Optimize your exercise performance by tracking glucose

Exercise, glucose, and metabolic flexibility: how monitoring glucose levels with CGM can help you improve fueling, endurance, performance, and recovery.

Article highlights

  • Physical exercise can improve the body’s ability to maintain stable and healthy glucose levels.
  • Increased “metabolic flexibility,” or the body’s ability to shift between various fuel sources efficiently, is linked to positive health outcomes.
  • The timing and type of exercise can have an effect on how the body taps into energy sources, ideally shifting towards fat usage for energy over glucose.
  • Continuous glucose monitoring (CGM) gives you visibility into the relationship between your specific exercise routines and your glucose levels, allowing you to track the metabolic progress of your hard work.


Exercise Leads to Healthier Glucose Levels

Physical exercise–including walking, jogging, high-intensity interval training, and resistance training with weights—can change our cells’ ability to take up and utilize glucose, and in turn, increase our metabolic fitness.

The positive effects of exercise on metabolic fitness include an increase in glucose transporters traveling to the lining of cells (GLUT4 channels), allowing more glucose to enter and lowering circulating glucose—without additional insulin. Exercise may also improve the function of pancreatic beta cells (which produce insulin) and insulin activity, increase the amount of fat we burn between meals, and increase the number of mitochondria we have in our cells (where the cell burns fat and glucose to make energy).

A large body of research shows that the amount of exercise required to produce these benefits may be surprisingly low: Moderate aerobic activity for just 30 minutes at least 3 times per week over 8 weeks improves insulin resistance and glycemic control, including fasting glucose levels.

These adaptations are just part of the story, though. Tracking glucose levels can also help you optimize “metabolic flexibility,” your body’s ability to utilize different fuel sources. By understanding when and how these fuel sources are accessed, we can make choices that lead to greater endurance and better performance—in the gym and in everyday life.

Exercise and “Metabolic Flexibility”

Our bodies have two major sources of energy: glucose (sugar) and fat. There are advantages to each “fuel,” but ideally, we can easily switch between the two without major mental or physical disruption. The term “metabolic flexibility” refers to this efficient switching process.

Plenty of factors affect this capability, including the intensity and duration of exercise routines, our physical state, and what we eat; however, there is evidence that low carbohydrate diets may be best for promoting metabolic flexibility.

An important difference between glucose and fat as a primary fuel is the storage capacity of each in the body. A 155lb person with 10% body fat can store the equivalent of about 1,800 calories of glucose in the form of glycogen (enough to fuel about 90-120 minutes of continuous exercise) but has about 63,000 calories available in the form of fat.

You might be familiar with the term “bonking,” when an athlete suddenly depletes their glycogen stores and experiences severe fatigue and possibly even hypoglycemia, a sometimes-dangerous condition when glucose levels in the body fall too low. The mechanism is simple: The athlete has simply run out of the finite amount of glucose that their body can store.

Conventional wisdom once held that high-carbohydrate glucose-spiking diets were necessary to optimize exercise performance, and “training tables” full of pasta and bread were a regular feature for athletes. This made sense at the time because it was well known that depleting stored glucose would lead to fatigue, so the goal was to replenish glucose with high-carbohydrate meals.

As the science progressed, however, we learned that activating and optimizing fat-burning pathways could be a more successful strategy than loading up on carbohydrates. The ease with which our bodies can convert food to body fat means that calories stored as fat are a nearly unlimited source of energy for long-duration exertion. If we can shift our metabolic processes to efficiently use this fat instead of sugar, we may find that we can optimize athletic performance and endurance.

A diet low in carbohydrates can help promote this metabolic shift: Athletes who follow these diets adapt to burn fat (fatty acid oxidation) at significantly higher rates during prolonged exercise. This has important implications, particularly for long-duration fitness events, and the ability to oxidize fat has been correlated with performance in Ironman competitions (>8hr events).

In fact, physical training alone can shift the body toward higher rates of fat oxidation, a sign that it may be the preferred energy pathway when the human body is pushed for higher performance. A study of endurance athletes showed that even without a change in diet, their bodies adapted to favor usage of fat rather than glucose. These highly trained athletes showed a three-fold increase in fatty acid oxidation, perhaps explaining their improved capacity to perform high-intensity activities compared to recreational athletes.

In contrast, insulin-resistant obese individuals don’t utilize fatty acid oxidation pathways as efficiently as lean individuals. Indeed, emerging research suggests that glycemic control, insulin sensitivity, and metabolic flexibility are all important determinants of an athlete’s ability to efficiently utilize stored fat as energy.

Figure 1: During exercise, athletes on low-carb diets use significantly fewer carbohydrates and significantly more fat for energy than high-carb athletes. HC = high-carb diet group; LC = low-carb diet group.

Figure 2: During exercise at 55% peak power output, individuals in the low-carb/high-fat diet group (light grey line) burned more fat and fewer carbohydrates than the mixed macronutrient diet group (black line). Black line = mixed macronutrient diet; Light grey line = low-carb/high-fat diet.

When athletes consume a low-carbohydrate diet, keeping glucose (and insulin) levels low, they develop an enhanced cellular ability to utilize fats. The subsequent production of ketones and glucose precursors offers an abundant source of fuel. Fat-burning byproducts like beta-hydroxybutyrate have also been shown to increase gene expression of health-promoting antioxidants and reducing tissue-damaging reactive oxygen species in the body, potentially making exercise recovery speedier as well.

Use Of Fuel Type Shifts By Activity Type

Whether your body uses primarily glucose or fat is also affected by how intensely you’re exercising. In most people, the switch in energy use from glucose to fats happens when endurance exercise is moderate, or below about 60% VO2max. However, in high-intensity anaerobic activities, or those above 80% VO2max, glucose generally becomes the predominant fuel source.

Figure 3: Fatty acid oxidation as a fuel source tends to decrease as exertion increases beyond a threshold.

Interestingly, circulating glucose tends to rise during brief bouts of intense exercise (>80% VO2max) and even more so for the hour after exercise ends. This is thought to be because high-intensity exercise stimulates the secretion of specific hormones (catecholamines, including “the stress hormone” epinephrine), which stimulate up to 8-fold increased glucose production in the liver. The muscles, however, only increase their use of glucose by about 3-4 fold, and the result is a supply/demand mismatch where there is excess glucose in the blood. This elevated glucose signals a rise in insulin, at which point muscles take up the excess glucose to replenish stored glucose (glycogen), and we see a concurrent fall in glucose back to baseline values.

If you’re tracking your glucose using CGM, you might be worried by this apparent glucose rise during high-intensity exercise. You shouldn’t be. Despite the acute rise in glucose, high-intensity training actually improves both fasting glucose and insulin sensitivity in as little as two weeks. Both of these adaptations lead to better metabolic flexibility and glucose control.

Two Ways That Glucose Awareness Makes Exercise Even Better

  1. By optimizing the timing and type of your exercise routine:

We know that post-meal glucose spikes are a risk factor for heart disease, stroke, and vascular damage. Glucose oscillations also cause “oxidative stress,” the process through which free radicals in the body cause tissue damage. Tracking glucose gives you the tools to reduce these spikes by helping you determine the best type and timing of exercise to support stable glucose levels.

You might be a committed daily runner, which clearly benefits your health and fitness. But after tracking your post-meal glucose levels, you might find that you’re not getting the results you’d like. Consider the timing and duration of those runs.

A study compared 3 exercise timing regimens (20 minutes of jogging before each meal, versus 20 minutes of jogging after each meal, versus short bursts of jogging for 3 minutes repeated 20 times a day), with all regimens adding up to 60 minutes of activity per day. The study found that the scenario with 20 short bursts of jogging throughout the day was significantly more effective in reducing post-meal glucose spikes.

It’s not just jogging: The superior effect of short, frequent bouts of activity on metabolic control has been shown in other studies as well. Another study looked at walking for a discrete 30-minute period once per day versus walking for just 1 minute 40 seconds every 30 minutes during waking hours. While both groups walked a grand total of 30 minutes, the study showed that the frequent short walks were significantly more effective at reducing post-meal glucose peaks and insulin levels.

While you likely don’t have a schedule that allows for 20 jogs every day, armed with this information and a means of easily measuring your glucose, you might shift your schedule to get the most benefit in the time you have. In particular, you might try to fit activity throughout your day instead of in one big chunk.

Additionally, exercising in a fasted state can also promote metabolic flexibility and fat-burning capacity. A study from 2019 in overweight and obese men showed that exercising before eating breakfast leads to increased use of fat for energy during the workout, reduces post-meal insulin elevation, and increases insulin sensitivity over 6 weeks.

  1. Increasing how often you exercise

Research suggests that real-time individualized feedback on glucose levels can inspire people to exercise more. A study in a diabetic population has shown that wearing a CGM as part of an individualized counseling program reduces average blood glucose and weight levels significantly, perhaps due to increased focus and motivation brought on by seeing the improvements as they happen. Additionally, CGM is associated with individuals upping the intensity level of their exercise, from “sedentary” and “light activity” to more “moderate intensity” and can lead to significant increases in total exercise time per week.”


The relationship between glucose and exercise is complex, but research shows that there are steps we can take to optimize the impact of activity on our metabolic health. Even if you’re not an elite athlete, these choices may help you burn more fat, increase endurance, lower post-meal glucose levels, and even inspire more intense and frequent workouts.

A metabolic fitness tool like Levels can unlock the power of biometric data to give you insight into how specific behaviors, foods, and routines affect your glucose levels, giving you the biofeedback to understand and optimize your exercise routine and benefit more from your hard work.