It’s been very well established that carbohydrates drive exercise performance.

The quantity of carbs needed is dependents on the duration x intensity of exercise, whilst factoring your bodyweight. i.e. bigger people have a greater capacity to store carbs.

We know that the total AMOUNT of carbs is important, but what about the TYPE of carbs? Reintroducing, the Glycemic Index debate…

The Glycemic Index (GI) classifies carbohydrates into low, medium and high based on how quickly they increase blood glucose concentrations (blood sugars) once consumed.  I.e. high GI foods increase blood sugars more rapidly compared to medium and low GI.

This classification model was originally used in diabetes education and treatment, but does it have a role in enhancing athletic performance?

Previous research examining the impact of different GI meals before exercise have resulted in a mixed bag of results – some research shows low GI is better for performance, where other say high GI is better, where some say there’s no difference…

For race day strategies, I’d highly recommend that you eat med-high GI carbohydrates due to their (1) higher energy density, (2) lower fibre content and (3) lower food volume. These foods will reduce any nasty and unwanted gut issues, fit racers preferences better and will undoubtedly improve performance. I.e. who wants to race on a belly full of sweet potatoes?

That being said, should you change your carb choices during the training week to improve performance and enhance adaptations within the muscle?

The current recommendations would suggest that it probably doesn’t matter. However, the aforementioned studies were only 1-5 days in duration where it’s unlikely you’ll see significant adaptations to the muscle in this time frame. But what about studies that last for weeks as opposed to days in duration? Would you see improvements then? i.e. something that reflects your NORMAL diet?

To answer this question, Durkalec-Michalsi et al (2018) recruited a group of young endurance runners and provided them with one of the following diets for 3 weeks:

Diet A) Medium GI, high carb diet: 3190kcal, 485g carbs, 109g protein, 90g fat, 38g fibre.

Diet B) Low GI, high carb: 3174kcal, 480g carbs, 110g protein, 90g fat, 58g fibre.

As you can see, the diets are pretty much identical apart from their fibre intake, which is most associated with lower GI foods. The runners were also instructed to slightly undercook their carb sources (al dente) in the Low GI group, where they were instructed to slightly overcook in the medium GI group to decrease/increase the GI content of their meals respectively.

They then tested how the two diets would impact higher intensity performances; time to exhaustion, distance ran in 12 minutes and maximal work done on an incremental cycling test.

After a fourteen day break, they repeated the same experiment but followed the other dieting strategy.

On a whole, the low GI diet appeared to be more advantageous for athletic development as they saw greater improvements in the 12 minute running test (~70m increase) and time to exhaustion in the incremental cycling test (~20 seconds).

On face value, these results don’t sound ground breaking. But if you apply them to real world race scenarios, they suddenly become very meaningful….and all you have to do is eat different types of carbohydrates for 3 weeks? What about 12 weeks? Or for the rest of your athletic career?

Interestingly, both diets improved anaerobic threshold and there was no difference between diets.

It’s worth noting that the runners ate their low GI or medium GI meal two-three hours before the fitness tests – Research has previously shown that eating a high GI meal prior to exercise can cause a sudden drop in blood glucose after 10-20 mins of starting. Therefore it was speculated that the low GI meal resulted in more stabilised blood sugars and subsequently greater performances.

Long story short, a 3-week high carbohydrate diet improved medium-high intensity performance. However, greater performances were seen when runners ate mostly low GI foods. This is perhaps not due to any superior adaptations of the muscle, but more as a product of having more stabilised blood sugars.

High GI foods such as bagels, Rice Krispie Squares, energy gels and carb powders do have their place in a runner’s diet. However these should not make the foundation of the diet, they should be seen as supplementary foods to further support higher training loads.

Therefore, I wouldn’t typically recommend these foods if your energy and training demands aren’t overly high, I would advocate the lower GI approach with higher fibre intakes.

That being said, once training loads increase, that’s when we start ‘supplementing’ with Jam bagels and all the other goodies found in an endurance athletes toolbox.

Whilst you’re here…If you find this content interesting, but don’t quite know how to apply it to maximise your performance and take your athletic development to the next level, I can 100% help you through my Athlete Coaching Programme. Click the shiny orange button below to find out more.



Subscribe to my newsletter now to claim your free guide to shred body fat without compromising performance.