Podlogar et al. (2022) Addition of Fructose to a Carbohydrate-Rich Breakfast Improves
Cycling Endurance Capacity in Trained Cyclists. IJSPP.
TLDR: Liver glycogen stores are depleted when we wake up in the morning. Providing multiple sources of carbohydrate may enhance liver glycogen replenishment. Combining rice + fructose (vs. rice + glucose) for breakfast increased time to exhaustion in trained cyclists. Adding fructose-rich foods at breakfast may be beneficial for performance in training and racing.
The current guidelines for pre-race breakfasts are pretty vague: consume 1-4 grams of carbohydrate per kg body mass.
Two factors immediately jump out: (1) this is a broad range - should you aim for closer to 1g or 4g per kg of body mass? And (2) what types of carbohydrates are optimal?
Researchers from the UK, Sweden, and Slovenia teamed up to try and answer the second question: does the type of carbohydrate you consume in your pre-race breakfast impact on your ability to perform?
One of the key factors that we need to take into account with pre-race breakfasts is that we haven't eaten anything for the previous 8-12 hours since dinner. We have two primary stores of carbohydrate in our body - muscle and liver glycogen, and when we wake up after an overnight fast, our muscle glycogen stores are stable but our liver glycogen stores can be depleted by ~25%.
Given that liver glycogen content is an important factor for endurance capacity, designing pre-race breakfasts to top-up liver glycogen stores seems like a logical step. Research looking at post-exercise carbohydrate consumption has shown that consuming a mixture of carbohydrates (e.g., glucose + fructose vs. glucose-only) enhances liver glycogen re-synthesis.
The researchers used this information to design their study. They wanted to find out if providing a mixture of carbohydrates (glucose + fructose) for breakfast provided an additional benefit over a single source of carbohydrate (glucose-only).
What did they study?
8 male trained cyclists
VO2max = 62.1 ml/kg/min
VT1 = 239 W (3.3 W/kg)
RCP = 293 W (4.1 W/kg)
VT1 is similar to LT1, and RCP is similar to FTP or critical power.
All the cyclists completed a graded exercise test to determine VT1, RCP, and VO2max.
On two separate occasions, the cyclists came into the lab having completed an interval session the previous evening and had the following breakfasts containing 2 grams of carbohydrate per kg body mass (the carbohydrate from rice/powder were provided in a 2:1 ratio):
Rice + GLUCOSE powder
Rice + FRUCTOSE powder
After breakfast, the cyclists rested for two hours before starting the testing session. The researchers decided to use time-to-exhaustion at VT1 as their protocol. They decided on this because in many races riders are required to ride at a moderately hard intensity for prolonged periods of the race. Generally, whether or not you can maintain this pace (or get dropped) plays a large role in your finishing result.
During the trials, they measured a range of physiological variables every 15 minutes including carbohydrate/fat oxidation, heart rate, blood glucose/lactate, and RPE.
Here is a visual overview of the testing protocol.
What did they find?
The main finding from the study was that the participants cycled for 5% longer after consuming the Rice + FRUCTOSE breakfast (2 hr 17 min), compared with the Rice + GLUCOSE breakfast (2 hr 10 min). Everyone improved with the Rice + FRUCTOSE breakfast, some by 1% and others by 8% - but there were individual differences in the effectiveness of the breakfast.
Interestingly, they didn't observe any differences in carbohydrate or fat utilisation at any timepoints between the two trials. What they did find was that cyclists utilised 9% more total carbohydrate in the Rice + FRUCTOSE trial compared to the Rice + GLUCOSE trial.
The researchers suggest that the fructose in the breakfast helped to increase liver glycogen storage, which delayed the depletion of these stores during exercise, allowing the cyclists to ride for longer before exhaustion.
Essentially, the cyclists used carbohydrate at the same rate in both trials, but the Rice + FRUCTOSE breakfast allowed more glycogen to be stored in the liver, which meant they could continue using this carbohydrate for a longer period of time.
It should be noted that they didn't directly measure muscle or liver glycogen in this study, so the conclusions about the mechanisms that are driving the performance improvements do need further investigation!
So, how can we use these findings to benefit training and racing?
For key training sessions (high-intensity intervals, hard long rides etc.) or prior to racing, ensuring that liver glycogen stores are topped up is important. The results from the study suggest that adding fructose-rich foods at breakfast can help to achieve this.
An easy way to do this is to add foods such as a spoonful of honey or jam to a bowl of cereal, porridge or toast in the morning. A glass of fruit juice or a handful or Haribo will also provide a source of fructose too. Or if you really want to replicate the strategy they used in the study, you could mix fructose powder with rice for your breakfast!