Why You May Want to Fast Before Working Out


The notion of eating before a workout has long been a cause for debate among the fitness community, and compelling arguments have been made for both sides.

A team at the University of Bath seeks to put the matter to rest by conducting the first study to ever examine the effects of eating (or not eating) on fat burning during exercise.

The study claims that eating after a workout is more effective for burning fat than eating beforehand. Credit: Flickr, CC BY 2.0

Published ahead of print in the American Journal of Physiology – Endocrinology and Metabolism, this study brought in volunteers to undergo blood tests to reveal how adipose tissue responds to the presence or absence of carbohydrates. The participants were all male and classified as overweight.

The participants were asked to walk for 60 minutes at 60 percent of their maximum oxygen consumption. On one occasion they walked after fasting, and at another time they consumed a high-calorie breakfast that was rich in carbohydrates. The breakfast was eaten two hours prior to exercise.

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Before and after each exercise session, the participants had blood taken to assess the change in gene response in adipose tissue. There was a very distinct change in each instance, showing that consuming food does create a significant effect on fat burning on the genetic level. When the men fasted before exercising, the PDK4 and HSL genes decreased. On the other hand, these same genes increased when exercising after eating. These two opposite effects led the researchers to believe that exercising fasted could have more favorable outcomes when it comes to losing stored fat.

In the fasted exercise sessions, the increase of PDK4 could mean that the body was turning to stored fat as fuel for the workout, and effectively burning fat in the adipose layer. HSL, in like kind, is typically increased when the body begins to use fat as a source of fuel.

“Adipose tissue often faces competing challenges,” wrote the corresponding author of the study, Dylan Thompson. He goes on to explain that adipose tissue “is busy responding to the meal and a bout of exercise at this time will not stimulate the same [beneficial] changes in adipose tissue. This means that exercise in a fasted state might provoke more favorable changes in adipose tissue, and this could be beneficial for health in the long term.”

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In other studies, it has been difficult to properly identify whether or not food consumption has a direct effect on a person’s results. The Journal of the International Society of Sports Nutrition published a work on body composition for people consuming a nutritional shake before or after a workout. They did not see a dramatic difference in either group, but saw that both groups lost weight. However, this new study from the University of Bath is the first to directly examine the gene response.

Of note is that this study fed the participants only a high-calorie, high-carbohydrate breakfast prior to training. Further research may examine the effect of the other macronutrients on gene response, such as if a person consumed a high-protein or high-fat meal before exercise. As of now, exercising fasted can burn more fat than eating beforehand.