How continuous glucose monitoring (CGM) impacts athletic performance and the work of a sports dietitian


Article highlights

  • Food is the fuel our bodies run on. Knowing the difference between “regular fuel” vs. “premium fuel” helps understand which one is better for performance
  • My experience using a continuous glucose monitor (CGM) for 2 months brought me to the conclusion that CGM can offer insights for both athletes and Dietitians to help understand how food affects performance and energy
  • CGM shows real-time results to athletes so they can make the necessary adjustments to optimize their performance

CGM through the eyes of a Dietitian

Before using a continuous glucose monitor (CGM), I wondered how someone like myself who does not have diabetes would benefit. I have a healthy pancreas, and my diet mainly consists of high-fiber carbohydrates, fruits and vegetables, lean proteins, and healthy fats. So how would it benefit me?

Using CGM helped me figure out:

  • How to optimize my performance
  • How to optimize my energy
  • How to optimize my recovery

As a sports Dietitian who’s worked for the University of Florida and then a major league baseball team, I often speak about carbohydrates and performance. It’s one of the first lessons I cover when doing 1:1 virtual coaching sessions with my clients.

“Using a CGM is almost like having a gas gauge on your body to see how you’re utilizing fuel for your performance.”

When I talk to my athletes about carbs, I ask, “Is there a button I can press on your body to reveal a gas gauge that measures how fueled you are?”

My virtual clients say, “No.”

However, using a CGM is almost like having a gas gauge on your body to see how you’re utilizing fuel for your performance.

Different types of fuel for performance

Imagine two types of cars: a high-end sports car like a Lamborghini or a Ferrari and a lower-end car like a Toyota Corolla or a Honda Civic. The Lamborghini and Ferrari take premium gas while the Corolla and Civic take regular gas.

If you were to put regular gas in one of the high-end cars, it wouldn’t run as well. But if you were to put premium gas in one of the low-end cars, it would run better.

How does this compare to athletes?

Most of the athletes I work with are Ferraris, who want to gain that competitive edge. Even if you’re an everyday athlete trying to optimize your performance, knowing what fuel to have will get you to the next level in your training.

That’s where nutrition comes in, specifically carbs.

I think of carbs as premium and regular. When recognizing the difference between the two, remember that premium carbs are high in fiber, low in sugar, while regular carbs are high in sugar, low in fiber. Premium carbs are slower to be digested, less likely to cause a big elevation in blood glucose levels and have health-supporting fiber. Most of the time, this is the better type of fuel to consume because large repeated large glucose spikes are not generally healthy for the body and can zap energy and lead to inflammation, oxidative stress, and metabolic disease over the long run.

Upon reading a nutrition label, my clients learn to take the total amount of carbohydrates and divide it by the total amount of fiber. Anything with a 5:1 or less ratio is premium fuel, anything between 5:1 and 10:1 is like the middle-grade gas, and anything 10:1 and above is regular fuel.

Athletes doing marathons or even interval training are often advised to have regular fuel. This is thought to give them a competitive edge and boost their performance due to the fast-acting fuel. You typically want regular fuel when doing high-intensity exercise because that’s when your body needs the most fast-acting carbohydrates to refuel your muscles and give them energy for optimal performance. All other times you want premium fuel. (Note: Some athletes follow low-carb training regimens to become better adapted to using fat for fuel during workouts, a strategy sometimes called “carb cycling” to generate metabolic flexibility).

Why athletes need to be conscious of what they eat

Athletes train multiple times a day. They’re getting very little recovery time and typically eat a lot of processed and individually packaged foods. They need to recognize whether or not those foods are hurting them or helping them optimize their individual performance.

Imagine finishing a race and noticing your energy go down. If you could monitor your glucose with a CGM device coupled with the Levels app, you’d know when to fuel up before your blood sugar drops so you can continue optimizing your performance.

Real-time results using CGM to optimize performance

By using Levels, I was able to test my theory on carbohydrates to see if high-fiber foods/premium fuel actually helped me perform better. Not only was I able to confirm what I teach, but I also came to understand that most of the foods I eat help me stay fueled.

For example, I found that rice and beans work great for me because I didn’t hit a post-meal spike, which means having my favorite foods actually optimized my performance. We do not want a post-meal spike because high post-meal spikes are linked to worsened glucose control over time, development of obesity, diabetes, cardiovascular disease, and impaired exercise and cognitive performance. When I ate these foods that did not cause spikes, I found that I felt more focus throughout the day and no slumps in the afternoon.

Unsweetened dried mango, however, sent my blood sugar through the roof. Knowing that, I’d have the dried mango and a protein shake post-workout to maximize my recovery. Maximizing my recovery meant ingesting rapidly absorbed carbohydrates to refuel my muscle stores and protein to repair the muscles. If I were looking for a quick boost, I’d grab some fruit. If I was trying to maintain my energy throughout the day, I made sure to have a high-fiber snack like granola with Greek yogurt with berries.

Once during my first month of the Levels experience did I feel my energy crash. After having a dried mango (regular fuel), I saw my glucose go up in real-time, only to come crashing down and resulting in a post-snack energy slump. This helped me be more aware of how “regular fuel” can sometimes lead to poor performance.

I also noticed how the CGM picked up on the 4 Rs of recovery (the recovery strategy I teach clients):

  • Refuel your body with fast-acting carbs. I typically suggest granola, fruit, and toast.
  • Repair with protein since you were just ripping and tearing your muscles.
  • Rehydrate with some sort of liquid from the sweat you lost.
  • Rest to ensure your muscles recover properly for your next workout.

Levels showed me that the amount of carbs and protein I had post-workout was what I needed for my recovery. Knowing your own individualized recovery strategy ensures you’re getting what your body needs, and Levels can help accomplish that.

Not only did I have a better workout the next day, but in the moment, my body reacted best because those carbs and protein stabilized my blood sugar after it had spiked and then dropped from doing HIIT.


I can tell you from first-hand experience that CGM is something I would love to use with more athletes. It’s the first product I’ve seen that can address if performance and energy really are affected by what you eat or if there’s another underlying issue. As a dietitian, this can enhance the specificity of the advice I can give to athletes and allows athletes to have more control and clarity over their fueling and recovery strategies.


Author Bio

Tony Castillo is a Registered Dietitian and Nutrition coach who helps recreational athletes lean out and enhance performance without diets, endless supplements, or overhauling their whole life.

Tony graduated from Florida International University in Miami with a Bachelor’s in Biology, Bachelor’s in Chemistry, and a Master’s in Nutrition and Dietetics.

Tony worked at the University of Florida at the collegiate level and then the Toronto Blue Jays with elite professional baseball athletes, and does virtual private practice on helping recreational athletes become elite performers. Tony teaches his virtual clients how to manage their weight and increase performance through behavior changes without counting calories or macros.

Having both parents from the Dominican Republic, Antonio is fluent in both English and Spanish. His journey began in middle school when he was overweight. This continued throughout high school until he decided to jump on a “diet,” which resulted in unsustained weight loss.

Throughout college, his weight fluctuated due to a lack of nutrition knowledge. This led him to see that his true passion was to understand how nutrition played a role in the human body. Today, his passion is to teach others how healthy lifestyle modifications optimize performance in ALL areas of life through nutrition.

For more information on Tony:

Email: [email protected]