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During exercise, as carbohydrate utilization increases and muscle glycogen stores begin to decline, potentially blood glucose levels can begin to drop. Low blood glucose levels can result in a brief spell of dizziness, confusion, nausea, headache, or blurred vision – all of which could impact exercise performance. Our bodies have evolved to keep blood glucose levels within a tight range by releasing counter-regulatory hormones, both at rest and during exercise. But a few susceptible athletes may experience “rebound hypoglycemia” or “transient hypoglycemia.”
Certainly, everyone experiencing these disturbing symptoms should check with a medical professional to ensure everything in the body is functioning at optimal levels. After ruling out major causes, it may well be that the culprit is rebound hypoglycemia.
What is rebound hypoglycemia?
If hypoglycemia does occur during the first stages of exercise, it is associated with carbohydrate ingestion 30 to 60 minutes before exercise. Why? Eating carbohydrate increases blood sugar and an insulin response to it by increasing insulin levels. When exercise starts, the skeletal muscles are also pulling on the blood sugar. The result? A rapid decrease in blood glucose at the start and early stages of exercise. Additionally, there may be a decrease in the concentration of free fatty acids available for fuel, due to the effect of insulin inhibiting fat breakdown.
In the 1970’s and 80’s, researchers spent a significant amount of time examining the effects of exercise on glucose levels, particularly in endurance exercise. Much of this work focused on the impact of the pre-exercise meal prior to prolonged exercise. The goal was to determine if you could manipulate the composition of the meal to prevent hypoglycemia. They found that there is an increased risk of rebound hypoglycemia if exercise follows the meal too closely, but only in few (not all) moderately trained endurance athletes. The one factor in common for the individuals who experienced rebound hypoglycemia was an enhanced insulin response to the pre-exercise carbohydrate ingestion.
Managing rebound hypoglycemia
The good news is that within 10 minutes of exercise, everything is normalized, and there is no reported effect on subsequent endurance performance. But try telling this to an athlete that experiences the dizziness, lack of power, nausea, and general feeling of lack of control at the beginning of a race!
There are specific things to do if you are one of the unlucky few that experiences this transient hypoglycemia.
- Time your food intake so that you have at least an hour before the start of your event. You do not have to be one of the people that is slurping down a gel or energy chews 15-20 minutes before the start, as this is too close to the start to try to “top up.” This is a sure-fire way to perturb your blood glucose and have a hard start to a race!
- Research shows that if you do need to eat, do it just before you warm up. Then take a good 20 minutes to warm-up, as this exercise will help normalize insulin and exercise-induced glucose uptake. When you hit the start line, you will already be in a good metabolic state.
- Consider your caffeine use. Caffeine increases blood glucose clearance, so if you are using caffeine, the timing of caffeine ingestion is very important. Prior to the start of a race, you will need to eat with your caffeine consumption, knowing the half-life (peak) of caffeine is around 90-120 minutes post consumption. In this, if you plan your pre-race meal appropriately, you will have finished food and caffeine consumption a minimum 60 minutes prior to the start of the event. Again, to help normalize blood sugar fluctuations, give adequate time for a 20-minute warm up.
Rebound hypoglycemia is a rare occurrence, but it does happen. Being aware of why and how to implement strategies to reduce the transient effects on blood glucose will ensure a strong start and finish to your event.