Your body’s energy systems

How your body burns
fat and carbs during exercise

The way your body uses energy and burns fat largely depends on the type of exercise and the length of time you exercise. 

If you understand these energy systems, you can design your exercise routines to maximise performance and help your body burn fat more effectively.

Swiss Ball Bar Glute Hip Thrusts

The body’s three main energy systems are the ATP-PC, Glycolytic, and Oxidative.

When we eat, the foods and liquids are digested and broken down into the macronutrients, which are the carbohydrates, protein, and fats.

These ‘macro’s are further assimilated into more simple molecules, which are glucose (from carbohydrates), amino acids (from protein), and fatty acids (from fats). These molecules are absorbed into the bloodstream and transported to various cells throughout the body.

The body will use some of these molecules to then make energy molecules. The energy molecule of the body is called adenosine triphosphate (or “ATP” as it’s commonly known) and is made inside your cells as it is required.

Adenosine triphosphate (ATP) is the ‘energy currency’ molecule that allows your body to use and store energy. ATP is needed for activity, metabolism, and brain function.

Your body has 3 systems to produce ATP: the phosphagen system, the glycolytic system, and the oxidative system. Which system you primarily use depends on your activity and intensity. 

AT very high intensity exercise
your body uses the phosphagen system

The phosphagen system produces very fast-acting, immediately available energy. This is what the muscle uses first for generating ATP.  As the body can only store a very small amount of ATP, the phosphagen system uses the creatine phosphate molecule (CP) to make more ATP. 

CP allows your body to make ATP very quickly, rather than using the slower carbs or fat pathways. You can build your CP stores in two ways, either from your diet, by eating red meat and or taking a creatine supplement or our body also makes CP from amino acids in your liver, kidneys, and pancreas. 

Muscles can’t store very much CP and it’s readily used up within the first 10 seconds of high- to extremely high-intensity activity. This can be enhanced somewhat by creatine supplementation, loading and maintenance dosages.  For muscle activity longer than 10 seconds, your body uses the glycolytic system to make more ATP.

FOR high intensity exercise your body
switches to the glycolytic system

The glycolytic system uses carbohydrates—blood glucose and muscle glycogen—to produce ATP. 

Glycolysis, which is the process of burning carbs, has two mechanisms, simply called “fast” and “slow.” Fast glycolysis uses no oxygen and can make ATP from glucose quickly, sustaining activity for 2–3 minutes until slow glycolysis has enough time to get started. Fast glycolysis is somewhat inefficient, producing only a small amount of ATP from glucose. 

Slow glycolysis requires oxygen and takes longer to make ATP because the process involves many more steps than fast glycolysis. While it lacks speed, it efficiently produces more ATP per molecule of glucose—or muscle glycogen—than fast glycolysis does. 

So, you need both fast and slow glycolysis to produce ATP: the former to meet your immediate need for energy, and the latter to sustain energy.

Low to moderate-intensity exercise
your body uses the oxidative system.

It’s the only energy system that burns fat.

Older Male Resting with Weight Lifting Bar

When you’re not exercising, about 70% of your ATP is produced from fat and 30% from carbs.

When you start low- to moderate-intensity activity, fast and slow glycolysis kick in, and the ratio shifts to nearly 100% carbs. 

Fat oxidation is the slowest but most efficient method of producing ATP. It produces the most ATP per molecule of fat burned. Since it’s such a slow process, fat loss during exercise requires you to exercise for a long time. A quick 15-minute session will use some fat, but it won’t maximize fat loss. 

High-intensity exercise and short-duration, moderate-intensity exercise almost exclusively use CP and carbs to produce more ATP. 



weight management

Exercise’s contribution to weight management is based on how you use each of the three energy systems, especially if your goal is to lose weight, “burn fat,” and improve your body composition. 

It’s not about ‘how many calories you burned” but the question you should be asking is “what energy system am I using to burn calories?”

time available to exercise is a big factor that is overlooked

The limiting factor is the time available to do the exercise to burn the fat. You need the most efficient way possible to get into the fat burning energy system. 

You need the ‘2 Tanks System!

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