Updated: Oct 6, 2021
During my PhD, I did one particular study in which I had highly trained athletes eat both a high carb diet and an ultra-low carbohydrate diet (similar to the ketogenic or Atkins diet) for 14 days. Two striking results came out of this study.
Result Number One
After almost two weeks of <80g of carbohydrate per day, one night of carbohydrate loading was enough to produce equal or better performances compared to the high carbohydrate diet. This was a big finding because the 10km cycling TT I used required the athletes to exercise above their anaerobic threshold. At this intensity, carbohydrate is the main energy source used by the working muscles. This meant that despite eating <80g of carbohydrate per day the athletes were able to replenish their glycogen stores with just one day of carbohydrate loading, rather than the 3-4 days previously recommended.
The reason the athletes were able to store so much glycogen in just one day is due to a phenomenon referred to as glycogen super-compensation. Put simply, glycogen super-compensation occurs when the muscles have been starved of carbs and become extra-absorbant when the carbs are finally reintroduced. This results in greater glycogen storage compared to carb loading on top of a high carb diet.
Result Number Two
There was a massive shift in the fuels the athletes used, both at rest and during exercise. In just 10-14 days athletes were burning fat at rates similar to those on a full-time low-carbohydrate diet (which I also researched in another study). See image as a reference.
Note: This study involved two dietary interventions (low-carb and high-carb) followed by an exercise test in which participants completed with either a sports drink + ketone supplement or a sports drink only. The rationale behind this was to determine whether a ketone supplement could replicate the benefits of a carbohydrate-restricted diet, it could not. PLB = placebo, supp = ketone supplement.
What I learnt from this study was that no matter how reliant on carbohydrate you are, with just 14 days of dedicated carbohydrate restriction you can induce rates of fat-burning which are similar to that of seasoned low carbers.
Real World Application
Short carbohydrate restriction protocols like this are being utilised by many professional endurance athletes. At the 2016 European College of Sports Science conference, I attended a presentation by the Team Sky sports scientist. In his presentation, he described how a ketogenic diet is applied for 10-14 days anywhere from 1-4 weeks out from big races to increase fat burning and body fat loss. After completing my own study in this area I can see why they’d use such a system.
A protocol I use with many professional and amateur athletes is to cut carbs from 2 weeks before a target race. I'll have them limit carbs for 10-12 days until 2 days before the event. 2 days out I'll have them increase their carbs to 6-7g/kg for the day. The next day, the day before the race, they'll eat 4-5g/kg for the day. By restricting carbs for the 10-12 days leading into the event they are able to boost their fat burning capacity, and in most case avoid overeating and weight gain. Introducing a majority of the carbs 2 days before the event allows us to reduce the carbs back down to a level that shouldn't negatively affect fat burning rates while still ensuring maximal glycogen storage the day before the race.
Dr Will O'Connor
Educating and inspiring athletes to train smarter