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

Sandy writes:  “A lot of people are promoting the idea that fats (and not carbs) are the body’s preferred source of energy. Is there any truth to this is or is this just the latest fad?”

One of the cool things about human metabolism is that we have an amazing ability to adapt to whatever kind of fuel is available. Although the body turns first to carbohydrates to provide fuel for the muscles and brain, if carbohydrates are not available, it can also convert fat (either from the diet or from the body’s own fat stores) into energy.

As to what type of fuel works best, I think it depends on what you are asking your body to do. Think? Sleep? Lose weight? Go long periods without eating? Run faster? Run longer? Lift more?

I’ve talked before about the pros and cons of low-carb diets for weight management and about the effects of diet on brain function. But what about low-carb diets for athletes?

Should Fuel Your Workout with Bread or Bacon?

Traditionally, athletes have regarded carbohydrates as the best choice for fueling athletic performance because these can be most quickly converted into glucose. The downside is that you have to refuel regularly. Fat contains more calories and is more slowly converted into energy, so you don’t have to refuel as often.  But it’s not as good for quick bursts of energy.

Fueling with carbs is like adding a lot of small, dry twigs to a campfire: you’ll get an instant blaze but to keep the fire going, you’ll have to keep adding more twigs. Fueling with fat is more like throwing on a big log. It’ll take a lot longer to get going, but it’ll also burn for longer. I don’t know about you, but when I’m building a campfire, I want some of each.

But some are now arguing that a high fat diet is actually better for athletes than a high carb or mixed diet. And it’s true that if you deprive your body of carbs for long enough, your body will become much more efficient at converting fat to energy. This is sometimes referred to as being “keto-adapted” or “fat-adapted.”

There are some amazing examples of keto-adapted athletes pulling off feats of physical strength and endurance while eating almost no carbohydrates. Once again, it’s incredible how adaptable the body is! Then again, these folks would probably be exceptional athletes no matter what they were eating.


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.