High Fat Low Carb Diet and Endurance Performance: Friend or Foe?

Unless you have been asleep under a rock somewhere, you would have heard about the re-emergence of the controversial Low Carb High Fat (LCHF) diet and its supposed beneficial effects upon endurance performance. High profile athletes, from various sporting backgrounds, anecdotally report improved performance with a LCHF diet. Thanks to today’s social and traditional media, news of the latest ergogenic tool with hopes of performance enhancement spreads like wild fire. You may have tried the LCHF diet, but how well is this approach supported by research and expert opinion?

The LCFH diet was a key topic for debate at the 2015 Sports Dietitians Australia (SDA) conference. Louise Burke (Head of Sports Nutrition, AIS) led a hot discussion, presenting 30 years of research into the manipulation of carbohydrate and fat intakes and evaluating the impact on athletic performance. So what does the evidence conclude, and what is considered “best practice”?

This article will answer some important questions, and provide coaches and athletes with practical evidence-based advice to determine whether the LCHF diet is suitable for them.

First, let’s look at fat adaptation. How does this work?

It is widely accepted that carbohydrates are the preferred fuel source during moderate and high intensity activity. However, carbohydrates – stored as glycogen within the muscles and liver- provide a relatively limited pool of energy. Glycogen depletion is a key contributor towards fatigue and performance decline during events lasting longer than 90 minutes. This has been the driving reason behind traditional nutrition approaches for endurance training, which have focussed on educating athletes and coaches on the most effective ways to refuel with carbohydrates before, during and after endurance events.

By contrast, our body’s fat stores may be capable of providing a relatively abundant pool of energy to tap into. Is it therefore possible to become better at deriving fuel from fat, thereby conserving glycogen stores and enhancing endurance performance?

Diets low in carbohydrate (<2.5g/kg/day) and high in fat (~65-70% of energy intake), coupled with regular endurance training, will deplete muscle glycogen (1). Studies have shown that training in a glycogen-depleted state does increase fat oxidation, which in turn spares glycogen stores during subsequent activity (1). More recently, researchers have considered how the increase in fat oxidation occurs, identifying enhanced gene expression of the stress response, substrate utilisation and mitochondrial biogenesis as the underlying physiological changes (2). This raises the further question of whether the LCHF diet and fat adaptation improves endurance performance?

To date, the evidence has not established any performance enhancement with a LCHF diet, despite an increase in fat oxidation during a glycogen-depleted state. In fact, most studies have suggested the opposite. There appears to be an increase in perceived effort required for training, combined with an impaired response to training when the LCHF diet continues for periods longer than 4 weeks (1).

During the SDA conference, Louise concluded her presentation by saying that the LCHF diet reduces metabolic flexibility and the athlete’s ability to rapidly oxidise carbohydrates during high intensity activity, and in turn impairs performance. This also includes the strategic activities that occur during events, such as the breakaway, the surge during an uphill stage, or the sprint to the finish line - all of which depend upon the athlete’s ability to work at high intensities (1). Louise did point out that, while the evidence does not support enhanced performance in competitive athletes, there may be situations where there are some advantages or, at least, no disadvantages. This however, excludes activity that requires high intensity or maximal effort.

Through the evolution of the LCHF diet and the proven fat adaptation response, researchers have considered the concept of “training low, but competing high” (2). This is where selected training sessions are completed with reduced carbohydrate availability, but glycogen is restored immediately before competition.  

“Training low” can, however, can come at a price. Performing long duration and/or high intensity sessions while carbohydrate-depleted can increase susceptibility to illness and infection, given the role of carbohydrate and exercise-induced immune suppression (2).  To add to this potential burden, it is important to recognise that exercising with low carbohydrate availability can cause muscle protein breakdown and a loss of muscle mass.

While optimal practical strategies are not yet known, some preliminary expert advice has been given to help maximise the benefits and minimise the drawbacks of “training low”. This advice includes:

  • Practicing “training low” in conjunction with sessions undertaken with normal or high       carbohydrate availability, so that capacity to oxidise carbohydrate is not impaired while racing.
  • To help maintain intensity during sessions, try carbohydrate mouth rinsing, and consume caffeine before or during exercise.
  • Consume high quality protein (E.g. 20g WPI) before, during or immediately after training, to help prevent protein breakdown.
  • To maintain healthy immune function, “training low” should be avoided during exhaustive workloads (2).

Carbohydrate requirements vary greatly between athletes, events and timing in the training and competition cycle. It’s important to understand individual needs, and avoid prescribing a “one-size- fits-all” approach. Current best practice recommends the periodisation of carbohydrate intake to match the fuel needs of the training cycle, as well as considering the importance of performing well in that particular training session, and the aim to fat adapt to exercise with low carbohydrate availability (3). An Accredited Sports Dietitian is well positioned to assist athletes in determining the best approach for their individual nutritional needs.

While current research shows no performance enhancement with a LCHF diet, scientists and sports nutrition experts are eager to build upon this knowledge and established practice, in order to provide a range of optimal approaches for coaches and athletes seeking elite performance. Watch this space!


Andrea Sparrow APD ASD AEP


1.       Burke LM. “Fat adaptation” for athletic performance: the nail in the coffin? J Appl Physiol 100: 7-8, 2006.

2.       Bartlett JD, Hawley JA, and Morton JP. Carbohydrate availability, and exercise training adaptation: Too much of a good thing? Euro J Sport Sci, 15: 3-12, 2015.

3.       Burke LM. Re-Examining High-Fat Diets for Sports Performance: Did We call the Nail in the Coffin Too Soon? Sports Med, 2015