Energy requirement for Exercise:101

When we start our workout session our body starts producing energy at a very faster rate as compared to the time when our body is at rest.

When we workout our muscle flex and contract faster, our heart and lungs also perform their respiratory function rapidly, have you ever wondered where this energy comes from ??

All this process needs a tremendous amount of energy. 

energy, running

Before we go through the deep details, you must know how our body produces energy and what happens to it.

This article will take you through what mechanism goes on in our body during workouts that results in the formation of energy, how the different amount of calories we get from eating a variety of foods that our body uses for different types of exercises.

You will know why fatigue occurs and how to reduce it and how you can get more results from workout by changing your diet.

What is Energy ?

Although, we can’t see energy but we can feel it through body heat and in terms of physical work. Energy is produced in every cell of our body by splitting the chemical bonding of Adenosine Triphosphate.

Carbohydrates, fats, protein and alcohol that we get from different kinds of food sources are broken down by our body by means of different biochemical processes that result in energy formation.

What is Adenosine Triphosphate – ATP ?

ATP is the molecule that is consisting of adenosine and three phosphate molecules attached to it forming Adenosine Triphosphate.

When this chemical bonding of ATP molecules breaks down it is converted to energy and mostly into heat.

Energy requirement for Exercise:101

This is the reason why you feel warmer during workout. Once this ATP molecule breaks down it gets converted to Adenosine Diphosphate.

Again this Adenosine Diphosphate gets converted to Adenosine Triphosphate. This Biological process continues every time.

Inter-conversion of ATP and ADP

Adenosine Triphosphate contains adenine which is a base and that is attached to Ribose sugar, and these are attached to three phosphate molecules.

These three phosphate are combined with two high energy bonds which are known as Phosphoanhydride Bonds

When there is a requirement of extra energy this Phosphoanhydride Bonds will break and ATP will get converted to ADP by the process Known as Hydrolysis.

Additionally, this ADP also breaks down further and forms AMP which is known as Adenosine monophosphate.

Energy requirement for Exercise:101

AMP again gets attached to a free Phosphate molecule in our body to form ADP and finally to ATP, which is the final stage.

Again the breakdown starts and releases energy. This interconversion of ATP, ADP and AMP continues whenever a biological process goes on for energy requirement.

Where does the energy come from ?

 Our body stores only a small amount of ATP  which is just enough to carry out the basic function of our body at rest when we start a workout session the demand for ATP increases and needs to replenish at the faster rate. 

There are basically 4 important source of energy for our body :

  1. Carbohydrates
  2. Fats
  3. Protein
  4. Alcohol

When we eat various foods or drinks they break down and convert into various building blocks components.

Carbohydrates are broken down into simple sugar or Glucose for instant energy purposes, Fats are broken down into fatty acids and are stored in our body for long term energy purposes.

 Proteins are broken down into amino acids for producing new cells and muscle growth.

Proteins also break down and provide energy when the carbohydrates and fats store get used up totally. And alcohol is mostly absorbed into the bloodstream. 

Our body can not use hundred percent of available energy into power, most of the energy gets converted into heat.


Why do we get different amounts of energy from different foods ?

Different foods we eat contain different amounts of carbohydrates, fats, protein and alcohol mixture ratio.

Each of these macronutrients  have different energy values and when they break down they provide different amounts of energy.

However, fat is the most dense energy content macronutrients because it provides twice the energy of carbohydrates, protein, fats and alcohol. But fats are not a good form of energy for workout.

Energy Content of different macronutrients ( 1 gm approx. )

  1. Protein – 4 calories
  2. Carbohydrates – 4 calories
  3. Fats – 9 calories
  4. Alcohol – 7 calories

How does the body store Carbohydrates ?

Carbohydrates are broken down into simple sugars which are known as Glucose. Many glucose molecules combine together to form glycogen, when it is stored mostly in our muscles along with water. However, our body stores only a limited amount of glycogen.

 About 400 gm of glycogen is stored into the muscle cells and 100 gm is stored into our liver which makes a total storage of glycogen 500 gm.

These 500 gm of glycogen can provide upto 2000 calories which is more than enough to carry out normal body function and daily activities of an individual without eating the whole day.

The Glycogen that is stored in the muscle cells are used in workout sessions and the glycogen that is stored in the liver is used to maintain the normal blood sugar level of our body. 

These glycogen store factors are one of the reasons why people on their fast few weeks of low carbohydrate diet trends to lose more weight.

This weight loss is due to the loss of glycogen storage and water mass.

How does the body store Protein?

Proteins are not stored in our body like carbohydrates, they mainly when consumed break down into amino acids and are mainly used for building muscle, new cells and Organs.

However, proteins can be used as energy sources if needed but that can result in muscle breakdown.

How does the body store Fats?

Fats are mainly stored in the fatty acids in adipose tissues which are over the whole body. About 300 to 400 gms of fats are stored into our muscle but our body stores a maximum amount of fat around the internal organs and under the skin.

However, the amount of fat that is distributed and stored in our body mainly depends upon genetics and hormonal balance of an individual. 

Female hormones like Estrogen trends to store more fats than the male hormone which is testosterone.

Individuals whose body stores fat around the abdominal portion have a greater risk of heart diseases then the people who store around hips and thighs.

Which energy is the most important for Workout ?

Carbohydrates, Protein and Fats are the major source of nutrients from foods. However they all are capable of providing energy during workout. 

However, protein does not contribute as a good energy source. It provides a small amount of energy only when carbohydrates and fats storage is finished and there is still some energy requirement. You can take as an example, during your intense workout.

However, on the other hand alcohol is not a good source of energy during your workout, no matter how energetic you feel normally.

Don’t think working out intensely will help you in breaking down alcohol and use the energy during workout and in that way you will be able to prevent fat storage in your body. 

Liver has a specific enzyme that will only help you to break down the alcohol at the specific rate, You can’t fasten the process by working out intensely.

Can Protein contribute to the body’s energy source ?

Protein is usually not the good source of energy for workout, but your body can use protein as a source of energy when you are training intensely for a long time and the glycogen store of your body is depleted.

Usually, protein contributes 10 percent of the total body’s fuel mixture.

Moreover, protein can be used as energy when you are in the state of semi-starvation or if you are following the low carbohydrate diet because of low glycogen stores. 

People think on a low carbohydrate diet they will be able to force their body to burn more fat, but unfortunately this is not the case. On this kind of diet, the body will actually withdraw energy from your muscle and you will notice muscle loss.

So, this collateral loss, you will be losing muscle as well fat. Sometimes it can result in raising health issues when a low carbohydrate diet is followed for a long time.

How is energy produced in our body ?

Our body uses mainly three systems to regenerate energy for different kind of activities :-

  1. Phosphagen system
  2. Glycolytic and lactic acid system (Anaerobic system) 
  3. Glycolytic and lipolytic system (Aerobic system)

How do these systems work ?

  1. Phosphagen system –

This energy system usually uses Adenosine Triphosphate and Phosphocreatine to produce energy. However, the energy produced by this system is for a very short period of time after which our body has to change the pathway to another system for producing energy.

Energy requirement for Exercise:101

The phosphagen system usually works by breaking down phosphocreatine into Creatine and Phosphate and transferring the free phosphate to Adenosine Diphosphate to form ATP.

This system can generate ATP at a much faster rate but it doesn’t last long.

  1. Anaerobic glycolytic system –

In this system our body breaks down  muscle glycogen in the absence of oxygen to generate ATP and lactic acid.

In this anaerobic condition 1 glucose molecule will produce 2 ATP only, which is insufficient for energy requirement.

Energy requirement for Exercise:101

In this process of energy production our body’s glycogen stores get used up quickly but this system gives an advantage of releasing quick jerks of energy when energy requirement is high. 

Beside providing energy quickly at the time of requirement this system also builds up lactic acid with time this is the reason you get the burning feel after you finish the workout sets and you cant contract your muscles further.

  1. Glycolytic and lipolytic system (Aerobic system)

This system works by breaking down fats ( lipolytic glycolysis ) and carbohydrates (glycolysis)  by using oxygen.

This system is unable to generate energy as fast as the Phosphagen system or Glycolytic and lactic acid system (Anaerobic system). 

When you are working out for a longer time period our muscle glycogen store will be finished. Then the body starts to use blood glucose by breaking down the liver glycogen store as well as breaking down fats ( lipolytic system )in our body for supplying energy.

When you are doing intense workout your body relies on the above two systems and as soon as you complete your workout set, the body’s energy production pathway changes to Aerobic system. 

Energy requirement for Exercise:101

This happens so, because the energy demand will be decreased and our body will get enough time to transfer oxygen from lungs to muscle and to break down glucose for the formation of ATP.

Anaerobic systems only use glycogen for generating energy but this system uses both fats and glycogen.

In this system 1 glucose molecule can produce 38 ATP and 1 fatty acid molecule can generate up to 200 ATP but this process is much slower so it is not good for sudden energy requirements.

Muscle Fiber and Energy Production

Our body is made up of many different fiber types which can be classified into two categories and that are fast-twitch fiber and slow twitch fiber. 

Every individual has a different ratio of muscle fiber distribution. Whichever muscle fiber it can be at some point of activity level they use all three energy systems. 

But fast-twitch fiber uses specially phosphagen and  anaerobic systems and slow-twitch fiber uses mostly Aerobic energy systems.

Our body decides which energy system to use on basis of :-

  1. Workout Intensity
  2. Workout time period
  3. Your fitness level
  4. Food taken before workout 
Energy requirement for Exercise:101

 What happens when we start a workout ?

When we start our workout initially energy is being produced in our body anaerobically for the first few minutes until your breathing system and heart rate adjust with the energy demand of the body.

Due to this suddenly high demand of energy is generated by anaerobic process lactic acids start to build up in our working muscle and thus increasing the muscle acidity level.

After sometime when the heart rate adjust with our training intensity then there is enough supply of oxygen to the muscle so the anaerobic energy pathway changes to aerobic energy system pathway and additionally the lactic acids that are formed in the muscle are released in the bloodstream as there is a enough supply of oxygen. 

If you are gently raising up your training intensity level the oxygen will match with your energy demands and there will be less chance of fatigue.

After the first few minutes of anaerobic energy production the slower functioning aerobic system comes into effect.

Initially in aerobic energy production carbohydrates are mostly used in the form of glycogen further when the workout continues for a long time and maintaining the gentle intensity level, our body starts to break down fat in fatty acids for the production of ATP to use as energy.

 As you continue to workout with time there will be an increase in oxygen supply and our body will use less carbohydrates and more fats.

Initially when you are starting your training session make sure to warm up properly. If you start your training very intensely at the very beginning there will be a high energy demand and less oxygen availability of your muscle that will result in lactic acid formation which is known as fatigue. Soon you will be tired, you will be exhausted and can’t maintain your intensity further.

What is fatigue ?

Fatigue in biological terms can be explained as the inability to perform muscle construction at any given time with that of work load. Due to fatigue you will find your workout to be more harder to perform and you can’t maintain the intensity.

Always remember to maintain a pace between your energy demand and oxygen supply to inhabit the formation of fatigue.

Why does fatigue develop during Anaerobic workout ?

During intense workout there is maximum energy output and fatigue develops due to the shortage of ATP and phosphocreatine depletion.

In simple words it can be said as the demand of ATP exceeds the available ATP. During 30 seconds to 15 minutes intense workout fatigue can be caused by different mechanisms. 

The rate of Lactic acid production exceeds the rate of lactic acid removal into the bloodstream. As a result, there is a gradual increase in muscle acidity level and the acidic environment prevents you from further muscle contruction. 

This is the reason that you experience the burning effect in your muscle. Actually, the burning sensation is the kind of safety mechanism of our body to prevent muscle cell damage due to acidic environment creation by the formation of lactic acid.

Why does fatigue develop during Aerobic workout ?

When you are training for a longer period of time the muscle glycogen level gets used up fully and the liver glycogen store starts to breakdown to provide your body with energy supply. 

Our body’s glycogen store is limited to some point, after that all the glycogen stores get depleted and it will result in fatigue formation due to muscle and liver glycogen depletion and very low sugar level.

After the glycogen depletion fully, then comes the aerobic lipolytic energy system in effect and fats start to break down into fatty acids to generate ATP. When our body enters this energy production pathway, most of the fuel is supplied by fats. 

As you know, our body stores more amount of fats than glycogen, so you might be thinking you can carry out your workout for longer with high intensity.

But this is not true, because the conversion of fats to fatty acids and then to ATP is not fast enough to make up with high energy demand of the working muscles. These usually cause fatigue.

Now, if you slow down the workout intensity and give your body enough time for energy production the other biochemical  changes in your body will cause fatigue. For example, raise the level of serotonin which will make you feel tired.

How can you delay your fatigue development ?

Whatever activities you do or whatever energy production pathways your body uses, the glycogen use is common in every way. SAo the amount of your muscle and liver glycogen stores will have a direct effect on your workout intensity and performance.

So take high glycemic carbohydrate foods for your pre workout meal to increase the glycogen store and this you will delay your fatigue development.

Another way to delay fatigue is to rate at which your muscle uses glycogen stores by gradually increasing your training intensity.

For example, when you are at the gym and about to start your session, start with warmup and stretching sets without you going for heavy workout sets.

Hope you like the article, now make sure to comment below and let me know what you think about this topic. Also share the article with your gym buddies.

Also read

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Worried about fat loss?Here’s the effective ways to lose fat


Surajit Jana.

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