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

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.

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.

References

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.

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