Blood Sugar and Running
Runners are a peculiar group. We can tackle a 10-mile run without hesitation, yet we groan our way up a flight of stairs or complain about having to park “so far away.” Many of us dedicate so much energy to our runs that we feel drained for the rest of the day—but it doesn’t have to be this way!
This topic is admittedly a bit selfish. I want to learn how to make my weekends more enjoyable, where the long run isn’t the main event that leaves me stuck in bed recovering for hours. Research from the Mayo Clinic highlights that primary exercise headaches—those not caused by underlying health conditions—often stem from poor hydration, inadequate fueling, intense exertion (leading to rebound blood vessel dilation during recovery), or prolonged muscle tension.
While sleep undeniably plays a crucial role, I want to focus on how hydration and fueling around a run influence how you feel afterward. Optimizing these aspects can transform the post-run period into a productive, energized time rather than one spent recovering on the couch. This information is valuable for everyone: neglecting these strategies not only impairs running performance but also limits your ability to tackle daily life—making it harder to achieve goals, participate in activities, or simply feel present and engaged.
Consider companies that provide employees with a fully stocked kitchen. It feels like a generous perk, but the real genius lies in the productivity boost it offers. With the right items and proper use, such kitchens can enhance employee efficiency—and, presumably, company profits as well.
Blood sugar is another trendy topic. Depending on the context, discussions about it might revolve around diabetes, weight loss, mental alertness, or athletic performance. Context matters! For instance, it would be unwise for a marathon runner to avoid carbs simply because an algorithm suggested it would stabilize blood glucose. My focus is on blood sugar’s role in mental alertness, specifically within an athletic population. I’ll also touch on related factors like electrolytes and caffeine, keeping the primary emphasis on the importance of blood sugar.
SUGAR
This past October, I had the chance to wear a Lingo glucose biosensor for a few weeks. I was excited to analyze my own data, identify trends, and make meaningful changes to improve both my running performance and daily life. Unfortunately, I went through two sensors in a short time before they fell off, leaving me without enough valid data for analysis. Despite this setback, I remain intrigued by the potential of continuous glucose monitors (CGMs) and the growing number of companies offering personalized analysis and lifestyle recommendations.
However, blood sugar management is not one-size-fits-all. For instance, athletes preparing for an event might need higher blood sugar levels to fuel their efforts, making generic algorithm-based recommendations less ideal. Blood sugar goals should be individualized, tailored to your activity level, metabolic needs, and performance objectives.s ideal. Blood sugar goals should be individualized, tailored to your activity level, metabolic needs, and performance objectives.
For fueling recommendations, I suggest exploring Featherstone Nutrition. She offers a wealth of free resources specifically designed for runners, including guides on carbohydrate loading and fueling strategies before, during, and after runs.
Before Running
One area I was eager to explore with the CGM was whether I experience reactive hypoglycemia in the mornings. This occurs when blood sugar rises after eating but crashes about an hour later. My typical morning routine includes coffee and a muffin, followed by a run an hour later. However, I sometimes feel lightheaded at the start of my run, despite thinking I’ve fueled appropriately. This dizziness can make my run feel weak and uncoordinated. If I don’t consume a quick sugar source—like juice or energy chews—before heading out, I often feel drained and less in tune with my hunger cues for the rest of the day.
One tip for managing reactive hypoglycemia: If you forget a quick sugar source before your run, doing a few surges (short bursts of faster running) about a mile in can help. Surges stimulate epinephrine release, which raises blood sugar and can temporarily alleviate dizziness.
Research supports the connection between stress hormones and blood sugar fluctuations. A recent (yet to be peer-reviewed) study examined stress-induced hyperglycemia during high-level athletic competition. The researchers observed sharp blood sugar increases before and during the race (due to excitement, anxiety, and physiological demands), followed by a drop after the event. Interestingly, blood sugar levels remained elevated for three days post-race compared to the three days prior, suggesting that both perceived stress and physiological strain drive these changes. I’ll be talking more about this in an upcoming post on stress!
I leverage this information by purposefully raising my blood sugar during times of low energy, even when I’ve consumed enough calories through surges. Contrary to popular belief, running often raises blood sugar due to the hormonal response and energy demands of exercise. This elevation can persist for some time after running, which may surprise those who assume running always lowers blood sugar.
During Running
A study by Inamura et al. (2024) found that glucose fluctuations during a 100-mile ultramarathon negatively correlated with performance (rho = −0.612, p = 0.012). Faster runners maintained higher and more stable glucose levels, particularly early in the race, by consuming carbohydrates strategically. In contrast, runners with significant glucose fluctuations performed worse, emphasizing the importance of stable blood sugar management for endurance events.
Another study by Flockhart and Larsen (2024) highlights that carbohydrate supplementation during exercise protects blood sugar stability. The availability of carbohydrates is a key predictor of hypoglycemia severity post-exercise. Low blood sugar during or after a run is associated with fatigue, brain fog, and reduced performance.
Interestingly, even a sugary mouth rinse—not consuming the sugar, just tasting it—can temporarily increase alertness and performance. The hypothesis is that the body anticipates incoming energy, "allowing" for increased effort. However, this effect is short-lived, as actual calories are required to sustain glucose levels. Consistent fueling, through regular snacks and meals, is critical to maintaining energy balance and preventing hormonal disruptions caused by prolonged energy deficits, even if your overall intake balances out by the end of the day.
Takeaway: Fueling during your run stabilizes blood sugar, improves performance, and helps you feel physically and mentally better afterward!
After Running
Failing to refuel after running can lead to a post-exercise blood sugar dip, as your body continues to draw on glucose to meet recovery demands. This can result in brain fog, low energy, and a missed opportunity to optimize recovery.
Personally, I often experience reduced appetite after a hard workout or long run. This is normal for physiological reasons, such as the rise in lactate, which may cause nausea, and the decreased blood flow to the GI system during intense exercise. Despite this, the post-run window is critical for recovery.
Quick, digestible carbs paired with protein can help replenish glycogen stores and support muscle repair, even if eating feels unappealing. Prioritizing recovery nutrition not only restores energy but also sets the stage for improved performance in future training sessions.
Most days, you’ll find me chowing down within 15 minutes of finishing a run on a giant smoothie bowl made with bananas and protein powder, topped with granola—a balanced mix of hydration, carbs, and protein. However, on especially hard or long running days, I sometimes find myself entering a state of brain fog an hour later.
For me, agitation from thirst is usually my first clue that I need water and pretzels (salt) to raise my blood sugar, increase blood volume, and restore homeostasis. These hard and long-run days often remind me how easy it is to underestimate energy demands. Ultimately, you need calories—real energy—to sustain glucose levels.
Balanced energy intake with expenditure is the best way to maintain energy levels. Consistent snacks throughout the day are crucial. Even long gaps where energy balance is net negative can lead to hormonal disruptions and fatigue, even if your total energy intake and expenditure balance out by the end of the day.
The Power of Proper Fueling: Energize Your Runs, Optimize Your Day
Hard workouts and long runs don't have to leave you wiped out. With the right nutrition, hydration, and recovery practices, you can feel energized and ready to tackle the rest of your day.
Stable blood glucose is key: Managing blood sugar levels before, during, and after your runs is essential to avoid brain fog, fatigue, and poor recovery.
Proactive fueling prevents crashes: Proper fueling strategies can prevent post-run energy dips, ensuring you feel strong and clear-headed throughout the day.
Listen to your body: Symptoms like lightheadedness or brain fog often indicate low blood sugar or dehydration—addressing these promptly with carbs, fluids, and electrolytes can quickly restore balance.
Consistent energy intake matters: Even short-term energy deficits can disrupt hormones and leave you feeling drained. Regular snacks and meals help maintain energy balance.
References:
Flockhart, M., Larsen, F.J. Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health. Sports Med 54, 247–255 (2024). https://doi.org/10.1007/s40279-023-01910-4
Inamura, N., Taniguchi, H., Yoshida, S. et al. A comparative observational study of carbohydrate intake and continuous blood glucose levels in relation to performance in ultramarathon. Sci Rep 14, 1089 (2024). https://doi.org/10.1038/s41598-023-51048-6
