Should Runners Use a Continuous Glucose Monitor?
Runners love data. Pace, heart rate, power, and now glucose. Continuous glucose monitors (CGMs), built for diabetes management, have crossed into endurance sport. When I read about CGMs two years ago in Dr. Peter Attia’s excellent “Outlive: The Science & Art of Longevity,” the devices were available only by prescription for people with diabetes. But recently, they became available over the counter for non-diabetics, so I created an account at Levels, ordered CGMs, and underwent a battery of tests from a blood draw.
Written by Scott Benbow
Edited by Pavlína Marek
(Editor’s note: Levels is not affiliated with the San Francisco Marathon. Scott Benbow isn’t sponsored by any products in this article and all opinions are his own.)
Dr. Attia identifies two phases to CGM use. He calls the first the “insight phase”, where you learn how different foods, exercise, sleep, and stress impact your glucose readings in real time. In the second phase, which he calls the “behavior phase,” you apply what you learned in the insight phase to modify your behavior around certain foods. However, with his patients, Attia also found that CGMs activate the Hawthorne effect, the phenomenon whereby people modify their behavior when they are being studied.
Low & Stable Wins the Race
Attia posits that keeping your glucose levels “low and stable” is healthier than spiking them repeatedly and for long periods of time.
The Levels mobile phone app lets you maintain a log of the food you consume and to see when glucose spikes occur and how long they last. Succinctly described by Dr. Casey Means, keeping your blood sugar over the course of a lifetime in a low and healthy range is probably the best thing you can do for health and longevity.
While the article is not intended to be a deep dive into the science, the result of low and stable glucose levels is better insulin sensitivity. Repeated and lengthy glucose spikes, on the other hand, can lead to unhealthy insulin resistance and metabolic syndrome.
You’ve probably seen CGMs in the wild. I have been noticing them on the upper arms of runners for the past year in marathons and half marathons, and debates about competitive fairness rage on. The device consists of a microscopic filament that is implanted on the back of the arm. It transmits data about glucose levels to the user’s mobile phone.
Used wisely, CGMs can help you personalize fueling; used blindly, they can send you on a wild goose chase after laggy numbers.
What a CGM Actually Measures (And Why It Can Lag)
CGMs read glucose in interstitial fluid, not directly in blood. During steady aerobic work, interstitial readings often trail blood glucose by several minutes, with accuracy getting worse when glucose is changing quickly (brought on by surges, bonks, or big hits of sugar). CGM values are trends, not absolutes.
Attia warns that it’s important to remember the limitations of a CGM. He points out that a person could arrive at low and stable CGM results by eating bacon three meals a day seven days a week. While I am not a nutritionist, I can recognize that such a diet, while delicious, would not be very healthy.
What did I discover?
After two weeks of continuous glucose monitoring, the results have been remarkable. My typical breakfast, 150 grams of almonds and a bowl of low-glycemic steel-cut oats, hardly moves the needle, despite the carbohydrates in the oatmeal. A sashimi bento box with no rice and a serving of wakame (seaweed salad) spiked my blood glucose by more than 100 points, which was surprising until I discovered the restaurant added sugar to the seaweed. Attia and others assert that results will vary among individuals. Your glucose response to sugary seaweed might not be as problematic as mine was.
Like Attia’s patients, using a CGM to assess my glucose levels is beginning to have a positive impact on my eating behavior. In a future article, I will describe the changes I am making to my diet and exercise as a result of using a CGM.
About Scott Benbow
Scott Benbow is a San Francisco Marathon Ambassador, attorney, nonprofit specialist, and passionate SFM runner. He lives in San Francisco and runs the hills of our incredibly beautiful city with us every year.