Because your body uses different pathways to create energy, and each pathway relies on different macronutrients, it's important to consume fat, carbohydrates, and protein in your diet.

Nutrients get converted to ATP based on the intensity and duration of activity, with carbohydrate as the main nutrient fueling exercise of a moderate to high intensity, and fat providing energy during exercise that occurs at a lower intensity.

Fat is a great fuel for endurance events, but it is simply not adequate for high-intensity exercises such as sprints or intervals. If exercising at low intensity (or below 50% of max heart rate), you have enough stored fat to fuel activity for hours or even days, as long as there is sufficient oxygen to allow fat metabolism to occur.

As exercise intensity increases, carbohydrate metabolism takes over. It is more efficient than fat metabolism but has limited storage capacity. Stored carbohydrate (glycogen) can fuel about two hours of moderate to high-level exercise. After that, glycogen depletion occurs (stored carbohydrates are used up). If that fuel isn't replaced, athletes may hit the wall or "bonk."

An athlete can continue moderate to high-intensity exercise for longer by simply replenishing carbohydrate stores during exercise. This is why it is critical to eat easily digestible carbohydrates during moderate exercise that lasts more than a few hours. If you don't take in enough carbohydrates, you will be forced to reduce your intensity and tap back into fat metabolism to fuel activity.

In fact, carbohydrates can produce nearly 20 times more energy (in the form of ATP) per gram when metabolized in the presence of adequate oxygen than when generated in the oxygen-starved, anaerobic environment that occurs during intense efforts (sprinting).

The three main energy systems the body uses to create ATP are: the ATP-CP energy pathway (or phosphagen system), glycolysis, and aerobic metabolism.

The ATP-CP energy system powers very short bursts of exercise, and supplies up to 10 seconds of power and energy to your body.

The ATP-CP energy system works by using ATP and creatine phosphate (CP) to give your body fuel. While ATP provides about 2 to 3 seconds of energy, the CP provides 6 to 8 seconds. Together, they can provide enough energy for a quick 10-second sprint.

The ATP-CP, or phosphagen, system is the first energy pathway that is used during exercise. This energy pathway is quickly depleted and allows for a quick burst of fuel to lift heavy weights or perform a short sprint.

Energy pathways in the body can adapt as you increase your fitness. With appropriate training, these energy systems become more efficient and allow you to exercise at a higher intensity for longer periods of time.