The Performance-Wrecking Flaw of Low-Carb and High-Carb Diets

Learn how and why eating too few or too many carbs can ruin your performance.

Your body is an amazing machine. It turns the food you eat into fuel you need to live on a day-to-day basis and to perform at your max capacity during training and while competing in sports.

A series of complex processes occur to ensure your body is properly fueled for the different activities you perform throughout the day. However, if you're not careful with your diet, you can alter these natural processes, making your body a less efficient machine and ultimately impairing your performance.

How Your Body Uses Fuel

Your body uses carbohydrate and fat for fuel. It also uses protein, but not as a main fuel source. During high-intensity activity, carbohydrate is the go-to fuel source because it provides fast-acting energy for strength, speed and power moves.

Fat is used as the primary energy source during low- to moderate-intensity activity, which is why the so-called "fat-burning zone" is typically an activity like a slow jog. Your body never uses carbohydrate or fat exclusively. It shifts toward one or the other, but it's always using both.

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It's possible to determine in a lab what fuel source you're using. A metabolic cart measures the ratio of oxygen consumed to carbon dioxide exhaled, determining your Respiratory Exchange Ratio, or RER. A value of .7 indicates your body is using primarily fat, whereas a value of 1.0 indicates your body is primarily using carbohydrate.

According to Dr. Mike T. Nelson, an exercise physiologist and owner of, our bodies are designed to be metabolically flexible, meaning they can shift fuel sources depending on activity intensity. However, it's also possible for our bodies to be metabolically inflexible.

When Your Diet Causes Metabolic Inflexibility

You can cause your body to become metabolically inflexible by consuming either a high-carb or a low-carb diet. This was demonstrated in several studies in which people performed low-intensity exercise and the amount of fat they used varied greatly among them.

"Studies show that at very low intensity, how much fat a person burns during exercise varies greatly," says Dr. Nelson. "If we have a person do low-intensity work, another person won't necessarily use the same amount of fat."

Individuals have different crossover points, when the intensity of their work causes them to switch from one fuel source to the other. And this is likely caused by their diets.

RELATED: Caution: Low-Carb Diets Are Not for Athletes

Dr. Nelson refers to the fact that some runners load up on carbs even when they are going for a short run and not necessarily performing at a high level. He explains, "They're so used to taking in so many carbohydrates that their insulin levels are always high. So they literally push the crossover point to the right relative to the same percent of their max. They basically are tuned to run mostly on carbohydrate."

This also happens to individuals who cut carbs. Dr. Nelson continues, "There are people who use very low carbohydrate diets and they lose the ability to use carbohydrate well. They may have increased their ability to use fat, but they also lost their ability to use carbohydrate."

For athletes, this causes a number of problems. If your body is unable to cross over and use the proper fuel for the activity you're doing, it will be difficult to perform at a high level. For example, if your body is tuned to use carbohydrate during fat-burning workouts, it will be hard for you to burn fat. If you're tuned to use fat as a fuel source because you have a low-carb diet, it will be difficult to perform at your max strength, power and speed—especially over the course of an entire game.

What You Should Do

The key is to have a balanced diet. Low-carb diets are renowned as poor choices, and this only furthers that claim—especially if you're an athlete. If you're trying to lose weight, you will be tuned to use fat, but it will be difficult to have quality workouts.

On the flip side, high-carb diets and loading up on carbs before low- to moderate-intensity activity lasting less than 90 minutes can have the opposite effect. You will be less able to use fat, and if you run out of carbohydrate during your workouts, you will hit the proverbial wall.

RELATED: How to Incorporate Healthy Fats Into Your Diet

Dr. Nelson provides a simple formula to solve your woes: "If it's a high-intensity day, then eat carbohydrates because that's what your body is going to use. If it's a fat-burning day or a recovery day, cap your heart rate at around 130 and lower the amount of carbohydrate so you can increase your body's ability to use fat."


Nelson, M. T., Biltz, G. R., & Dengel, D. R. (2015). "Repeatability of Respiratory Exchange Ratio Time Series Analysis." Journal of strength and conditioning research. National Strength & Conditioning Association. Published ahead of print.

Goedecke, J. H., Gibson, A. S. C., Grobler, L., Collins, M., Noakes, T. D., & Lambert, E. V. (2000). "Determinants of the variability in respiratory exchange ratio at rest and during exercise in trained athletes." American Journal of Physiology-Endocrinology and Metabolism279(6), E1325-E1334.

Burke, L. M., Angus, D. J., Cox, G. R., Cummings, N. K., Febbraio, M. A., Gawthorn, K., ... & Hargreaves, M. (2000). "Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling." Journal of Applied Physiology89(6), 2413-2421.

Stellingwerff, T., Spriet, L. L., Watt, M. J., Kimber, N. E., Hargreaves, M., Hawley, J. A., & Burke, L. M. (2006). "Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration". American Journal of Physiology-Endocrinology and Metabolism290(2), E380-E388.


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