Making Energy How our Bodies Work

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This article was edited and updated on November 5, 2015.

How do we get energy to do the things we do?

The simple answer is: We make it! It’s called ATP.

ATP (Adenosine Tri-Phosphate) is, quite literally, the energy we use to keep ourselves alive. They are manufactured by our body’s cells — and we need many of them every single second — for just about every imaginable task, including growth, movement, and metabolism.

The production of ATP’s takes place within the power house of the cell; oxygen and fuel (glucose or fat) are essentials for the manufacturing of them. Simply put: More oxygen and fuel to the cells means more manufacturing of ATP which means more energy.

We can manufacture ATP in one of three ways and we use all three systems, depending on the body’s needs: (1) The ATP/CP System; (2) the Anaerobic Lactic Acid System or (3) the Aerobic System.

The ATP/CP System: For Fast, quick energy

The Production of ATP from glucose and fat takes time, so the body needs to have an immediate source of energy.

Creatine, a nitrogenous organic acid, occurs naturally in the body and helps to supply energy to our muscles. With the help of a special molecule, it produces ATP.

But our bodies have only a limited supply of Creatine, so this method of ATP production only lasts for up to 20 seconds.

It is used for short-term activities such as sprinting or in start/stop sports like hockey or rugby. It is an important system because it provides us with quick energy when needed.

The Anaerobic Lactic Acid System: Using Glucose without Oxygen for Energy

Since the ATP/CP system lasts only for twenty seconds, we need another way to make ATP so we have energy that lasts longer.

The Anaerobic Lactic Acid System breaks down glucose and changes it into ATP without the use of oxygen (hence, the name anaerobic).

Unfortunately, only about 5% of the energy contained in the glucose is actually released, and several by-products are produced, including Lactic Acid, water, and carbon dioxide. This system produces less energy per molecule of glucose than the aerobic system: one molecule of glucose burned anaerobically produces 2-3 ATP per cell.

At first glance, this system may seem inefficient and never preferable, but that is not the case. As with the quick-energy system, the Anaerobic Lactic Acid System is needed for our normal, day-to-day activities. It doesn’t burn fat, but ... it’s still very useful. Even though not as much ATP can be furnished by glycolysis alone, it is a significant source of ATP when muscular activity continues for any length of time. It is often a natural bridge for energy production between the ATP/CP System and the Aerobic System.

The Aerobic System: Making Use of Oxygen and Fat Cells

In the first few minutes of any strenuous activity, energy must be made anaerobically. The Aerobic System comes into play when the amount of oxygen coming into the body is meeting the demand of the muscles.

As the heart and breathing rates increase, the demand for oxygen will be met and we create ATP aerobically. We will still be burning glucose, but much more ATP can be produced (36-40 ATP per molecule). Once oxygen is present in the muscle cells, fatty acid oxidation begins to take place, creating up to 100 ATP. As the exercise carries on, we will begin to burn more fat instead of glucose. Since we cannot sustain intense exercise for long, it makes sense to exercise at a milder level for an extended period. As time goes by, the body will steadily burn more fat and less glycogen.

This efficient process occurs because fat as the main fuel has a much higher caloric density than glucose, allowing us to release far higher amounts of energy. Thus, stored body fat, combined with oxygen, becomes an excellent concentrated source of energy, generating ATP much more efficiently than glucose -- producing roughly six times more energy per gram.

What does all this mean?

At any given time, a mixture of carbohydrates, fat (and even a little protein) is being used to fuel our energy levels. The fuels for energy release are both time and intensity related. At rest the body uses approximately 40% glucose and 60% fat. With mild intensity endurance exercise the body uses approximately 50% each of carbohydrates and fat. At high intensity, during very short duration (20 seconds) the body uses 95% glucose.

Any relatively intense, short-term exercise is fueled by what is in your muscles. We do aerobic exercise for the cardiovascular benefits: it enables our lungs, heart, and blood vessels to do a better job converting oxygen into ATP. If you exercise with little or no intensity, like walking, you're using your aerobic system to produce energy, but you're not doing aerobic exercise. To get aerobic benefits, the exercise needs to be at a pace that you can maintain for at least 10 or 20 minutes and that feels "comfortably difficult."

The cardiorespiratory section of a fitness class is designed to make your body produce energy, but you shouldn’t avoid using free weights simply because we don’t burn as much fat during anaerobic exercise. The more muscle you have, the more you increase your metabolism. This allows more fat to be burned, even at rest! The more muscle tissue you have, the more little furnaces you have in your body to burn fat. Not to mention that strong muscles will help stabilize joints and prevent serious injury to the joint. It is important to work all the different energy systems, because doing so will prepare your body for daily activities.

Note: Information for this article was taken from a course by Body Blueprint.

See also:

I am a BCRPA-certified fitness instructor in Vancouver, BC. I teach four classes at the West End Community Centre in Vancouver, BC, mostly designed for the older adult. The Inevitable Disclaimer: Everything published here expresses only my opinion, based on my training and research. What you do with the information is entirely your own responsibility. I am not liable for any injury you suffer that seems to be related to anything you read here. Always consult your doctor before beginning an exercise program. For other articles, return to the table of contents.

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