The topic of low carbohydrate high fat diets (LCHF) or ketogenic diets for athletes is still hotly debated. I posted some thoughts in a blog recently but a few days a paper was published in the esteemed Journal of Physiology that studies the effects of different diets on metabolism and performance in elite athletes and one of these diets was a LCHF diet.
The study was performed at the Australian Institute of Sport in the lead up to the 2016 athletics season which included qualifiers for the Rio 2016 Olympics. They called it project supernova.
In this study they used very well trained athletes (21 race walkers) who were following a structured training program in 3 week blocks. The athletes were allocated to one of 3 diet groups. Some athletes completed more than 1 training block and thus also more than 1 diet).
A high carbohydrate diet: Overall macronutrient composition 60-65% of energy from CHO, 15-20% protein, 20% fat; Similar daily CHO intake, with CHO consumed before, during and after training sessions. The diet represents sports nutrition guidelines from 1990s.
Periodised CHO availability: Same overall macronutrient composition as HCHO, but spread differently between and within days according to fuel needs of training as well as an integration of some training sessions with high CHO availability (high muscle glycogen, CHO feeding during session) and others with low CHO availability (low pre-exercise glycogen, overnight fasted or delayed post-session refuelling). This strategy represents current guidelines and evolving evidence around benefits of strategic training with low CHO availability.
A LCHF diet: 75-80% fat, 15-20% protein, <50g/day CHO.
Measurements were performed before and after the 3 week period and included performance measurements as well as metabolic measurements.
The study demonstrated that there were NO benefits of a ketogenic diet versus a high carbohydrate, or periodised nutrition approach in elite endurance athletes. In fact, performance of high intensity exercise was not improved by 3 weeks of intensified training in the ketogenic diet group, while athletes consuming the other diets made substantial performance improvements.
The study also confirmed early findings that the LCHF diet resulted in reduced economy. In other words: more oxygen is needed to perform the same amount of work.
The consistent finding in all LCHF studies is that fat oxidation is increased when carbohydrate intake is decreased. This is not surprising or new as it was first observed in the 1920s and many times thereafter. The question is, is this an advantage? Or could this be a disadvantage?
There are three ways to look at the increase in fat oxidation. One can look at an increase in fat oxidation in the light of improvements of fat oxidation. (generally this is believed to be a positive effect). This is a common interpretation but not necessarily the correct one.
Another possibility is that the increase in fat oxidation is the result of impairments in the ability to oxidise carbohydrate. Generally, such impairments would not be a positive adaptation because it is known that for every athlete no matter how long they have been on a LCHF diet, at high intensity carbohydrate is required as a fuel. Fats cannot be used anaerobically.
Of course it is also possible that at the moderate intensities it does not make a lot of difference where the fuel is coming from. This is perhaps why we see in ultramarathons (where he average intensity is relatively low) athletes being successful with very high carbohydrate diets as well as very low carbohydrate diets.
So what can we conclude from all the studies to date?
Fat oxidation is increased if we reduce carbohydrate in the diet
There are adaptations in the muscle if we deprive the body of carbohydrate that favour fat metabolism and such changes have been seen even within 5 days. Some of the adaptations take place rapidly (others may take longer)
There are no well controlled studies in humans that show performance benefits of carbohydrate restriction
If you are an elite athlete and need to compete at high intensities, the LCHF diet does not seem to be the way to go. Instead it may be best to periodise nutrition with strategetically planned low carbohydrate intake and high carbohydrate intake (Although in this study that approach was not superior to a high carbohydrate intake).