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What to Eat Before a Workout? A New Study on Fasted Cardio


Noa is dancing enthusiast, a Zumba instructor, a mom and a PhD in Bioinformatics.


Do you want to know what to eat before a workout? A new study supports the claim that skipping breakfast and performing physical activity on an empty stomach boosts the amount of fat burned during exercise. This is the first study to demonstrate that eating before exercise affects post-exercise fat cells gene expression, and the researchers suggest that fasted exercise is likely to have long-term health benefits on our fat tissue.

In this study the researchers recruited overweight participants, making the research findings particularly relevant, given that ~62% of the UK population is overweight.

What is Fasted Cardio?

To make your regular cardio a "fasted" cardio you simply need to skip breakfast and do your workout on an empty stomach, first thing in the morning. In order to understand the rationale behind fasted workouts we need to consider what happens in our body after a meal.

When food is consumed it is first broken down into simpler molecules that our cells can utilize. Those molecules are then released into the blood stream, and need to be shuttled into cells. That is where Insulin comes in, helping to transport these molecules into our cells. Thus, Insulin levels increase after eating and can remain elevated for several hours. Once our body finishes digesting and absorbing the nutrients from the food, insulin levels drop to a low baseline level.

If we add exercise to the equation we can think of our body as functioning in one of two distinct states:

  1. "Fed" workouts - exercise performed in times when insulin levels are elevated and food is still being processed and absorbed.
  2. "Fasted" workouts – exercise performed when insulin is low and food is no longer being digested and absorbed.

Can Fasted Cardio Burn More Fat?

Another aspect of Insulin is that it suppresses the breakdown of fatty acids. When Insulin levels are high our body doesn't use fat as an energy source. Thus, after a meal is consumed your body suppresses its fat-burning mechanisms because energy is readily available from the food you just ate. Once the food is completely digested Insulin levels decline, telling our body that it is time to start using stored fat for energy.

It therefore makes sense that fasted cardio is often endorsed as a way to speed up fat loss. When you skip breakfast and exercise on an empty stomach your Insulin levels are low and your body is forced to burn more fat. On the other hand, if you eat a meal before working out, your Insulin levels are elevated and you will burn less fat.

Fasted Cardio Makes Evolutionary Sense

If we think of this issue in evolutionary terms we realize that our ancestors did not have access to food first thing in the morning. Upon waking up they would initially need to go hunting and gathering. It was therefore the norm for exercise and exertion to precede the consumption of food.

Criticism of Fasted Cardio

The following video explains some of the problems that may be encountered when trying to implement the concept of fasted training:

  • If you wake up feeling drowsy, weak and hungry and try to force yourself to go for a run on an empty stomach, you may simply not have enough energy to complete your workout session.
  • If, on the other hand, you were to eat a small snack before your morning exercise, you may be able to exercise harder and for a longer duration. A higher intensity workout could result in burning a larger amount of calories.
  • When doing fasted cardio our body may start generating energy from breaking down our muscle tissue and converting it into fuel. This process is known as catabolysis.

The New Study: Eating Before Workout Affects Fat Cells Response to Exercise

A new study from the University of Bath in England was published in The American Journal of Physiology, Endocrinology and Metabolism. One interesting aspect of this study is that the researchers decided to focus on regular, overweight, people rather than athletes. Previously, research into meal timing focused mostly on studying how meal timing affects athletic performance. Far less is known about how meal timing and working out might influence the health of average individuals.

Recent studies indicated that the fat tissue in our bodies is not an inert lamp of lard. Rather, our fat cells are constantly busy, producing and excreting a large variety of substances that affect the organs and systems in our body. The scientists from Bath University suspected that eating before exercise can affect the behavior of fat cells and the production of these substances.

In order to check this hypothesis, they recruited 10 sedentary and overweight but otherwise healthy young men. The assumption is that the lifestyles of these men are probably representative of those of the general population.

The experiment was conducted on two separate mornings. The workout was identical on both days: a one hour walk at moderate pace on a treadmill.

On the first morning the participants were asked to skip breakfast and not eat anything before the workout. Therefore, the 10 men exercised on an empty stomach, after an extended overnight fast.

On the second morning, the men enjoyed a substantial, 650-calorie breakfast, supplied by the scientists. The meal included some toast, jam, cereals, milk and orange juice. The breakfast was consumed about two hours prior to the treadmill workout.

On both days, blood samples were collected at regular intervals to observe changes in metabolic parameters and in the concentrations of particular cell signaling proteins that are secreted by adipose cells.

Fat tissue samples were taken at two time points: just before the start of the exercise and one hour after the walk was completed. The collected samples were analyzed and compared.

The Results

Significant differences were found between the different samples:

  • When comparing the pre-workout samples from the two mornings it was found that blood sugar and insulin levels were significantly lower in the first day, when breakfast was skipped.
  • More fat was burned during the walk that was performed on an empty stomach, with the low blood sugar levels.
  • The researchers also looked at the total number of calories burned in each walk. On average, the men burned slightly more calories on the second walk, the one performed after eating a hearty breakfast.
  • Multiple genes that produce proteins that can improve insulin levels and blood sugar regulation were significantly activated in the fasted workout relative to the fed workout.


The finding about the elevation of multiple genes associated with blood sugar and Insulin regulation is most interesting. It suggests that walking on an empty stomach could potentially improve our overall metabolic health.

This study offers the first evidence that eating changes the response of fat tissue to exercise. Several genes involved in lipid metabolism, insulin signaling and glucose transport behaved differently in fat cells when exercise was performed in a fed versus fasted state.

Limitations to the Study

It is important to note that this is a very small scale study that included a total of 10 individuals and only two exercise sessions. Several important issues still remain unanswered and awaiting further research:

  • It is not yet clear whether the significant changes observed in gene expression and fat burning during a fasted workout will necessarily translate into lingering health improvements.
  • What about other forms of meal timing? For example, would skipping lunch before an afternoon walk produce similar effects?
  • The researchers studied only healthy young men. They did not recruit women, claiming that it is problematic to control for the impact of the menstrual cycle on metabolism.
  • The results also did not suggest that fasting before exercise accelerates weight loss. In fact, in this study, the men burned a slightly larger number of calories when the exercised after eating a big breakfast. Although, you may like to keep in mind that the extra amount of calories burned in the fed workout was negligible compared with the 650 calories consumed in the large breakfast.

Is it Better to Workout on an Empty Stomach?

Professor Dylan Thompson, the director of health research at the University of Bath and senior author of the study, said that many questions must still be examined before scientists can propose recommendations about eating before exercise. However, he added that he would not be surprised if subsequent data reveal that exercising on an empty stomach has multiple health benefits.

Is Fasted Cardio Right for You?

It is important to listen to your body and learn what works for you. If you wake up genuinely hungry and need to put something in your stomach before you can master the energy to work out, then that is the best option for you.

If, on the other hand, you get up and your body is ready to go, then maybe you should give fasted cardio a try. It could boost your morning workout and help you achieve your fitness goals.

If you would like further suggestions on improving your workouts, read 10 Ways to Burn More Calories in Zumba Class and How Coffee Can Boost Your Workout Performance.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


Queen-Of-The-Worl on January 13, 2018:

Actually its Queen-Of-The-World, but whatever

I have found for myself that I get extremely hungry if I don't eat about an hour before a work-out. I feel ill as if I will pass out and my work-out fizzles and concentration goes to zero.

Drinking water does not offset this feeling. So I make sure I always have fuel in the tank.

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