Carbs or Fat: Which Is the Body's Preferred Fuel Source?

Forget carb-loading. Some swear that a low-carb diet is the way to promote athletic greatness. Does the body actually run better on fat or carbs? Nutrition Diva dives in.

Monica Reinagel, MS, LD/N, CNS
3-minute read
Episode #343

What Is Metabolic Flexibility?

In a recent article in the Examine Research Digest, exercise physiologist Mike Nelson argues that elite athletes (as well the rest of us) benefit from being able to switch easily back and forth between carbs and fat as a fuel source, something he refers to as “metabolic flexibility.”

Our body’s ability to adapt to a variety of fuel sources is a great advantage. But as all you yogis out there know, you don’t maintain flexibility by holding your body in one rigid, extreme posture. Flexibility requires moving back and forth between postures, stretching in one direction and then the other.

Athletes who follow low carb diets for an extended period of time become less able to utilize carbohydrates as fuel. This is usually not to their benefit. Losing your metabolic flexibility can also increase your risk of Type 2 diabetes.

Food Is More Than Fuel

Let’s also not overlook the fact that diet supplies us with more than just calories or cellular energy. It’s also a source of nutrients that protect and repair our organs and tissues, friendly bacteria and the nutrients that sustain them, and much more. While fat can replace carbs as a source of energy, a high fat diet is not going to provide the same range and balance of other nutritional factors.

It's certainly possible to sustain the body on a very high-fat diet for the short term, but I worry a bit about the long-term effects of such a lopsided diet. I’d have the same concerns about a very low-fat diet. A diet that delivers a balance mix of fats, protein, and carbohydrates from a variety of whole foods seems to me like the best way to promote optimal function and flexibility for the long haul.

What do you think? Post your comments below or on the Nutrition Diva Facebook page. I look forward to hearing from you.


Corpeleijn E, Saris WH, Blaak EE. Metabolic flexibility in the development of  insulin resistance and type 2 diabetes: effects of lifestyle. Obes Rev. 2009Mar; 10(2):178-93.

Kelley DE, Mandarino LJ. Fuel selection in human skeletal muscle in insulin resistance: a reexamination. Diabetes. 2000 May; 49(5):677-83.

Nelson, MT. Metabolic flexibility: The argument to use both carbs and fats. Examine.com Research Digest 2015 June; 8: 12.

Volek JS, Noakes T, Phinney SD. Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci. 2015;15(1):13-20.

Image Courtesy of Shutterstock.


About the Author

Monica Reinagel, MS, LD/N, CNS

Monica Reinagel is a board-certified licensed nutritionist, author, and the creator of one of iTunes' most highly ranked health and fitness podcasts. Her advice is regularly featured on the TODAY show, Dr. Oz, NPR, and in the nation's leading newspapers, magazines, and websites. Do you have a nutrition question? Call the Nutrition Diva listener line at 443-961-6206. Your question could be featured on the show.