Reactive hypoglycemia and how to avoid it
If you’ve ever experienced a pre-workout “bonk,” where you feel dizzy, shaky, and energy depleted within the first 30 to 60 minutes of your run despite properly fueling, you might be experiencing reactive hypoglycemia. Here we look at what it is and how to avoid it.
Some runners experience a sudden onset of fatigue, dizziness, and shakiness within the first 30–60 minutes of training. While it might feel like early glycogen depletion (or “bonking”), for athletes who are adequately fueled, the cause may be reactive (or rebound) hypoglycemia.
What is reactive hypoglycemia?
Reactive hypoglycemia (also called rebound hypoglycemia) is a short-term drop in blood glucose levels that can occur soon after starting exercise (within 30 minutes but up to 60). This often happens when a high-carbohydrate meal is eaten 30–90 minutes before running. For most people, this dip is temporary, and blood sugar levels recover as the body adjusts.
Why does this happen?
After eating a high-carbohydrate meal, your blood sugar (glucose) begins to rise. Your body releases insulin to help move glucose into your cells where it becomes available for use as energy. But when you start exercising, your muscles start using glucose, causing blood sugar levels to drop further. The result: your blood sugar drops quicker than your body can stabilize it.
Estimates of how many athletes experience reactive hypoglycemia vary, with studies suggesting anywhere from 8% to 30%. The science is still unclear about why it happens to some athletes over others (there are conflicting studies on causes). But one common denominator is the timing of pre-exercise carbohydrate consumption.
For most runners, it’s more annoying than dangerous, but it’s always wise to consult your doctor. And while it’s not the same as “bonking”, the symptoms can still be uncomfortable and frustrating. Thankfully, there are two very easy things you can do to reduce the likelihood of reoccurrence.
How to avoid reactive hypoglycemia
Reactive hypoglycemia is consistently linked to the timing of pre-exercise carbohydrate consumption. Changing when and what you eat before you run can reduce the likelihood of an occurrence. Here’s what you can do to avoid early onset fatigue:
Ensure the meal you eat 2-3 hours before your run contains a mix of fats, proteins, and carbs. Fats and protein will help slow carbohydrate absorption.
Avoid eating carbohydrates 60-75 minutes before your run.
Eat your pre-run snack as close to the start of your run as possible (within 15 minutes of starting, but ideally immediately before).
If you frequently experience reactive hypoglycemia, it can be good practice to take in a few energy chews right before your run regardless of what you ate before.
What to do if you experience reactive hypoglycemia
While experiences vary, some athletes can recognize the onset of reactive hypoglycemia from how the first 10-15 minutes of a run feels. They may describe an unusually light or “floating” sensation. The recommended response is the same: Stop running and consume approximately 15g of carbohydrate (about 1–3 energy chews, depending on the brand). Sit or walk slowly until symptoms subside. If they persist, take another 15g. Relief is typically rapid, allowing most runners to continue without further issues.
Resources & References
Study: “Pre-exercise ingestion of carbohydrate and transient hypoglycemia during exercise,” International Journal of Sports Medicine (PubMed).
Study: “Prevalence of hypoglycemia following pre-exercise carbohydrate ingestion is not accompanied By higher insulin sensitivity,” International Journal of Sport Nutrition and Exercise Metabolism (Pub Med)
Study: “Association between pre-exercise food ingestion timing and reactive hypoglycemia: Insights from a large database of continuous glucose monitoring data,” European Journal of Sport Science (PubMed)
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