A performance dietitian’s perspective on CGM: a critical tool for dialing in sleep, exercise, nutrition, and energy


Article highlights

  • During my Levels metabolic awareness experience, I discovered that my blood sugar is affected by different factors including food, exercise, and sleep.
  • Continuous glucose monitoring can help you personalize your nutrition to ensure you’re getting the proper fuel and maintaining stable energy.

I played a lot of sports growing up, ranging from swimming to soccer to football to weightlifting. One of the things I was told was that I needed to do low-carb diets in order to lean out and get that body-builder physique.

I applied this approach throughout most of my life until I got to college, sticking with foods like protein shakes, protein bars, and cauliflower rice. Little did I know that those low-carb diets weren’t fueling my body in an optimal way. I actually ended up breaking my foot from lack of carbs, lack of sleep, and overuse of supplements. I would estimate that I was getting about 50-80g of carbohydrates per day at my lowest. I didn’t understand how that was all affecting my body.

Eventually, I learned that carbs are my body’s preferred source of energy. I spent years eliminating them from my diet when they should’ve been there all along. While everybody processes energy differently, for my body, I know that carbohydrates are an efficient and effective source of fuel.

“Levels provided me with real-time data on what different foods did to my blood sugar. Knowing this information is beneficial so you can prepare yourself for the day.”

Between my education and professional experience, I’ve learned a lot about nutrition. Using Levels allowed me to actually apply what  I’d learned in school about food and what it does to your body, and see instant feedback on it.

Working with elite athletes at the University of Florida, I learned about the fuel gauge analogy: the idea of having a button on your body that will reveal a gas gauge when pressed to measure how fueled you are. Levels can be that gauge.

Using Levels to Understand My Blood Sugar

My experience using Levels revealed three major surprises:

  • How blood sugar affects my energy
  • How certain foods affected my blood sugar
  • How sleep affected my blood sugar

How Blood Sugar Affects My Energy

One of my favorite and most eye-opening moments I had while using Levels was with my energy levels.

I teach the athletes I work with about premium carbs and regular carbs because they play different roles when it comes to energy. Premium carbs are high in fiber, lower in sugar. They’re slower to be digested and less likely to cause large spikes in your glucose levels. Regular fuel, which is low in fiber and high in sugar, is best had when doing high-intensity exercises so your body can use those fast-acting carbs to fuel your muscles.

There was one afternoon during my second week using Levels when I noticed a change in my energy. It felt as though I’d hit a wall. Instead of working, I just wanted to lay in bed and nap.

When I swiped my CGM, my blood sugar was low. I realized I had hit a post-meal spike from my lunch (homemade pad thai and a Fiber One brownie), and then plummeted. Since the brownie was loaded with fiber (which is generally thought to help with glucose control), it was surprising to see my body react the way it did.

Once I made this discovery, I ate a piece of dried mango and my energy went up. While this helped in the short term, I came to realize that if I had avoided the big spike and subsequent dip (often called reactive hypoglycemia — the body’s exaggerated response to a big glucose load), I could have avoided the energy low. It was totally clear to me that big glucose swings affect my energy, and that pad thai and a brownie might be too many simple carbs for me in one sitting.

How Food Affected My Blood Sugar

One day after having rice and beans, my Levels report revealed this meal did very little to my blood sugar. My levels actually maintained a fairly steady level, most likely because the beans were full of fiber and counteracted the fast-acting carbs in the rice. In contrast, eating a banana on its own made my blood sugar spike then crash down.

Levels provided me with real-time data on what different foods did to my blood sugar. Knowing this information is beneficial so you can prepare yourself for the day. Since the banana caused a crash, I know not to have one before going into a consultation with a client. For athletes, they’ll be able to know what they should and shouldn’t eat before doing any sort of physical activity.

A feature of Levels that I liked in particular was their Zone Scores, which help identify how different factors such as food, sleep, and exercise, affect your metabolism and glucose response. Learning and understanding these effects can help you determine how to combine different foods, as well as foods and different activities, to minimize glucose swings.

While using Levels, I found that homemade protein pancakes with almond flour generated a perfect zone score, meaning very little glycemic response A meal that generated my worst Zone Score  a combination of corn, asparagus, and salmon burgers.

How Sleep Affected My Blood Sugar

Sleep can affect insulin sensitivity, carb metabolism, and even protein synthesis. After using Levels, I can confirm that this is true.

Levels released colored lines to indicate whether your blood sugar is in an optimal range or not. A blue line means low and a red line means high, so you want to stay green. When I changed my sleep schedule for a couple of days, these lines went from steady all day to peaks and valleys. Through this experiment, I learned that lack of sleep hindered my performance. Hindered performance led to higher post-meal spikes and carb intake, which causes more oxidative stress and inflammation.


Through my own experience, I was able to monitor how different foods affect my energy and how sleep impacts the ways in which my body uses food as fuel. It helped me optimize and personalize my performance through nutrition, and I can take these new learnings and apply them to the work I do with clients to make a bigger impact on their lives.

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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 degree 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, Tony’s 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]