Is Fasting During Workouts Safe?

If you purposely withhold calories during a workout, proceed with caution. Photo: www.shutterstock.com

Think twice before you “starve” yourself during and after workouts.

Several years ago I used to run on occasion with a female friend who twice qualified for the U.S. Olympic Trials Marathon. One day when I met her at her house for a run I brought over a canister of a sports drink that I was then getting for free and wanted her to try. When she mixed up a bottle in the kitchen I noticed that she used only half the recommended amount of powder. Bemused, I asked why.

“It’s 120 calories per serving,” she said. “I don’t want all that.”

I was surprised. My philosophy on the use of ergogenic aids during workouts had always been to take in as many calories as I needed to maximize my performance. The idea that the calories I consumed during runs or rides might hinder my efforts to get leaner for racing never crossed my mind. But after witnessing my friend’s “workout fasting” I began asking around and learned that many endurance athletes intentionally restrict their calories during training to promote fat loss.

Is there any validity to the fear that taking in sports drinks, gels, and so forth in workouts makes it more difficult to shed excess body fat? Should you, at least in some circumstances, intentionally take in fewer calories than would be required to optimize your workout performance? Let’s look at the science.

Athletes who fast or are tempted to fast during workouts operate on the belief that the calories in ergogenic aides simply supplement the calories eaten during the rest of the day and thereby increase the day’s total calorie intake. But this is not the case. Studies have shown that when athletes consume carbohydrate during exercise, they eat less during the rest of the day. So by using a sports drink or whatever during workouts you get the advantage of better performance without the disadvantage of increased total daily calorie intake.

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The other fear that lies behind the choice to restrict carbohydrate intake during workouts is the fear that doing so reduces the amount of fat burned during the workout. This is true. Your body will burn more carbs and less fat in workouts during which you consume carbs than during workouts in which you fast. But this does not mean that using a sports drink during workouts will make it harder for you to shed excess body fat. With respect to losing body fat, what matters is not the type of calories you burn during workouts but how many calories you burn, and you will usually burn more calories in carb-fueled workouts because you will be able to work harder in those workouts.

The reason it doesn’t matter whether you burn primarily fat or carbs during workouts is this: During the hours that follow a workout in which you burn mostly carbs, your body will burn a lot fat as it spares carbs for use in replenishing your depleted muscle glycogen stores. And during the hours that follow a workout in which you burn mostly fat, your body will burn a lot of carbs as it spares fats for use in replenishing your depleted intramuscular triglyceride stores. Research has consistently shown that the most effective workouts for fat loss are high-intensity interval workouts that burn mostly carbs. Why? Because the body burns a ton of fat after such workouts. So don’t worry about the fact that your body will burn less fat during carb-fueled workouts. You’ll come out ahead in the long run.

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So does all of this mean you should never intentionally restrict carbohydrate intake during workouts that are long enough for carb consumption to make a difference (roughly one hour and up)? No. There are benefits associated with occasional workout fasting, but they have nothing to do with getting leaner. It so happens that some of the positive physiological adaptations to training are triggered by depletion of the body’s internal carbohydrate stores. When you consume carbs during a workout, your body’s carb stores become less depleted and there’s less stimulus for positive adaptations. In addition, it has been shown that performing longer workouts without taking in carbs increases the body’s fat-burning capacity during exercise, which aids performance in long-distance races.

It’s not necessary to withhold carbs in every long workout to maximize fitness gains and fat-burning capacity, but it’s a good idea to do it occasionally.

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About The Author:

Matt Fitzgerald is the author of numerous books, including Racing Weight: How To Get Lean For Peak Performance (VeloPress, 2012). He is also a Training Intelligence Specialist for PEAR Sports. To learn more about Matt visit www.mattfitzgerald.org.

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