Food before exercise: What does science say?

Most of us like to have breakfast before we exercise in the morning, but what happens if we don’t eat anything first?

The issue goes beyond weight loss and exercise timing to involve blood sugar, insulin sensitivity, and other — perhaps unexpected — aspects of your health.

In today’s short episode of ZOE Science & Nutrition, Jonathan and Prof. Javier Gonzalez ask: Should we exercise on an empty stomach?

Mentioned in today’s episode:

Lipid metabolism links nutrient-exercise timing to insulin sensitivity in men classified as overweight or obese from The Journal of Clinical Endocrinology & Metabolism

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Episode transcripts are available here.

Is there a nutrition topic you’d like us to explore? Email us at, and we’ll do our best to cover it. 


[00:00:00] Jonathan Wolf: Hello and welcome to ZOE Shorts, the bite-sized podcast where we discuss one topic around science and nutrition. I'm Jonathan Wolf, and this week, we have a new expert joining me.

Javier Gonzalez is a professor of nutrition and metabolism at the Center for Nutrition, Exercise, and Metabolism at the University of Bath, in the United Kingdom. Javier is also one of our more recent members of the ZOE Scientific Advisory Board.

And Javier researches how our bodies use the energy that we get from food and what happens to this energy when we exercise — or if we don't use it at all. And since today's subject is exercising on an empty stomach, it seems like you're the perfect person to talk to about this, Javier.

[00:00:42] Javier Gonzalez: Well, thanks for having me on the show, Jonathan. I'm really excited to talk about this topic. Uh, we all know the benefits that exercise can have in our body, and many of us are trying to incorporate some form of movement into our daily routine, but it's often quite difficult to find time for the exercise. These moments are precious and therefore it's important that we maximize the health improvements of every second that we spend jogging, swimming, or lifting at the gym. And we're usually thinking about what we eat around exercise, but less so about when we eat it.

[00:01:12] Jonathan Wolf: So I'm really looking forward to talking about this and I really want to know, should we exercise before or after a meal?

And does this timing, you know, really affect our health?

[00:01:22] Javier Gonzalez: Well, we have some research that looks at both sides of this, and the results are really interesting and it might even change the way you exercise forever.

[00:01:28] Jonathan Wolf: Well, in that case, we better get started.

Now before we discuss how best to exercise. I have a favor to ask you listener. 63% of people that watch this podcast haven't hit the subscribe button, and 11% haven't yet hit the bell to turn notifications on. We want this podcast to reach as many people as possible as we continue our mission to improve the health of millions. So if you ever enjoyed this podcast, please hit the subscribe button and turn notifications on. Doing this small favor will really help.

[00:02:02] Javier Gonzalez: So Jonathan, I'd love to start telling you about a study that I ran with some colleagues a few years ago. If you picture a group of 30 men who were either overweight or had, we randomized them to three groups. Some of them did a moderate-paced bike ride at our lab, and then we gave them breakfast.

Another group had their breakfast first, and then they rode their bikes. And some of them didn't do any exercise at all.

[00:02:27] Jonathan Wolf: Okay. I'm now imagining a lab full of middle-aged men in lycra on exercise bikes.

[00:02:32] Javier Gonzalez: Yep. And Jonathan, do you know what the most common reason why people want to know if they should exercise on an empty stomach or not?

[00:02:40] Jonathan Wolf: So I think it might have something to do with fat burning.

[00:02:43] Javier Gonzalez: That's absolutely right. The main reason that people are interested in this is whether they can burn more fats if they exercise before they eat in what we would call the fasted state.

[00:02:54] Jonathan Wolf: Ok, so it sounds like your study might be able to answer exactly that question.

[00:02:57] Javier Gonzalez: Well, before I tell you what we found, I really want the listeners to know contrary to what they might read on the internet, the question of whether we exercise before or after food is actually much more than just about fat. Right. That sounds intriguing. I will tell all, and you're right. We did look at how much fat our study participants burned during the bouts of exercise.

[00:03:18] Jonathan Wolf: Okay, Javier, I'm on the edge of my seat. What did you find?

[00:03:21] Javier Gonzalez: So simply put, the men who exercised before breakfast burned about double the amount of fat than the group who exercised after breakfast.

[00:03:32] Jonathan Wolf: Amazing. Double the amount of fat burnt. I mean, that sounds like we've got a clear winner here, right?

[00:03:36] Javier Gonzalez: Oh, there is a caveat. Why do you think people want to burn more fat?

[00:03:40] Jonathan Wolf: I imagine the most common reason is to lose weight?

[00:03:42] Javier Gonzalez: Exactly. But it's a common misconception that burning fat through exercise will necessarily lead to weight loss. You may remember our discussion in a previous podcast on whether exercise or nutrition are more important for weight loss and that there isn't a lot of good evidence to show that exercise, whether that's before or after you eat, is a reliable way to lose weight. What we do have evidence for is that the timing of your exercise can have really important health effects in other ways.

[00:04:12] Jonathan Wolf: Ah, okay, Javier. So what you're saying is that the question of whether Ishould exercise before or after I eat isn't really about weight loss, but it's about what's best for my health?

[00:04:20] Javier Gonzalez: Yes, exactly. And I want to come back to our exercise study to tell you why. It's to do with our blood sugar.

[00:04:27] Jonathan Wolf: Right, so after we eat, most of the sugar in our blood comes from the carbohydrates that we've eaten, right? We've covered that on a lot of podcasts and they're broken down in like our stomach and our intestine. And then this sugar, which I think you as scientists often call like blood glucose, right? Is moving around in our blood.

[00:04:42] Javier Gonzalez: That's correct. It's in our blood waiting to be either used or stored away. And when we start doing exercise, This blood sugar or blood glucose, as we would call it, suddenly has a job to do and our body encourages this sugar to be transferred out of our blood and into our muscle cells when it can be put to use.

[00:05:03] Jonathan Wolf: And presumably that's important because if blood sugar isn't being sort of efficiently removed from your bloodstream and you have like higher than normal levels all the time, then you're going to increase your risk of type two diabetes?

[00:05:15] Javier Gonzalez: Yes, absolutely. And there's loads of studies to show that exercise can help move sugar from our blood into our cells.

And in fact, muscle glucose uptake can be 50 times faster when we're exercising compared to when we're sedentary. That's absolutely huge.

[00:05:32] Jonathan Wolf: That is huge. And so you're saying basically if we're doing a bunch of exercise, suddenly your muscles are gonna sort of suck all of this blood sugar out of my blood and into our muscles and, and use it for something Javier?

[00:05:43] Javier Gonzalez: There is certainly plenty of evidence to show that if we time our exercise correctly, we can potentially lower the blood sugar peaks that many people get after they eat, and potentially avoid some of the undesirable effects with very high glucose or blood sugar excursions.

[00:05:59] Jonathan Wolf: Okay, Javier, so, so tell me the question everyone wants to know, which is, when's the perfect time?

[00:06:02] Javier Gonzalez: So we know that blood sugar peaks at about 30 to 60 minutes after a meal, and a particular paper from 2016 reviewed all of the literature on this subject and came to the conclusion that doing exercise within the first hour, so 30 to 45 minutes after a meal, can efficiently blunt these glucose surges.

[00:06:22] Jonathan Wolf: Well, that sounds fantastic, and it seems to me that now we are getting closer to answer to whether to exercise before or after food.

[00:06:30] Javier Gonzalez: Not so fast, Jonathan.

[00:06:32] Jonathan Wolf: You just keep teasing me. So what are we missing?

[00:06:37] Javier Gonzalez: We need to take a look at insulin.

[00:06:39] Jonathan Wolf: Okay. Can you give us a brief explanation? What is insulin?

[00:06:43] Javier Gonzalez: Insulin is a hormone that's released from an organ known as the pancreas. And one of its main actions is to transport blood glucose into muscle cells. And a key point here is that people can have very different sensitivities to insulin. Someone with a high insulin sensitivity will need to produce less insulin to move the same quantity of glucose from the blood into their muscle cells.

However, if you are less sensitive to insulin, and this includes many people who have obesity, for example, you'll need to produce more of the hormone insulin to do the same job.

[00:07:20] Jonathan Wolf: And so is this low sensitivity to insulin dangerous?

[00:07:24] Javier Gonzalez: Potentially, yes. Another way of saying that someone has low sensitivity to insulin is to say that they are insulin resistant or that they have insulin resistance, and this means that their pancreas will make more and more of the hormone to try to get the cells to respond. And when the pancreas can't keep up, your body will struggle to control its blood sugar levels and this can ultimately lead to type two diabetes.

[00:07:47] Jonathan Wolf: Okay, so that's not a good trajectory, and we all know that type two diabetes has had this huge growth and like, you know, is a very dangerous disease. How is this linked to exercise?

[00:07:57] Javier Gonzalez: Well, the good news is exercise can improve your insulin sensitivity. And this brings us back to our study with the men on the exercise bikes.

[00:08:06] Jonathan Wolf: I knew we would get back to the men in their lycra or on the bike. So what did you find, Javier?

[00:08:11] Javier Gonzalez: So the group that exercised before breakfast showed an improved insulin response after the training. They had to produce less insulin to control their blood sugar levels, and this suggests that they would have lower risk of diseases such as type two diabetes

in the longer term, but the men who performed exactly the same exercise after eating breakfast did not show this improvement in blood insulin response.

[00:08:35] Jonathan Wolf: I think that's amazing. So they were doing exactly the same thing. They were still eating breakfasts, but just by changing the timing of the breakfast and not changing anything about what they're eating, just the timing, there was this really big change. And that sounds like that could be quite important for people like me who get very high sugar peaks after eating.

[00:08:54] Javier Gonzalez: Yes. And interestingly, you may remember that I said earlier that the men who exercised before eating also burned twice the amount of fat. And we think that there may be a link between this increased insulin sensitivity and the increased fat burnt and better health outcomes. But I should say this study was only in men. And so we definitely need to also look at this research in a group of female volunteers.

[00:09:21] Jonathan Wolf: So look Javier, that's amazing. There was also a lot of information there. So, what's your verdict on this question? Should you exercise on an empty stomach or have breakfast first?

[00:09:31] Javier Gonzalez: So, I think there are some benefits to exercising after you've eaten. You can have an immediate beneficial effect on lowering those blood sugar peaks, but if we want to get some of the longer term improvements from exercise training, it can be useful to do at least some exercise before you have breakfast and that seems to improve some of the longer term benefits, including our insulin response, potentially fat burning, which may relate to other health outcomes, but not necessarily weight loss.

[00:10:02] Jonathan Wolf: And Javier, you first told me about this study a little while ago, and, um, I actually said, you know what, I'm gonna try this. So I do three, um, exercise sessions a week with a, with a trainer.

And I have one relatively early in the morning and I had always basically eaten this really big breakfast before because like, like an hour of exercise was like, that's pretty intense. I need like lots of food or I'm not going to be able to do this. And I have actually started, um, doing this without eating. And I think the thing that is really shocking and I think will surprise a lot of listeners is I was able to do the session with absolutely, the same level of performance as I had done when I was eating. And of course I'm not, some Olympic athlete, anyone looking at me will see this.

So the point is like for sort of a regular person, interestingly, I could lift the same weight, I could do the same amount of activity, and my body was just basically producing blood sugar anyway. So what was going on, Javier? And are you surprised to hear that?

[00:10:58] Javier Gonzalez: Yeah, I'm not surprised at all, but it's, uh, reassuring to hear. We, we know from, um, certainly a lot of the studies that we and others have done, that, um, if we're eating a normal diet and we wake up in the morning, we've got enough. Fuel easily for a

typical exercise session up to an hour or even an hour and a half. And it's only really your elite athletes who are training for multiple hours a day,

um, that kind of skipping breakfast or saving breakfast for after exercise can actually impact on, on their training session For most people, we've got enough fuel there from the day before.

[00:11:29] Jonathan Wolf: So I'm now like mix and matching. I'm not doing it maybe as regularly as I should 'cause I still quite like having breakfast at my regular time. I have to admit, I'm, I'm not very good at the intermittent fasting, but I have been really struck, and every time I do this, I'm thinking about like, I'm really doing something about my, um, blood sugar control, which I, I know is weak. So I would say as a, as a sample of one, I am sold on the story that you're telling Javier.

That's great to hear. Well, look, fantastic. Thank you for this brilliant advice. I think it's a very straightforward answer to a very complicated question. If after this episode you'd like to learn more about your own blood sugar, how it responds to food and exercise, then you may want to try ZOE's personalized nutrition program. You can learn more about it and get 10% off by going to I'm Jonathan Wolf.

[00:12:14] Javier Gonzalez: And I'm Javier Gonzalez.

[00:12:15] Jonathan Wolf: Join us next week for another ZOE Podcast.