Sports woman peeling a banana

Key Topic

Sports dietitians frequently suggest eating before exercise, but does the research really support its effect on performance? Today, we will explore this concept by examining both endurance and resistance training.

Endurance

Theory

  • There are a few theoretical reasons it can matter.
  • Consuming carbohydrates before a workout can boost glucose and glycogen levels, providing readily available fuel that may help sustain higher intensity training for longer.
  • There is even evidence that carbohydrate mouth rinses can help. This supports the argument that this might matter.
  • There could also be benefits related to electrolytes or even other nutrients, rather than just carbohydrates playing a role.
  • In saying that, how you FEEL plays a large role in performance. For example, if you eat too close or too far from a session, you might not feel as good. So timing and food choices do matter. 
  • Clients who feel that eating before exercise worsens performance often do so because they’ve eaten too close to their session or chosen less optimal foods.

Endurance

  • A general guideline from the Academy of Nutrition and Dietetics suggests consuming 1–4 g/kg of carbohydrates 1–4 hours before exercise to support performance.
  • It makes a lot of sense that it would help performance if the exercise is above a certain intensity and duration.
  • Going through a bunch of studies actually testing this though:
  • Study 1: 2.5g/kg carbohydrates pre-exercise vs placebo during a time to exhaustion run at 70% of VO2 max found a noticeable improvement.
  • Study 2: 3g/kg carbs vs 0.5g/kg carbs 3.5 hours before a time to exhaustion test. Once again the higher carb group outperformed. 
  • Study 3: 70g of carbs from rice 2 hours pre and 74g carbs during vs no carbs for a 21km run. The carb group outperformed.
  • I could keep going but too be honest it seems clear that in almost all studies, if the intensity is high enough and duration is long enough, adding carbs prior to a session helps performance. 
  • Note that not every session is about performance though e.g. a zone 2 run doesn’t need to be optimally fuelled to the same extent that a harder session would.

Resistance Training

Theory

  • The same concepts apply, it’s just that resistance training does not have as much glycogen/glucose utilisation as endurance training. 
  • We often reference this particular study which found that 6 sets of leg extension to failure, depleted glycogen stores by ~40%.
  • That tells us two things: One is that if you are fully carb loaded, it would be rare to fully deplete glycogen by lifting weights.
  • But if you had low glycogen stores at the baseline, maybe this begins to matter.
  • Overall dietary context shifts how important pre-training carbs are.

Research

  • Two meta-analyses on this topic came out in 2022 with different findings that caused a lot of discussion.
  • One by Menno Henselmans found no difference in performance.
  • & Another by Andrew King and colleagues that found a difference under specific circumstances.
  • Those circumstances being: If the session is >45 minutes long and would otherwise have been occurring in a fasted state.

Additional Thoughts

  • How you feel impacts the session alongside the physiological aspects such as how much glycogen you have.
  • I
  • While we’ve primarily focused on carbohydrates, it’s worth considering other nutrients. For example, pre-exercise sodium has shown mixed effects on performance—while some studies suggest potential benefits, the overall evidence remains inconclusive.
  • We have evidence that magnesium levels DROP a lot during exercise and there is a case that has been made by some that having magnesium pre-exercise could offset that. Although, the practical impact on performance has not been measured in a sufficient way.
  • Dietary nitrates improve performance both with endurance and also higher rep resistance training.  

Summary

In most cases, it makes sense to include a portion of your carbohydrate intake before training. However, there are exceptions, such as if you simply prefer fasted training and aren’t focused on maximising performance in intense sessions. That said, ensuring you have adequate glycogen stores is still a sensible approach.

Relevant Blogs/Research

Relevant studies: