Is limiting your calories during workouts good for you? Matt Fitzgerald weighs the pros and cons.

Is limiting your calories during workouts good for you? Matt Fitzgerald weighs the pros and cons.

Written by: Matt Fitzgerald

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.”

Photo: Nils Nilsen
Photo: Nils Nilsen

I was surprised. My philosophy on the use of ergogenic or nutritional 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 caloric intake 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 who are tempted to fast, during workouts operate on the belief that the calories in ergogenic aids 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 carbohydrates during exercise, they eat less during the rest of the day. So, for instance, by using a sports drink during workouts you get the advantage of better performance without the disadvantage of increased total daily calorie intake.

The other fear that lies behind the choice to restrict carbohydrate intake during workouts is 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 mostly fat. And during the hours that follow a workout in which you burn mostly fat, your body will burn mostly carbs. 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.

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.

Matt Fitzgerald is the author of Racing Weight: How to Get Lean for Peak Performance (VeloPress, 2009,