A month of glucose tracking revealed surprising fueling mistakes and timing tweaks that could help runners perform better.
Optimal fueling is vital for performance, especially if you’re tackling longer distances like a half marathon, marathon, or even an ultramarathon. But while there is now a plethora of energy gels, drinks, and chews on the market, it can be hard to figure out exactly when and how much fuel you need to take.
That’s on top of knowing what to eat before and after a training session, too.“Glucose is the currency our body uses for carbohydrates,” says David Lipman, a health and human performance specialist. “It’s stored in the muscles and the liver as glycogen, and then we mobilize it to use it as glucose.”During moderate to intense activity, glucose is the body’s preferred fuel—and there’s plenty of evidence to show that taking in carbs during exercise improves endurance.Now, I’ve run a lot of marathons, and fueling is still one of the things I find difficult to nail—particularly as someone whose early 20s were dominated by diet culture, which advocated for eating less, rather than more.So, in an attempt to take the guesswork out of training, I wore a continuous glucose monitor for a month, a device that’s applied to the back of your forearm, tracking your glucose levels in real time. The idea is that you can see how the food you eat affects your training and devise an optimal fueling strategy to maximize your performance.Just to be clear, I wore a glucose monitor specifically designed for sports, not intended to be used by diabetics. While it uses the same technology, it has a range of 55-200mg/dL , whereas the range of diabetic CGMs goes beyond.Most glucose monitors last for two weeks before you’ll need to apply a new one. During this time, you’re encouraged to log “events,” including meals, workouts, and sleep, to establish your personal glucose performance zone , the range where you feel and perform at your best, with the aim to spend as much time as possible in this range when you’re training or competing.1. Simple Carbohydrates Have Their PlaceOne of the first things I noticed wasn’t that my glucose was rapidly spiking and falling after eating—quite the opposite. Working in health and fitness for several years, I’ve been taught to base most of my meals around slow-release carbohydrates and a good protein source. This helps to keep blood sugar levels steady, which is important for managing inflammation and promoting faster recovery.From a training perspective, however, it’s been a disadvantage because my glucose hasn’t been rising to the levels I need to perform at my best. I’ve had to learn to separate “fueling for general health” from “fueling for endurance sport,” eating larger quantities of carbohydrates—including more simple ones—ahead of a run to increase my glucose levels.I’ve also been leaving more time between eating and training , as I found that my usual window of 1-1.5 hours before a run didn’t give my glucose levels enough time to rise.Lipman stresses that “context is key” when it comes to understanding glucose surges and falls and that—as I’ve discovered—while they’re not so beneficial from a recovery perspective, they can be really important when it comes to training.2. Every Session Counts—Not Just the Weekend Long RunAfter each workout, the monitor encourages you to log your rate of perceived exertion—how hard you found the workout—and your energy levels out of 10. This self-reporting is to help you become more aware of how you feel during and after a training session.I began to notice that I was fueling my weekend long runs much better than my mid-week training sessions, often feeling strong and energized after these longer miles and weaker during the rest. One reason is that I was doing a lot of my mid-week sessions in a rush before work—often fasted.After seeing how low my glucose was during these sessions, I started introducing a small snack or gel mid-run for a boost of energy. Surprise, surprise, I was less exhausted afterward. I’ve also been doing my Wednesday speed sessions in the evening, instead of the morning, to give myself more time to fuel—I’ve found that I don’t start to fade until the final couple of intervals, and the session feels stronger overall.3. Timing Is EverythingMonitoring my glucose levels during a race was particularly game-changing. I signed up for a half marathon and followed my usual fueling strategy: breakfast two hours before, a caffeinated gel on the start line, then a gel every 2,5 miles. Afterward, my glucose chart revealed that I’d stayed within my GPZ—but I’d experienced a slight dip within the first 15 minutes of the race.On my next long run, I decided to experiment by moving my start line gel forward by 20 minutes. This seemed to even out the initial dip. This is called “rebound hypoglycaemia,” Lipman says: you eat, and your glucose rapidly goes up, but then a combination of exercise and your body’s insulin response brings it back down. Some individuals are more prone to this than others, Lipman adds—one case for tracking your glucose to spot if it’s happening to you. The Bottom LineA month of wearing a continuous glucose monitor has definitely taught me a lot about how I’ve been fueling—or rather, under-fueling—some of my training sessions. But while I’ve been able to introduce some changes, many of them are low-hanging fruit. If you want to figure out how environmental factors and other subtle influences impact your glucose, you’ll need more than 30 days. And at that stage, you’ll really be getting into that “1 percent improvement” category.Not to mention the major caveat: tracking your glucose doesn’t come cheap. Most biosensors will set you back over $100 for a month. Sure, they usually come with plenty of integrations with third-party wearables and apps , and the minute-by-minute glucose streaming is seriously impressive, but you really need to use them correctly to get the value.I’d encourage all runners to experiment with their fueling strategy—before, during, and after training sessions and races. Then, if you still want to get into the nitty-gritty , go ahead and tech yourself out.This story was originally published in Runner’s World UK.
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