How Can Athletes Avoid Low Blood Sugar During Exercise?

woman running

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You’re cruising along, crushing the pace in a group bike ride or race.  Then, all of a sudden, you feel like you are struggling.  Suddenly, your great pace comes to a screeching halt and it becomes a herculean effort to maintain a crawling pace.  You realize you haven’t eaten for the past two hours.  Many athletes refer to this horrendous state as “hitting the wall”.

Too many athletes have a similar story to tell.  “Hitting the wall” is a state in which your fuel reserves are depleted and you cannot produce energy fast enough to maintain exercise intensity.  Unfortunately, most athletes are unaware that their fuel reserves are getting low until they have reached this rock bottom. 

What Is Blood Glucose In The Context Of Exercise? 

blood sugar for athletes

When we exercise, we predominately use energy from two sources: fat or glucose. Fat already stored in our body is the primary way we power low-intensity exercise (under approximately 60% VO2max). This means that walking and chatting with a friend or a long easy bike ride uses fat as fuel.  However, fat molecules take longer to dissociate and become available for us to use as energy. Thus, when we need energy quickly, like during a sprint or all-out effort, fat is not an option. In these high-intensity circumstances (when exercise exceeds 60% VO2max) we turn to blood glucose as our main fuel source.  The higher the intensity of our workout, the greater the proportion of energy that comes from glucose.

We all have enough fat stored in our bodies to power plenty of hours of low-intensity exercise.  However, our endogenous supply of glucose is limited. We store glucose in the liver and muscles in the form of glycogen. When our blood sugar gets low the body can turn this stored glycogen into glucose and release it into the bloodstream to power our exercise and maintain adequate levels of sugar in the blood. However, the body can only store approximately 10-15 kcal of energy per pound of body weight. This amount equates to approximately 90 minutes of high-intensity exercise, and that is the maximal amount we can store. For individuals who are under-fueled to begin with or have back-to-back workouts, our glycogen supply will last even less than 90 minutes. The many athletes that frequently perform high-intensity exercise may be wondering what happens when we deplete our stored glycogen. One of two things happens: we “hit the wall” or we consume exogenous fuel.

What Is “Hitting The Wall?” 

low blood sugar in athletes

The situation discussed in the introductory paragraphs is very common for athletes. While many athletes write it off as being fatigued or having extended their workout too long, “hitting the wall” is entirely preventable. Athletes do not need to succumb to this drop in performance when they fuel properly. “Hitting the wall” occurs when we exercise at a high intensity long enough to burn through the glucose in circulation, and our stored glucose (aka glycogen). At this point, we have no fuel left to continue to exercise at the highest intensity. Thus, our bodies automatically throttle back to a lower intensity, where we can burn fat as fuel. In this way, we can continue to exercise but we will not be able to go as fast or as hard unless we consume glucose while working out. We will dive into some good mid-workout snacks in a future article but a general guideline is to look for foods that are higher in glucose and low in fiber (they will be quickly converted into fuel). A few popular options are watermelon, dates, grapes, or cereal.

Should You Eat Carbohydrates During A Workout? 

carbohydrate intake guidelines for elite athletes

Most athletes become aware that they need fuel after their performance starts to decrease.  The volume and timing of carbohydrate intake are determined by the duration and intensity of exercise (remember, we only need to consume carbohydrates if we are performing sustained, high-intensity exercise).  Research has established general guidelines that can serve as a great place to start as we work to perfect our fueling strategies (check out our chart for a quick breakdown). Extra carbohydrates probably are not needed for high-intensity exercise lasting less than 45 minutes, such as repeated sprints, tennis, some track and field events, cross-country skiing, and beach volleyball. There is likely enough stored glycogen within the body to power exercise lasting less than 45 minutes. However, if you start to feel fatigued, research has shown that swirling a 6% carbohydrate solution for 5-10 seconds can help to boost central nervous system function.

As the exercise duration increases (45-75 min) as is the case for soccer, field hockey, and lacrosse athletes, extra fuel is still probably not necessary. However, if an athlete is starting their workout with low glycogen stores then they may want to consider a carbohydrate mouth rinse or consuming 30-60 grams per hour of liquid carbs at 15 min intervals. Athletes who fall into this category may include athletes who are fasting, on a ketogenic diet, or completed multiple training sessions in a day. 

carbohydrate intake for triathletes

For triathletes, open water swimmers, or marathon runners that exercise at a high intensity for 75-180 min, taking on carbohydrates is recommended. If these carbs come in the form of a liquid, consume 30-60 g per hour at 15 min intervals. Gels can be eaten every 20-40 minutes. Solid food takes longer to get broken down by the stomach and can be consumed on an as-needed basis.  

Athletes competing in high-intensity exercise that lasts longer than 180 minutes, like road cycling or marathon running, will need to consume carbohydrates in order to perform optimally.  Research suggests 90-110 grams of carbohydrates per hour in a 2:1 ratio of glucose: fructose. This ratio will maximize the amount of fuel entering the cell by utilizing all three transmembrane carriers (GLUT2, GLUT4, and GLUT5).

Every athlete is different so carefully tracking carbohydrate intake and performance while making adjustments over time will help to dial in when and how much we need to eat during a longer high-intensity workout. However, there is an easier and faster way to get acquainted with your fueling needs.

How Do You Know If You Are Low On Fuel? 

continuous glucose monitor for athletes

Imagine the power of a device that could give you real-time biofeedback on your fuel stores.  You would have the chance to try different foods, see how your body reacts, and make educated decisions as to how much and when to eat during races and rides!  In this way, you would be able to tell when your blood glucose is getting low (well before you hit the wall).  This tool exists.  Actually, it has existed for a long time, and it’s called a continuous glucose monitor (CGM).  Formerly, CGMs have been marketed towards diabetics for biofeedback on maintaining healthy blood sugar levels.  Recently, individuals have realized the potential for CGMs in a non-clinical population.  These devices have the potential to help individuals become more metabolically healthy.  In addition, they have the power to help athletes refine their fueling habits in order to promote optimal performance. 

Putting It All Together

Nutrition is an easy variable to change that can have a tremendous impact on sport performance. Being aware of blood sugar changes throughout exercise can help to make educated decisions that can help to power you to the finish line. Avoiding glucose spikes will also help to circumvent the subsequent blood sugar crashes that leave us without energy and unable to adequately fuel our exercise needs.

Author: Dr. Colleen Gulick