In our last post we covered basic blood glucose response to exercise and next we will review how pre-exercise fueling can affect that. Remember that while fat stores in the body are abundant, carbohydrate is the main energy source for higher intensity and for longer workouts. Carbs are stored in your muscle and liver as glycogen, which with depletion can quickly lead to fatigue, delayed recovery and low energy availability if you aren’t intentional with fueling.
Supporting exercise with adequate carb availability starts with pre-exercise fueling. At rest, insulin is released following carb intake and the insulin response is dependent on the type and amount of carbohydrate ingested (glycemic load). Once exercise starts, muscle contraction induces blood glucose uptake and this can overlap with the insulin response from the pre-exercise feeding. In some situations, this could lead to a hypoglycemic state at the onset (first ~ 10 min) of exercise, or rebound hypoglycemia. While this may not be detrimental to performance, it’s possible some individuals could experience undesirable sensations in response to the drop in blood glucose.
One study compared the blood glucose response during steady state exercise (SS) and a subsequent time trial (TT) after manipulating the timing of ingesting 75 grams of glucose (on 3 separate occasions) 15, 45 and 75 min before exercise. Blood glucose was lower just before starting exercise and 5 min into SS when the carbs were ingested 45 and 75 min prior, compared to 15 min prior. However, after 5 min into exercise there was no difference in blood glucose whether pre-exercise carbs were taken 15, 45, or 75 min before. Additionally, there was no difference in RPE, HR, oxygen uptake, RER, power output or TT performance.
With a high glycemic index (GI), glucose is absorbed rapidly so the glycemic and insulinemic effects are more dramatic compared to many common pre-exercise feedings that are lower in glycemic index or load. When 75 grams of lower GI carbs are ingested 45 min prior to exercise, blood glucose still appears to fall lower than resting value at the onset of exercise, but then slowly rises after ~ 10 min of SS exercise. Because the blood glucose “spike” is higher after ingesting 75 grams of high GI compared to lower GI carbs, the actual drop in blood glucose is relatively larger after the higher GI pre-exercise feeding. That being said, there don’t appear to be differences in HR, average intensity, or TT performance whether pre-exercise carbs are high or low GI.
With the variations in pre-exercise feeding protocols and individual responses, it is difficult to provide a very specific set of recommendations for pre-exercise fueling.
If you can’t eat > 45 min before your workout, try taking in a small amount ( ~30 g carbs) just before (within 5 min of) exercise. (examples: 1 packet oatmeal, 1 banana, ½ cup granola, energy bar, piece of toast with honey, dried fruit.
Lower GI carbs taken in earlier may mitigate rebound hypoglycemia but also consider the timing of intake, amount, and whether or not your gut can handle those specific carbs/foods. Lower GI carbs or meals with fat and/or fiber may cause GI discomfort and impact exercise performance.
As with other fueling guidelines, it’s important to trial your race day pre-exercise fueling plan during training to determine what works best for you.
PMID: 3547009; 12500984; 12527976; 12527979; 12527977