Ask Eirik: Should I Exercise on an Empty Stomach?

woman-runningLast week I was interviewed by a Norwegian diet and lifestyle magazine. The interviewer was planning on writing an article about “vacation fitness” and asked me to share some diet and exercise tips that could help the readers of the magazine stay in shape during the summer. She was particularly interested in hearing my take on fasted exercise, as she, together with the rest of the people who write for and run the magazine, are arranging a trip to Greece later this year, on which the participants will partake in long hikes early in the morning, prior to eating breakfast.

Some of the people who had signed up for the trip were apparently concerned about the prospect of working out on an empty stomach, which isn’t really surprising, seeing as the longstanding dogma that it’s important to eat breakfast early in the morning is deeply ingrained into the public’s mind. In today’s edition of Ask Eirik, which I intend to keep fairly short, I thought I’d share some of the answers I gave the reporter when she asked me about fasted training, as well as talk a little about the evolutionary merits of fasted exercise.

My answer:

When I first joined a fitness center in my youth and started getting interested in strength training, I remember that there was a lot of talk about fasted cardio within the fitness community. Whereas some fitness enthusiasts and bodybuilders made the case that it’s much better to do cardiovascular exercise early in the morning, on an empty stomach, as opposed to later in the day, others argued that it doesn’t really make any difference whether you exercise before or after eating. I can also remember that some went as far as to say that fasted exercise is detrimental to health and/or suppresses muscle growth.

These days, I no longer pay that much attention to what fitness bloggers, bodybuilders, athletes, and other fitness enthusiasts say and write about fasted exercise; however, I suspect that the situation hasn’t changed that much from back when I got my first gym membership. If you go on Google, do a search for “fasted exercise” or “fasted cardio”, and proceed to look into the articles that appear, you’ll probably find that there’s no universal agreement among journalists, fitness bloggers, and other writers about whether or not it’s beneficial to exercise on an empty stomach.

All of this conflicting information is bound to make people confused.

I believe the debate about fasted exercise needs an infusion of evolutionary wisdom. We need to shed some evolutionary light on the fasted exercise issue. Most (not all) people who write and talk about fasted exercise are not evolutionary thinkers; hence, it’s not surprising that fasted exercise is a very contentious topic. The evolutionary angle doesn’t necessarily provide us with clear-cut answers as to whether we should exercise on an empty stomach or not; however, it does equip us with a foundation upon which we can build our studies and ideas.

The evolutionary angle

You don’t have to be an expert in human evolution to understand that our primal ancestors didn’t always eat a big meal prior to being physically active; all you need is a basic understanding of how hunter-gatherers live their lives. Contrary to popular belief, hunter-gatherers rarely experience long periods of famine (1). With that said, they obviously don’t have constant access to calorie-dense foods, like those of us in the industrialized world have. They have to work for their calories. They do a lot of walking, running, and digging in order to get a hold of food.

Some foragers perform the labor that is required to get a hold of food early in the day, and then eat later on. For example, Hadza men are known to hunt in the morning.

We obviously don’t know exactly how ancient hunter-gatherers lived; however, I don’t think it’s unreasonable to say that they regularly performed various types of physical activities on an empty stomach. You pretty much have to when you live in the wild, as physical activity is a prerequisite for food acquisition. Unlike us, our ancient forebears couldn’t just lean back, call a pizzeria, and get a newly baked ham and cheese pizza delivered to them when they were hungry.

When we think about it, it’s crazy that so many people these days are skeptical of fasted exercise, seeing as it is completely normal, from an evolutionary perspective, for hominins to be physically active while in a fasted state. There’s absolutely nothing to indicate that we are hard-wired to require an influx of food prior to being physically active.

The unique health benefits of fasted exercise

The idea that we humans are well-adapted to exercise on an empty stomach is supported by recent studies that have looked into the physiological effects of fasted vs. non-fasted exercise. I’m not going to delve into the science in this area in this article; however, I would like to bring up a couple of research papers, two of which I mentioned to the reporter of the diet and lifestyle magazine I talked about earlier.

The first paper I talked about with the reporter is a study that was recently published in the American Journal of Physiology – Endocrinology and Metabolism (2). It found that the expression of several genes involved in fat metabolism was increased during fasted training as compared to during non-fasted training. The investigators had the following to say in their concluding remarks:

This study provides the first evidence that the feeding status alters the response of adipose tissue to acute exercise. Several genes involved in lipid metabolism, insulin signaling and glucose transport were differentially expressed in adipose tissue when exercise was performed in a fed versus fasted state with either lower or opposing responses after feeding. Given the nature and direction of these differences, we propose that feeding is likely to blunt long-term adaptations induced within adipose tissue in response to regular exercise. (2)

That’s a pretty interesting study if you ask me. It adds strong supportive evidence to the notion that it’s beneficial to exercise on an empty stomach. It should be noted though that the meal the participants who ate prior to exercising consumed was fairly high in carbohydrate. It was a typical English breakfast, high in grains. If the meal had been more prudent and lower in carbohydrate, I’m sure the results of the study would have been somewhat different.

The therapeutic value of fasted exercise also extends into other areas of human health and well-being, which brings us over to the second paper I talked about with the reporter. I mentioned this article in my recent post entitled Our Brains Are Shrinking, but I didn’t fully get to explain what it’s about. Basically, the authors of the paper make the case that exercise can help suppress postprandial inflammation (3).

In other words, by exercising prior to eating, you can prime your immune system to respond appropriately to food intake. Food intake is associated with increased immune activation, which may be explained by the fact that food consumption is accompanied by increased exposure to bacteria and other potential dangers. By exercising prior to eating, you can enhance your immune system’s ability to respond properly to these potential threats and suppress the chronic inflammatory response that sometimes follow the intake of food, particularly energy rich, non-Paleo foods.

Before we wrap up, I’d like to briefly mention a couple of other documented health benefits of fasted exercise, so as to really bring the message home and show that there’s strong evidence to show that it’s not only possible to exercise on an empty stomach, but potentially highly beneficial. First of all, fat oxidation tends to be elevated during fasted exercise when compared to non-fasted exercise (4). This doesn’t necessarily mean that you’re going to lose a lot more fat if you regularly exercise on an empty stomach as opposed to in a fed state; however, it does add additional support to the idea that fasted exercise may be somewhat superior to non-fasted exercise in the context of body fat loss. In addition to finding that fat utilization is greater during fasted exercise than non-fasted exercise, an older study also showed that blood glucose concentrations remain within a normal range during exercise performed after a 23 hour fast (5). This is consistent with what one would expect to see if we humans are well-adapted to exercise on an empty stomach.

Second, the consumption of food is typically followed by a dip in energy levels, as the body enters into “digestion mode”. Energy levels are gradually restored as time passes; however, the risk of experiencing gastrointestinal discomfort may be higher during non-fasted exercise than fasted exercise, particularly if little time has passed between food intake and the exercise session. Third, it’s well-known that exercise favorably impacts insulin and leptin sensitivity and glucose and appetite control. These effects are likely particularly beneficial if they are harvested prior to eating, as they help fine-tune the body’s metabolic machinery and prep if for food intake.

The quote below nicely summarizes the unique effects fasted endurance exercise has on muscle biochemistry and metabolic health.

In healthy subjects, acute endurance exercise in the fasted state is accompanied by lower blood insulin and elevated blood free fatty acid concentrations, stable blood glucose concentrations (in the first 60–90 min), superior intramyocellular triacylglycerol oxidation and whole-body lipolysis, and muscle glycogen preservation. Long-term exercise training in the fasted state in healthy subjects is associated with greater improvements in insulin sensitivity, basal muscle fat uptake capacity, and oxidation. (6)

Exceptions and caveats

So, there are clearly many potential benefits to training on an empty stomach. With that said, both fasted and non-fasted exercise are obviously beneficial, as long as it’s not excessive. Moreover, not all types of training are effortlessly performed in a fasted state. In particular people who perform intensive, highly glycolytic anaerobic exercise may find that they perform better if they eat 2-3 hours prior to training than if they work out while in a fasted state.

Furthermore, it’s important to point out that health status is an important predictor of “fasted exercise success”. Someone who eats an unhealthy, carbohydrate-heavy diet and is chronically inflamed is going to find it more difficult to perform optimally in athletic events while in a fasted state than someone who is fairly healthy. Finally, perhaps needless to say, it takes time for the body to adjust to fasted exercise. One can’t just change one’s routines overnight and expect to be able to run like a tiger in the morning 🙂

Picture: Designed by Freepik


  1. Add to this: Listen to your body. I know people who feel weak working out without eating first. Often, that’s guys who eat huge amounts of food every day. Others, like me, feel stomach sick to work out after eating. So even if I’m hungry, work out first or I won’t work out at all.

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