Endurance Exercise Makes You Insulin Resistance..WTF!

A few week’s ago I wrote a post making the argument that movement and exercise is the most important thing you can do to help regulate your blood sugar.

The key argument from that post was that movement activates a receptor in our cells that allows them to take in sugar from the blood without the need to produce a hormone called insulin. This is important because due to a number of lifestyle choices many people make, they are becoming less and less responsive to insulin leaving them with elevated blood sugar which then leads to several adverse health outcomes as outlined in last week’s post. Having an alternate way to lower blood sugar that does not require insulin can be incredibly beneficial to one’s health.

Seriously, you need to read that post!

In this week’s post I am going to completely contradict what I said a few weeks ago! This week I am going to explain how endurance exercise makes your blood sugar regulation worse 🤦‍♂️.

How? Why?

Let’s find out!

When I first read the study Reduced Glucose Tolerance and Insulin Sensitivity After Prolonged Exercise in Endurance Athletes, 2023 my mind was blown.

Everything I have read about blood sugar regulation and exercise has said that any and all exercise is going to be better for blood sugar regulation.

This study is saying the complete opposite.

The study took 8 healthy non-endurance athletes and 9 (5 women and 4 men) endurance athletes and compared their glucose tolerance before exercise, after a high intensity interval (HIIT) workout and after a prolonged endurance workout.

The endurance group had several years of endurance training experience and had participated in endurance events lasting from several hours up to several days.

The endurance protocol used in this study involved cycling at 65% of VO2Max for 3 hours. The HIIT protocol was five 4 minute intervals at 95% of VO2Max with only 3 minutes of rest between each interval…not fun!

To assess how well each group was able to regulate their blood sugar, researchers gave participants an oral glucose tolerance test after a 14hr overnight fast, after the endurance exercise protocol, and after the HIIT protocol. An oral glucose tolerance test is basically 75g of glucose in liquid form that you drink. Over the next 1-2 hours the test tracks blood glucose and insulin.

Here are the results under all three conditions for both groups.

If we look at figure A of the glucose for the endurance athletes you can see their glucose after the endurance protocol (END PE in the figure). It is much higher than the overnight fast (RE) or the HIIT conditions.

While the non-endurance control group, figure B, also had elevated glucose after the endurance protocol however it ended up not being statistically significant. Researchers measured the “area under the curve” which represents the amount of space under the curve on the graph. As you can see in figure C the endurance protocol for the endurance athletes produced the largest area under the curve and it was also statistically significant.

If you are wondering what the “area under the curve” represents in this case, it not only represents how high their glucose got during the test, but also how long it was elevated for. When it comes to blood sugar, it’s not only how high but the length of time it stays elevated. This is why the area under the curve is a great way to assess blood sugar control.

The conclusion from this specific study is that endurance athletes have worse blood sugar control after three hours of endurance exercise when compared to after a HIIT workout or at rest. This effect appears to be specific to endurance athletes since non-endurance athletes do not seem to exhibit the negative effects on their blood sugar after the same endurance workout.

What is going on here?

We cannot say with 100% certainty but in my opinion this is an example of the human body adapting in order to “survive”.

Endurance athletes by definition spend a lot of time exercising at a low/moderate intensity. This type of exercise is primarily fueled by fat as opposed to carbs/sugar. Actually there is a great graph from the study which illustrates this nicely.

Here you can see how much more fat endurance athletes burn compared to the non-endurance controls.

It is not only the fuel utilization that the body is adapting to, it is the volume of exercise that endurance athletes tend to do.

Most humans have a virtually unlimited amount of fat to fuel exercise on their bodies whereas we only have about 2000 calories of carbs/sugar stored in the body. So if you are asking your body to exercise for a long period of time what fuel would be a better choice?

FAT!!! You will likely never run out of it no matter how long you are exercising for.

The other reason why using more fat is better for endurance athletes in this case is because certain parts of our body, like the brain, need glucose, they can’t use fat. If we burn all our glucose up from exercise we are going to be in big trouble.

Essentially what we are seeing here is the body intentionally becoming less metabolically flexible because of the demands being placed on it. The body is prioritizing fat utilization over glucose/sugar because that is the signal that is constantly being sent to the body by all the endurance exercise…its a survival adaptation.

OK, but is this something YOU need to worry about?

Well the question I would ask you first is are you an endurance athlete who is regularly doing multi-hour workouts?

If not then, you likely don’t need to worry about it (please keep reading though because there is a key takeaway at the end of this post for everyone!).

If you are an endurance athlete but are keeping workouts below 2 hours you are also probably still OK (Improved Insulin Sensitivity After a Single Bout of Exercise Is Curvilinearly Related to Exercise Energy Expenditure, n.d.).

If you are an endurance athlete and you are doing workouts longer than 2-3 hours you might at least want to be mindful about the food you are eating after your long workouts. It is common for athletes to “refuel” or celebrate after the completion of a long hard workout and this typically involves a lot of highly processed carbohydrates. The results from this study would suggest this may not be the best choice from the perspective of blood sugar control.

Does it mean you should eat no carbs though?

No, I don’t think so. I think you can probably eat some carbs, but stick to whole food carbs that are going to be slower to digest and combine them with protein and fat. You probably also would want to go for a walk after your meals as well to help better regulate your blood sugar.

If you are an endurance athlete who already has blood sugar regulation issues, then I might suggest you avoid carbs after your long workouts. Your blood sugar control is already compromised to begin with, adding on the suppression as a result of the endurance exercise is not going to help things.

The other question you might be wondering is how long does the effect of endurance exercise on blood sugar control last?

This study did not give us any data to answer that question, so we don’t know based on these results. However, my guess is that it is going to be an acute temporary effect. It took 3 hours of exercise to induce the changes in blood sugar regulation in this study, so reversing it should not take too long either. I would say within 24 hours things should return to baseline.

I realize that most of the people who are reading this post likely are not endurance athletes or if they are they may not be doing long multi-hour bouts of exercise, so is there a lesson to be learnt from this post?


I titled this post the way I did because I wanted to illustrate a point that I see happen all the time: it is easy to catastrophize any piece of information.

Someone could easily take the results from this study and make content that would make endurance exercise look pretty terrible. This is true of anything in human physiology though. Carbs, sugars, cholesterol, animal protein, plant protein, vegetables, saturated fats, vegetable oils, high intensity exercise, strength training….if you search long and hard enough you can find “facts proving” that anything is both good and bad for you.

It is frustrating, believe me I understand that.

Unfortunately we cannot take a headline, or social media post and trust it for what it claims on the surface, we need to dig deeper, we need to understand the context.

Yes endurance exercise could hinder your blood sugar regulation, but ONLY after several hours, and ONLY in endurance athletes. In addition, it’s ONLY a temporary effect. So while the headline of this post makes endurance exercise sound bad for you, it is likely a net positive when you consider the entire context.

I really like to dig deep to uncover the true application of research to the everyday person and their goals. I do that not only through my long form blogs but also through short emails I send out and weekly videos. Whether you like long or short form written content, or prefer to watch things via video, I guarantee you will be able to find practical advice you can apply to help you reach your goals by subscribing to my newsletter. Just enter your email in the form below and each week I will send you a few emails helping you decipher these confusing and often controversial topics.

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Improved insulin sensitivity after a single bout of exercise is curvilinearly related to exercise energy expenditure. (n.d.). PubMed. Retrieved June 2, 2023, from https://pubmed.ncbi.nlm.nih.gov/17635103/

Reduced glucose tolerance and insulin sensitivity after prolonged exercise in endurance athletes. (2023, April 5). PubMed. Retrieved June 2, 2023, from https://pubmed.ncbi.nlm.nih.gov/37017615/

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