Looking for Your Ideal Body Composition? Learn to Use Metabolic Flexibility

How the Body Switches Between Fuel Sources

By Michael Ormsbee, PhDFlorida State University
Edited by Kate Findley and proofread by Angela Shoemaker, Wondrium Daily

Do you find yourself snacking on cereal while you’re sitting at your computer? Often we reach for carb-filled snacks when we’re bored, but this isn’t the best strategy for obtaining our ideal body composition. Michael Ormsbee, Ph.D., explains how to choose the fuel that’s right for your activity.

highlighting the word metabolism on a page of a book
Metabolic flexibility involves the use of fat or carbohydrates as a fuel source as the body’s metabolic system responds to varying energy demands. Image credit: Feng Yu / Shutterstock

Why Metabolic Flexibility Matters

The human body adjusts to what you eat by burning more of that type of nutrient. For example, a healthy person should be able to use more fat for fuel when they eat more fat, such as when they have not eaten for a long time or during prolonged periods of exercise. This ability to shift easily from one fuel source to the other, based on the exact circumstance, is called metabolic flexibility.

Researchers are now investigating people who are unable to switch easily between fuel sources—those who might be classified as metabolically inflexible. We know now that being sedentary, overweight, or obese is associated with less metabolic flexibility. Essentially, someone who is obese may have more trouble burning fat as a fuel than someone who is physically active and lean.

“One really cool part about metabolic flexibility is that, in a lab setting, we can actually measure what fuel source—fat or carbohydrate—you are using,” Dr. Ormsbee said. “This is done by measuring the amounts of carbon dioxide and oxygen in your breath at rest or during exercise.” 

The ratio of carbon dioxide produced to oxygen consumed is called the respiratory exchange ratio (RER). When burning 100 percent fat for fuel, your RER is equal to 0.7. However, when burning 100 percent carbohydrate for energy, your RER is 1.0.

When walking slowly, you’re using primarily fat to produce adenosine triphosphate (ATP) because this is aerobic and would use the oxidative energy system. As a result, your RER would be at or very close to 0.7. 

When you increase the intensity of your walk to a jog or a run and move into the glycolytic energy system, you’ll use much more carbohydrate as the primary fuel source. Here, your RER would be at or near 1.0.

Adjusting Fuel to Activity

Basically, the faster and harder you go, the greater percent of carbohydrate that will be used to make energy or ATP. Keep in mind, though, that there will always be a mix of carbohydrates, fats, and proteins used to produce energy. The more metabolically flexible you are, the better you will be at using energy sources at the exact, right moment.

When you are doing low-intensity efforts—walking, working while sedentary, or watching TV—you would not need a large amount of carbohydrates; it simply doesn’t make sense metabolically. However, when you decide to be physically active—exert yourself to strengthen your heart, lungs, and muscles—then you might be better off eating carbohydrates.

In this way, you can take advantage of your metabolism and use the inherent metabolic use of fuels called bioenergetics. In this way, you’ll be less likely to store what you eat as excess body fat and you’ll improve your nutrient partitioning, particularly when food is not eaten in a huge serving size.

A Helpful Analogy

It might help to think of your fuel use and energy systems like a car with a series of gas tanks in it. This is an analogy that is often used in exercise physiology courses. 

Tank 1 would be very small but would produce ATP quickly, although you soon would run out of fuel. Then you move on to Tank 2, where you can go a bit longer because the tank is bigger. 

These first two tanks will deplete much of your stored creatine phosphate and your stored carbohydrate to use as a fuel. These get mostly used up before moving on to Tank 3, which is the largest but slowest tank. 

This tank uses fat for the largest part of its fuel supply. In order to make a real dent in Tank 3, you must first slow down and burn through Tank 2 or use a lot of fuel. Then you can rely on Tank 3 for the long haul.

Thus, you can see that knowing how to use your energy systems to your advantage is a huge asset when wanting to change your body composition. After digesting and absorbing the food you eat, it provides you with energy for everything you want to do. Aim to change your food intake and carbohydrate intake based on your activity level each day.

This article was edited by Kate Findley, Writer for Wondrium Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for Wondrium Daily.
Dr. Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University.

Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.