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How To Avoid a Pre-Workout Bonk

BY Carrie McCusker

Studies show that up to 30% of athletes experience reactive hypoglycemia. Here's what it is and how to prevent it before your next workout.

Feeling light-headed, weak, and/or dizzy can be signs of hypoglycemia, or low blood sugar. Also referred to as bonking, hypoglycemia is common at the end of a hard race due to glycogen depletion. However, some athletes might experience this sensation in a rested state at the beginning a workout. Reactive hypoglycemia, or a drop in blood glucose at the onset of exercise, could be to blame.

Obviously, it’s not ideal to begin a workout and immediately feel shaky, too tired, or like you need to eat. Fortunately, there are some easy things you can to prevent a pre-workout bonk.

What Is Reactive Hypoglycemia?

When you consume carbohydrates, your body releases insulin to help process it and keep your blood sugar stable. However, when you begin exercising, your body starts using those carbohydrates and a further decrease in blood glucose occurs. There was even a time when consuming carbohydrates directly before exercise was believed to have a negative impact on performance due to the double blood-sugar-lowering effects of these two processes.

While eating carbohydrates before exercise is now generally encouraged, there are adequate studies that note pre-exercise carbohydrate consumption may result in rebound or reactive hypoglycemia in some individuals (approximately 30 percent of athletes, according to one study). Unfortunately, the existing research doesn’t address why or how athletes become prone to reactive hypoglycemia — only that it occurs.

How to Avoid The Pre-Workout Bonk

There are some simple ways to minimize your potential for experiencing reactive hypoglycemia: choose carbohydrates with a low glycemic index; avoid insulin-spiking glucose; and add some fats to your snacks. All of these measures will help slow the rate at which your body processes carbohydrates.

The timing of your carbohydrate consumption pre-exercise can also make a difference. In one study, reactive hypoglycemia was most prevalent in athletes who consumed carbohydrates 75 minutes before exercise. When taken just 15 minutes before, few people were affected, and no athletes were affected when taking carbohydrates five minutes before exercise or during warm up.

In other words, if you’re going to load up on carbs before a workout or race, try to do it as close to your start time as possible — or even once you get rolling! Consuming carbohydrates during exercise has no hypoglycemic effects, and can even compensate for a lack of proper pre-fueling earlier in the day.

What to Do if you Experience Reactive Hypoglycemia

Rebound/reactive hypoglycemia can feel a lot like bonking, but it’s not the same thing. Unlike a true bonk, your muscles may well be stocked and ready to go. If you feel weak or unmotivated at the start of your workout (and you know that you’re healthy, adequately recovered, and hydrated), remember it’s possible to overcome that pesky pre-workout bonk with a little added carbohydrates.

Not everyone will experience reactive hypoglycemia, but if you do notice the symptoms, it’s important to experiment with the timing of your pre-exercise carbohydrate intake, as well as the type of carbohydrate you prefer. Also, remember you can always carry your carbohydrate source during your workout, as you won’t experience a drastic drop in blood glucose if you start consuming it after you are into your warm up.

About Carrie McCusker

Carrie McCusker is a level 2 TrainingPeaks coach and a lifelong athlete who enjoys bringing individual attention to every level of athlete. You can find her on Strava and Instagram or check out her coach profile at TrainingPeaks.

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