Are Low Carbohydrate/High Fat diets (LCHF) just another gimmick with little scientific evidence to support the growing number of anecdotal claims? La Velocita's Lori Hill investigates.
Sport Scientists have long known the role that muscle glycogen plays and particularly during high intensity exercise. As a result sports nutrition has been centred around this fact, with most endurance base diets focusing on a high carbohydrate intake (40-60%) both prior to and during exercise.
Over the last decade a number of questions have been raised that are in direct conflict to what we have known to be ‘true’ with regards to carbohydrate rich diets and submaximal exercise. Therefore, do athletes really require high carbohydrate diets and is it possible to utilise the abundant energy source we have stored in our body as fat? Or are the Low Carbohydrate/High Fat diets (LCHF) just another gimmick with little scientific evidence to support the growing number of anecdotal claims?
Whether an elite athlete or recreational cyclist, the majority of us are well aware of the importance that nutrition plays and the impact that it can have on our sporting performance and for many they are looking for any edge that will give them an advantage. The LCHF diet is not a new concept with studies in the 1990’s and early 2000s looking at the idea of how this way of eating could assist trained individuals by increasing their utilisation of fat during submaximal exercise and in turn improve endurance performance. Unfortunately in many of these studies, while the subjects showed an improvement in their capacity to burn fat in as little as 7 days, there was a failure to show clear performance benefits, particularly in endurance/ultra-endurance events, along with high intensity exercise which primarily relies on glycogen as fuel source.
Despite a significant amount of anecdotal evidence suggesting that a LCHF diet improves performance by both amateur and professional athletes, there is a considerable absence of any peer-reviewed data, which prevents conclusive findings. In addition to this while research has shown that modern humans require very little amounts of carbohydrates, with the human body able to adapt to an extremely low carbohydrate diet, there is very limited research that addresses sports performance outcomes while on a LCHF diet. Furthermore of those human studies that do exist many are limited by their small sample sizes, and have not addressed the endurance athletes’ high training load and the need for a quick recovery, along with cognitive factors such as their capacity to concentrate and hand eye-coordination in order to prevent injury on the bike. 
In events whereby the pace is a predictable, sub-maximal intensity, the LCHF diet has shown some promise however, for most cycling events where the terrain and average pace isn’t always pre-determined as a result of environmental conditions, breakaways and unplanned sprint finishes therefore to suggest that the LCHF diet is suitable for endurance events due to the submaximal intensity is somewhat misleading. As concluded by Phinney, in a study which looked at the performance of cyclists on the LCHF diet; fuelling of the brain and central nervous system who’s primary fuel source is glycogen must also be taken into consideration as recruitment of muscles for contraction, perception of effort, pacing strategies, and the execution of skills and decision making are all extremely important in ascertaining performance outcomes. Additionally in any events lasting greater than 1 hour, the focus of nutrition should be on methods that enhance carbohydrate availability for both the muscle cells and brain, as there is a direct correlation between low carbohydrate availability and both peripheral and central fatigue. 
Short term exposure to a LCHF diet has been shown to lead to the depletion of both muscle and liver stores of glycogen and does not show a significant increase in the use of fat as an energy source however; studies have shown that longer term adherence to a LCHF diet will lead to a number of physiological adaptations within the body which includes a marked increase in the use of fat as a fuel source. However, while there have been a number of anecdotal claims suggesting these metabolic adaptions in the body can help to facilitate submaximal exercise due to ketosis, (a process where fat stores are broken down for energy), the use of ketones as a fuel source for the body or as an exercise substrate is yet to be measured and therefore these claims are somewhat misleading.
As with any diet prescribed for an athlete, the perceived physiological benefits of a LCHF diet are very much individual with some athletes responding better to the use of fat adaption strategies. While all nutrition programs should take on a more individualistic approach it should be noted that the LCHF diet has limitations and particularly in regards to the intensity in which the athlete can perform.
At present there are only a handful of studies which have looked at the use of a LCHF diet in regards to sports performance, however of those conducted the small sample sizes used in each have not allowed for conclusive outcomes. The use of a LCHF diet has shown the capacity in some case studies to reduce an individual’s body fat, which is one of its main attractions, however with much of this ‘testimony’ unreliable and the evidence supporting the use of a LCHF diet limited, the use of these diets are likely to have a negative impact on one’s cycling performance.
Burke, L.M., (2015) Re-examining high-fat diets for sports performance; Did we call the ‘nail in the coffin’ too soon? Journal of Sports Medicine, Suppl 1: S33-49
Phinney S.D., (2004) Ketogenic diets and physical performance. Journal of Nutrition and Metabolism. 1:2
Volek J.S., Noakes T, & Phinney S.D. (2014) Rethinking fat as a fuel for endurance exercise. European Journal of Sports Science; 79:1-8