When you workout, your muscles power the movement of your body. They're like an engine, making your skeleton move. Like the engine in a car, your muscles need fuel. You get this fuel from the foods that you eat. It comes in the form of carbohydrates, protein and fat, which are broken down in your body.
Even when you're not exercising, your body is using energy. To stay alive your heart has to beat, your lungs have to fill with air and your brain has to function. When you workout, the need for energy goes up, especially when you're doing exercises like running and bike riding, that burn more calories than strength activities like weightlifting.
Protein, fat and carbohydrates are broken down into energy molecules that your body can store and save for later, or shuttle to your muscles for energy. Fat gets broken down into fatty acids, protein into amino acids and carbohydrates into glucose.
These are the building blocks of energy, which float around your bloodstream, waiting to be either used or stored. Your muscles will store fat in the form of triglycerides and carbohydrates in the form of glycogen, both of which will be ready to use when you start exercising.
Your body also stores fat in fat cells throughout your body. Glucose can be stored in the liver as well as muscles. Protein foods go directly into the muscles to help build and repair them, which makes muscle the largest storage of protein in your body.
When it's time to workout, your body has energy stored up for you to use. However, each form of energy has a different use.
Your body is always breaking down and using fat, glucose and protein. At the same time, it's also storing some away. The key is that when you need energy, you're going to use more than you store, so you end up having less than you started with.
Fat and glucose are the main sources of energy, and the amount that each is used changes depending on the intensity of your workout. The more intense you exercise, the more you use glucose.
Protein makes up a minuscule amount of the energy used, around 2 to 5 percent according to an article from the Riordan Clinic. Only in a case of extreme starvation, where your body is running low on fat and carbohydrate stores, would you use more protein for energy.
This is good news for people who want to build or maintain their muscle mass. It's also good for people who want to lose fat, because your body relies heavily on fat for fuel.
Changing Energy Usage
As your workout gets more intense, your body gets more and more energy from carbohydrates, because it's easier to metabolize. While the fat stores in your body can provide energy for a long time, they don't provide energy very quickly. If you're sprinting or lifting weights, you need energy immediately, and a lot of it.
Glycogen answers the call for quick energy because it breaks down quickly. As you keep sprinting, your burn through the reserves of glycogen that you have very rapidly, because it breaks down so quickly. Less glycogen in your muscles and glucose in your blood means more fat will be used.
Your body is forced to switch to using primarily fat when the byproduct of breaking down glycogen, lactic acid, builds up in your muscle. Lactic acid actually prevents your body from breaking down glycogen, so you have to slow down and let fat take over as the primary energy source until the lactic acid goes away.
Sparing Muscle Tissue
If you want to lose fat, you'll be happy to know that your body will pull from its fat stores to make up for the energy you lose from your cardio workout. However, you might be concerned that the same thing is happening to your muscle mass, especially if your goal is to build them up.
Thankfully, even though a small amount of protein is used to power your workout, you won't lose muscle mass by doing cardio. In fact, people who are sedentary can build muscle when they start doing cardio, according to a 2015 study published in Exercise and Sports Science Reviews.
Even if you've been working out for a while there's no need to fear that doing cardio will break down your muscle. A 2017 study in the Journal of Exercise Nutrition and Biochemistry showed that cardio actually has muscle-preserving effects in rats. It effectively blocks breakdown of the muscle and stimulates regrowth as the animals exercise. It can be extrapolated that the same mechanisms may happen in humans.
This muscle-preserving benefit works even if you don't eat immediately after your workout, according to a 2010 study from the American Journal of Physiology. The researchers had subjects go through a cardio workout consisting of 60 minutes on an exercise bike.
Afterwards, one group of subjects didn't eat for six hours. The researchers found that during their recovery period, even though they didn't eat, their muscles still grew.