Energy For Exercise: Metabolism, Energy Pathways And Adenosine Triphosphate (ATP)

In order to fully understand how a clean diet and exercise routine cause positive adaptations to the human body, it is important to understand the mechanics occurring at a cellular level. Metabolism is a word that most people understand to be associated with weight management; it is most commonly connected to diet and weight loss with people referring to it in terms of speed – ie a fast or slow metabolism. The word metabolism actually refers to all the chemical reactions that occur in the body.

Our body obtains energy released by the breakdown of organic molecules (food we eat), stores it as adenosine triphosphate (ATP) and uses it to support all activity. ATP is the cell currency of the body and most of its production occurs within the mitochondria of the cell. As you can imagine, the activity levels of body tissue (which is made up of cells) alter the metabolic requirements of the body and this is why our nutrient and oxygen demands alter depending on the level of activity we participate in and therefore the amount of energy we expend.

What is ATP?

A molecule of ATP comprises three phosphate groups, adenine and ribose. Two phosphate groups are attached to the nucleotide adenosine monophosphate. These two phosphate groups are attached by high energy bonds. When the bond between the second and third phosphate group is broken (forming ADP or adenosine diphosphate), energy is released which can be utilised for activity. This reaction requires an enzyme known as adenosine triphosphatase or ATPase. To sustain life, cells continuously generate ATP from ADP and then use the energy provided by ATP to perform vital functions such as the contraction of muscle or synthesis of proteins. The bond between the first and second phosphate group can also be broken to release energy (forming AMP or adenosine monophosphate), though this bond is stronger and more difficult to break (Martini & Nath, 2009).

The conversion of ADP to ATP is the most important method of energy storage in our cells. The reversion of ATP to ADP is the most important method of energy release.

ADP + Phosphate Group + Energy < - - > ATP + H₂O

Metabolism and Energy Systems

You have three exercise energy systems that can be selectively recruited, depending on how much oxygen is available, as part of the cellular respiration process to generate the ATP energy for your muscles. No singular energy system ever does 100% of the work; it will be distributed between the three energy systems depending on the intensity and duration of activity.

The majority of cells generate ATP by breaking down carbohydrate as follows:

C₆H₁₂0₆  +  6O₂  ->  6CO₂  +  6H₂O

Glucose + Oxygen -> Carbon Dioxide + Water

The complete catabolism of 1 molecule of glucose yields 36 molecules of ATP.

Carbohydrate metabolism involves glycolysis, ATP production and gluconeogenesis.

Although the body’s preferred energy source is carbohydrate, it is also able to catabolise (breakdown) lipids (predominantly fatty acids from triglycerides) and proteins (amino acids) in order to meet energy demands.

Lipid metabolism involves lipolysis, beta-oxidation and the transport and distribution of lipids as free fatty acids and lipoproteins.

Protein catabolism involves transamination and deamination, which is the exact opposite of what we are trying to achieve with protein shakes and amino acid supplementation to encourage protein synthesis.

The Alactic Anaerobic Energy System

This energy system is the first one recruited for exercise and it is the dominant source of muscle energy for high intensity explosive exercise that lasts for 10 seconds or less. It is this energy system which utilises creatine stored in the muscles.

Creatine is an amino acid derivative which is directly involved in the anaerobic energy cycle. ~95% of the creatine found in the body is present in skeletal muscle and the body is able to manufacture ~1g per day. Although it is not considered an essential part of the diet, it is integral in energy metabolism, protein balance and cell membrane stability.

Anaerobic energy:

In a reversible reaction, creatine ‘borrows’ a phosphate group from ATP (adenosine triphosphate) to form PC (phosphocreatine) and ADP (adenosine diphosphate). When energy is required in the first few seconds of activity, PC will return the borrowed phosphate group to form ATP. This cyclic process allows the muscles to rapidly expend and replenish energy but this maintenance of maximum energy output is short lived. During high intensity/power training, ATP re-synthesis fails to keep up with demands resulting in muscle fatigue. Supplementation of creatine raises the level available in the muscle for this energy cycle which enhances energy output during anaerobic exercise, allowing you to make more of your high intensity, short duration workouts, improve strength and increase muscle size.

The Lactic Anaerobic Energy System

This system is the dominant source of muscle energy for high intensity exercise activities that last up to ~90 seconds. Essentially, this system is dominant when your alactic anaerobic energy system is depleted but you continue to exercise at an intensity that is too demanding for your aerobic energy system to handle. Like the alactic anaerobic energy system, this system does not require oxygen. This system is not as efficient as the aerobic system and produces lactic acid as a by product of energy production.

Unlike the alactic system, this system does not rely on energy stored in the muscles. It has to directly recruit active cellular respiration to provide ATP energy. In other words, it follows a similar path to the aerobic system without oxygen, producing a by product which prevents it from being utilised for extended periods of time (Martini & Nath, 2009).

The Aerobic Energy System

The aerobic system is very efficient but it is also relatively slow to yield ATP in comparison to the anaerobic pathways, which is why it is utilised as the dominant energy system for low intensity, long duration activity. Unlike the other two systems where energy demand exceeds the supply, the energy demand placed on your muscles is equal to the energy supplied by the aerobic energy system, meaning that theoretically (as long as there is an adequate supply of oxygen and macronutrient stores) you could continue the activity for a lengthy period of time. For example, in a triathlon, marathon, long distance cycle or even an ultra-marathon!

The rate at which you are able to continue exercising is dependent on how efficiently oxygen can be delivered to and processed by your muscles to yield the ATP required. If you have been continuously exercising for 5 minutes, the aerobic system will be supplying the majority of the energy required.


Martini FH, Nath JL, Fundamentals of Anatomy and Physiology, Chapter 25: Metabolism and Energetics, Eighth Edition, 2009, 930-50.

About the Author

Job Role Sports Nutritionist and Social Media Coordinator Qualifications Bsc Sport and Exercise Science Steph has a competitive athletic background which spans 19 years. As a child she performed with the English Youth Ballet and had performed on the West End stage by the age of 10. Her enthusiasm for sport and fitness continued to grow as she did, encouraging her to learn more about nutrition and training. She began using her knowledge and personal experience to help others when she began coaching at the age of 16. From here, she went on to study Sport and Exercise Science at the University of Essex during which time she also received the Most Promising Newcomer Award from her University to mark her outstanding contribution to sport. During her first year of study she was introduced to partner stunt acrobatics and artistic gymnastics. After one year of dedicating herself to a lifestyle revolving around her sport, she was training with the best team in the UK who are currently ranked fifth in the world. Steph has worked in both the private and public sector coaching children and adults from grassroot to elite level as well as providing them with cutting edge advice on how to reach their goals. Steph has received awards for her choreography and has competed nationally and internationally meaning that she can back up her scientific knowledge with a wealth of experience. As our resident Sports Nutritionist, Steph is here to provide the most current and evidence based fitness, health and nutrition information to help you reach your health and fitness goals.


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