What is the best time to eat before exercise?
That depends. But here’s one thing we know: Eating within two hours of exercising risks rebound hypoglycemia and gastrointestinal distress. Not a good look when you’re trying (as usual) to hit your peak performance.
Our recommendation is loading 3-4 hours prior to exercise and avoiding eating 2 hours before.
It sounds like a very specific window, but here’s why:
4 reasons not to eat within 2 hours of exercise:
- Rebound hypoglycemia
- Insulin Release
- Gastrointestinal upset
- Replicating race day
Let’s go deeper.
All training is ultimately preparation for race day, right? So eating right before your workout because you forgot to eat during the day at work is just bad practice. The key here is to be planned and considered around your nutrition and fueling to ensure your training sessions go well and make sure you are ready for race day, not just in terms of your physical training, but also in terms of your nutritional plan and strategy.
To put it nicely: Training on a full stomach is… uncomfortable at best. Unless you are doing this with the intention of training the gut, it’s of no benefit. And it may well impair your performance. So make sure you eat far enough away from training; it’s key. And this may include altering what you intake to help quicker digestion. For example, avoiding fat, protein, and fiber can help keep your digestion comfortable. You can also use liquid sources of nutrition.
Insulin is released when you eat carbohydrates and protein together, but the insulin response to protein is much lower than to carbohydrates. Why? The rate of secretion of insulin into the plasma varies according to the metabolic state and the most important regulator is–you guessed it–glucose level. The insulin response has been studied by isolating pancreatic islets and exposing them to different concentrations of glucose (see below).(Frayn et al, 2019. Rothschild et al, 2020)
Insulin secretion rate shows a dose-response curve against glucose concentration. It starts when glucose >90 mg/dL and rapidly increases when glucose is between 140 and 180 mg/dL, above this insulin release peaks and plateaus.
Circulating insulin levels have many potential impacts. Perhaps the most significant and concerning is the potential for rebound hypoglycemia (see below), but additionally higher circulating insulin levels can impair fat oxidation rate. This is opposite to the goals of most athletes and recreational exercisers alike.2
As mentioned, the risk of rebound hypoglycemia (driven primarily by insulin) is greatest when eating within 2 hours of exercise. It is reported to occur in roughly 35% of the athletic population and manifests slightly differently in each person. That said, symptoms mirror traditional hypoglycemia: sweating, dizziness, and generally feeling weak or just overall terrible. In Figure 2, this is an indication of what rebound hypoglycemia can look like. This would be worsened by exercise starting anywhere from 08:35am onwards. This is at least partly for the same reason that walking after a meal reduces glucose rushes: The muscle takes up the glucose without insulin. The problem is that insulin is already there to do its thing, so you end up with hypoglycemia.
A case study:
A hungry runner wakes up for his usual Wednesday run. He usually does this fasted, but decides to grab a Powerbar quickly before he heads out because he knows he needs fuel to be running for 90 mins, especially if he is hungry. His glucose, pace, and heart rate graphs are below in Figure 3.
As can be seen in Figure 3 (above) glucose (white line) starts dropping just within 15 minutes of starting exercise. From there, it gets as low as 65mg/dL with a corresponding heart rate spike (red line) and pace drop(grey shaded background). This pace drop was the result of the runner needing to stop and walk due to symptoms (dizziness, sweaty and feeling terrible).
You can see things normalize a little while later, a good indication this was a rebound hypoglycemic event rather than another cause of hypoglycemia. This runner ate about 15 mins before running and not again during the run. Note the later pace drops (around 08:45am) were needed due to crossing roads, as explained by the runner.
Here are 5 questions to ask when planning your pre-workout meal:
- What does the workout entail? This will impact how important this meal is to your performance. The longer and higher the intensity of the workout, the more important this meal is and the more planning is needed for it.
- How are your fuel stores from previous intakes? Consider your last 24 to 48 hours worth of intake and training. Have you refueled well? Are you primed for this workout? Or do you need to have a bigger meal to ensure you have really topped up your fuel stores?
- What is the right timing? Giving yourself enough time to properly digest the meal is crucial to ensure you avoid gastrointestinal issues.
- What type of meal is this? This interplays with timing in that if you are eating far enough away from a workout, this can and probably should include protein and fats. If you are eating closer to a workout (3 hours or so) it should be carbohydrates only.
- How much are you planning to eat? The amount you eat, including the amount of carbohydrates, depends on how far away you are from your workout.
- Frayn K., Rhys E.. 2019. Human Metabolism. A regulatory Perspective. Chapter 6: Communication systems. Page 173-179.
- Rothschild JA, Kilding AE, Plews DJ. What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients. 2020; 12(11):3473. https://doi.org/10.3390/nu12113473
- Harrison, D E et al. “Properties of isolated human islets of Langerhans: insulin secretion, glucose oxidation and protein phosphorylation.” Diabetologia vol. 28,2 (1985): 99-103. doi:10.1007/BF00279924