The science of fuel

The way we fuel our bodies is becoming increasingly scrutinised. In this world of marginal gains, why wouldn’t we attempt to squeeze every last drop of performance out of our bodies? But what’s the best diet for you? What does the evidence say about the correct way to prepare for your next ride? 

Before proceeding any further it should be said that this is a generalised review of meta-analyses and this should not be taken as specific medical advice. 

The way you consume food surrounding a race is divided into three segments. Pre, intra and post event nutrition.   Let’s examine each of these sections individually to see what the evidence says.

Pre-race nutrition: The classic teaching for preparing for an endurance event of all descriptions is to pack your body to the brim with carbohydrates (CHO) and hope for the best. Now the theory behind this is simple, your body best utilises carbs during exercise, so why not give your body what it wants? But is physiology really that simple?  Well, potentially. 

The data actually supporting this theory isn’t as solid as you would expect it to be, and any benefit all but disappears for an event less than 90 minutes (1). However, at events greater than 90 minutes, a statistically significant improvement was observed, however the research in this area still remains incomplete and there’s definitely more work to be done.

So let’s go with the current suggestions, from a recent large-scale meta-analysis which examined 16 studies,  and says yes, if you’re competing for more than 90 minutes, you should be consuming pre-event CHOs. What type of carbs should you be eating, when should you be eating them, and how much should you consume? Well, that’s not entirely clear either.

CHOs are typically stratified by their glycaemic index based on their effect on your blood glucose levels (BGLs) relative to pure glucose. The amount a load of pure glucose raises your BGL is normalised to 100. A food that scores less than a 55 is classified as low GI, meaning for that same amount of CHO, at 2 hours after ingestion, less than 55% percent of that fuel source will be readily available in your bloodstream. Compare this to a high GI CHO which will be greater than 70. Now a lower percentage of glucose may not sound like a good thing, what’s actually happening is that fuel is being slowly released over a longer period of time. This has then led to theory that lower GI foods are more suited to endurance activity, as they maintain a more consistent level of a fuel source. Now, when exclusively considering the pre-race feeding stage, the evidence seems to suggest that it doesn’t actually matter if the carbs you eat are high or low GI.

A meta-analysis from 2017 (2) extensively investigated the role of low versus high GI foods in the pre-race setting. This review assessed 19 studies and 188 athletes, one of the larger reviews in exercise physiology and its conclusion was that there is no significant ergogenic benefit from consuming a low GI meal when compared to a high GI meal.

If you review these studies yourself, which I strongly encourage, you’ll note they also monitored a number of surrogate markers, including fat oxidation, insulin levels and respiratory exchange. While these can be useful, they can muddy the waters, and often do not directly correlate with improved outcomes, so keep your eyes on the prize, and remember that the only thing that matters is did this intervention (the type of carbohydrates consumed before the event) result in a clinically tangible benefit, in this case, a faster time. This analysis reports a ‘non-significant’ benefit for consuming low GI foods when no intra-event CHO’s are ingested. However this means that the likelihood that the data supporting this theory is due to chance is too high to be considered valid scientific evidence. 

So now we’ve established that it probably doesn’t matter what types of carbs you eat prior to the event? Again this is another source of controversy, and it actually confounds some of the data in the previous meta-analysis, but current guidelines place the optimal window for CHO consumption somewhere between 2-4 hours before your event (2). Theoretically, the further from the event you’re going to consume a meal, the lower the GI, however this remains to be adequately investigated.  And the dose? Between 1-4  grams per kilogram of body weight, and most studies which demonstrate evidence of CHO loading are closer to 2. (2,3) 

So as you can probably tell, as soon as you look into the advice of your mate who absolutely smashed that last crit, things start to get incredibly complicated. My recommendation would be to use this data as a springboard. Start with this and then change the recipe and find what works for you! 

As always, leave any questions in the comments and I’ll do my best to answer them! 

Pre Race Nutrition: The Science of Fuel

By Dr Tim Davis – Pushys Resident Medical Expert.


1 – Pöchmüller, M., Schwingshackl, L., Colombani, P. C., & Hoffmann, G. (2016). A systematic review and meta-analysis of carbohydrate benefits associated with randomized controlled competition-based performance trials. Journal of the international society of sports nutrition, 13(1), 1-12.

2 – Burdon, C. A., Spronk, I., Cheng, H. L., & O’Connor, H. T. (2017). Effect of glycemic index of a pre-exercise meal on endurance exercise performance: A systematic review and meta-analysis. Sports Medicine, 47(6), 1087-1101.
3 – Wu, C. L., & Williams, C. (2006). A low glycemic index meal before exercise improves endurance running capacity in men. International journal of sport nutrition and exercise metabolism, 16(5), 510-527.

Categories: Nutrition

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