Endurance Athletes & Sugar: Best Friends, Enemies, or Frenemies?


I can’t count the number of times that I have been asked about sugar as it relates to ultra-running.  Not just ultra-running either, endurance sports in general.  Aren’t all those gels and sugary drinks bad for you?  Am I going to become diabetic from taking in all this sugar?  Won’t I “crash” if I consume a bunch of sugar during an event?  Can I just eat real food during a race?  Why is my recovery drink full of sugar?

And the list goes on and on.  These are all very valid questions. The purpose of this blog is to answer these questions and others and clear up some of the confusion about sugar as it relates to endurance athletes.

Sugar During Rest vs Sugar During Endurance Exercise: The BIG Difference

If one hopes to understand sugar’s place in endurance sports, one must first understand the difference between consuming sugars during exercise vs. consuming sugars during rest.

Sugar During Rest

Excess sugar spikes your blood sugar, promotes inflammation and weight gain, and puts you at risk of various health issues.

During rest, it is beneficial to steer clear of all added sugars. Additionally, one should be mindful of high-glycemic foods such as white bread, potatoes, white rice, cereals, and fruits. I am not saying you need to eliminate these foods altogether, especially fruit.

Fruits are packed full of essential nutrients, and I highly recommend eating them every day. To lower the glycemic index of fruits and all foods, for that matter, something you can do is to add some healthy fat, protein, or fiber. For example, instead of a banana, you could eat a banana and a handful of walnuts. Instead of white toast and jelly, you could go for 100% whole grain toast with avocado. See the difference?

Sugar During Endurance ExerciseSugar during Exercise

During endurance exercise, your body uses sugar as an immediate fuel source. The sugars you consume are converted into ATP (energy) to fuel your working muscles. In fact, carbohydrates (sugars) are the most efficient energy source for athletes, endurance or otherwise. When consumed during exercise, there is no excessive blood sugar spike. Therefore, most health concerns associated with eating sugars during exercise are eliminated.


Sorry, I didn’t mean to yell. Okay, now we know that consuming sugar during rest compared to during exercise are two very different things. So let’s dive into the most common questions relating to sugar and endurance performance.

FAQs Regarding Sugar and Endurance Training/Performance

Are highly processed sugary gels, and carbohydrate drinks bad for me?  During rest, yes.  During endurance exercise, no.

Will I become diabetic from eating a bunch of sugar during endurance exercise?  No. You will use it as an immediate fuel source during endurance exercise. Therefore there is no excessive blood sugar spike and no reason for your body to become insulin resistant.

Will high quantities of sugar during endurance exercise result in an energy “crash?”  No. The crash one experiences after consuming high-glycemic food at rest results from rebound hypoglycemia (low blood sugar).  This happens as a result of a quick blood sugar spike and then a rapid drop below baseline.  As mentioned previously, since there is no rapid spike in blood sugar when used during endurance exercise, there is also no rapid drop.

Can I just eat “real” food during a race?  Of course, you CAN. The question is – can your stomach handle it?  The reason gels, candy, and sports drinks are often preferred is because they tend to be easier on the stomach.   When you’re engaging in endurance exercise, blood is directed away from the stomach towards working muscles, significantly slowing digestion.  The simpler the food, the less digestion is required to convert carbohydrates (sugar) into ATP.

Highly fibrous foods like whole grains, beans, vegetables, etc. (while extremely healthy during rest) are likely to cause stomach issues during endurance exercise due to their high demand on the digestive system.

Why are most endurance recovery drinks full of sugar?  There are two instances when higher amounts of simple/added sugars are beneficial for endurance athletes – during exercise and immediately following exercise.   Following endurance exercise, your muscles are like sponges, ready to replenish glycogen and take in nutrients.  Glycogen replenishment is crucial to endurance athletes for various reasons (see more on that here).

Intake of simple sugars immediately after an endurance workout ensures that you replenish glycogen in the 30-minute window of opportunity.  During this window, the harmful health impacts of simple sugar consumption are negated due to sugar immediately being used for glycogen replenishment.

Won’t all that sugar hurt my teeth?  Now, this is something that you do want to be mindful of.  Although many of the negative health consequences of sugars are eliminated during endurance exercise, this isn’t one of them.  Excess can be hard on your dental health.  I always brush immediately after a workout in which I supplement with sugary beverages and/or snacks.

During long ultramarathons, my crew has a toothbrush and paste. Brushing is planned at specific aid stations ahead of time.




It’s really no wonder people are often so confused when it comes to sugar and endurance exercise.  What is healthy during rest is not ideal during endurance exercise. What is ideal during endurance exercise is not healthy to eat at rest! Here are a few examples to further illustrate this point;


REST – 100% whole grains are best due to their higher fiber content and superior nutrient profile.

DURING ENDURANCE EXERCISE – Refined grains such as white bread and white rice are better options due to less fiber content and superior digestibility during endurance exercise.


REST – Vegetables are king. Eat as many as possible.

DURING ENDURANCE EXERCISE – Vegetables are a terrible idea.  Don’t eat them during endurance exercise.


REST – Legumes/beans, nuts, and seeds are healthy options.

DURING ENDURANCE EXERCISE – Legumes/beans, nuts, and seeds are not recommended due to their high fiber, fat, and protein content. Remember, fat and protein will slow digestion as well.


REST – Stay away from candy, added sugar, and junk foods.

DURING ENDURANCE EXERCISE – Candy and junk food are often full of simple sugars, making them a suitable choice during endurance exercise.  Added sugar is a simple form of energy that your body can digest and convert to fuel!


REST – Fiber is super healthy!

DURING ENDURANCE EXERCISE – Fiber sucks. Ok, that’s a little drastic and oversimplified. But, seriously, stay away from high-fiber foods during endurance exercise.

As you can see, the difference between what is best at rest and what is best during endurance training couldn’t be any more extreme!


As you can see, the answer to whether or not sugar is a friend or foe when it comes to endurance athletes is entirely determined by the situation.  When you’re at rest, reach for “real” foods like vegetables, whole fruits, 100% whole grains, nuts, seeds, beans, healthy fats (like avocado and fish), and high-quality protein sources.

When you’re in the heat of a training session, it’s time to do a complete 360 and reach for simple sugars.  Go for carbohydrate drinks, gels, chews, and if “real” food is desired, go for refined carbs like white bread and white rice instead of their more fibrous whole grain counterparts.  Even candy and soda are often used with great success!

Seriously, the green light on candy and soda?  You’re welcome.  Just remember to take extra care of your teeth.  Fats and protein will slow digestion and should be excluded for the most part during the majority of endurance events.  However, protein is necessary to prevent the excessive muscular breakdown in exceptionally long events such as ultramarathons.

Additionally, during ultramarathons, “taste fatigue” is a legitimate issue. Eating food that sounds appealing is recommended when simple sugars are no longer appealing.  Eating something is better than nothing.  As events increase in duration and length, they decrease in intensity.  When intensity decreases, your ability to digest real food improves.

I would absolutely recommend a significant portion of calories to come from real food with a more varied micronutrient makeup in multi-day events.


I’m not going to go through the whole friends or enemies thing when it comes to artificial sweeteners.  I’ll get to the point – they’re enemies! They suck!  Don’t use them!

SO MANY people look at a nutrition label to make sure there is no sugar, yet don’t think twice about grabbing something containing artificial sweeteners. The three most popular artificial sweeteners you are likely to see are sucralose, saccharin, and aspartame. They seem to be in everything, especially in the supplement world. Ultraverse Supplements does not use artificial sweeteners in ANY products and never will.

Why?  Artificial sweeteners negatively impact your gut microbiota.  This could be an entire blog, but let’s keep it short and sweet (no pun intended). Here are just a few things that studies suggest a disruptive shift in your microbiome can do to your overall health and well being;

  • It can increase glucose intolerance(Suez et al., 2014). Really?  The thing that you are using to replace sugar can make you intolerant to…sugar?
  • It can make you more prone to obesity (Turnbaugh et al., 2006).
  • It can increase diabetes risk (Qin et al., 2012).
  • It can increase the risk of metabolic syndrome (Janssen & Kersten, 2015).
  • It can disrupt sleep (Leone et al., 2015).

A healthy gut is obviously vital. Of course, a LOT more than merely the choice to consume artificial sweeteners goes into gut health. The entire diet plays a part. But for me, I see no upside to artificial sweeteners. Go for healthier options such as stevia, monk fruit, and natural sweeteners.


Disclaimer – Use common sense, and always listen to your doctor over a blog post. They know more about your personal health situation than anybody behind a keyboard. As someone who has a Graduate degree in Clinical Nutrition, I realize the variance that certain medical conditions create when it comes to optimal nutrition and supplementation. If you have underlying medical conditions, always check with your doctor before starting a new supplementation routine.



Janssen, A. W. F., & Kersten, S. (2015). The role of the gut microbiota in metabolic health. FASEB Journal: Official Publication of the Federation of American Societies for Experimental Biology, 29(8), 3111–3123. https://doi.org/10.1096/fj.14-269514

Leone, V., Gibbons, S. M., Martinez, K., Hutchison, A. L., Huang, E. Y., Cham, C. M., Pierre, J. F., Heneghan, A. F., Nadimpalli, A., Hubert, N., Zale, E., Wang, Y., Huang, Y., Theriault, B., Dinner, A. R., Musch, M. W., Kudsk, K. A., Prendergast, B. J., Gilbert, J. A., & Chang, E. B. (2015). Effects of diurnal variation of gut microbes and high-fat feeding on host circadian clock function and metabolism. Cell Host & Microbe, 17(5), 681–689. https://doi.org/10.1016/j.chom.2015.03.006

Qin, J., Li, Y., Cai, Z., Li, S., Zhu, J., Zhang, F., Liang, S., Zhang, W., Guan, Y., Shen, D., Peng, Y., Zhang, D., Jie, Z., Wu, W., Qin, Y., Xue, W., Li, J., Han, L., Lu, D., … Wang, J. (2012). A metagenome-wide association study of gut microbiota in type 2 diabetes. Nature, 490(7418), 55–60. https://doi.org/10.1038/nature11450

Suez, J., Korem, T., Zeevi, D., Zilberman-Schapira, G., Thaiss, C. A., Maza, O., Israeli, D., Zmora, N., Gilad, S., Weinberger, A., Kuperman, Y., Harmelin, A., Kolodkin-Gal, I., Shapiro, H., Halpern, Z., Segal, E., & Elinav, E. (2014). Artificial sweeteners induce glucose intolerance by altering the gut microbiota. Nature, 514(7521), 181–186. https://doi.org/10.1038/nature13793

Turnbaugh, P. J., Ley, R. E., Mahowald, M. A., Magrini, V., Mardis, E. R., & Gordon, J. I. (2006). An obesity-associated gut microbiome with increased capacity for energy harvest. Nature, 444(7122), 1027–1031. https://doi.org/10.1038/nature05414

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