Endurance training is key for Uphill Athletes to build the fitness and strength needed to move continuously for hours and days in the mountains and on the trails. In our earlier articles on fat adaptation and fasted training, we delved into how increased fat oxidation is a natural outcome of endurance training and that fasted training may actually hinder training adaptations, performance, and overall health.
Endurance athletes rely on both carbohydrates and fats as energy sources during prolonged exercise. Endurance exercise for longer than 90 minutes results in depleted muscle glycogen (carbohydrate) stores and requires carbohydrate intake in order to prolong performance and reduce the time it takes to reach fatigue (1, 2, 3).
Some studies have proposed that a low-carbohydrate, high-fat (LCHF) diet may enhance performance in ultra-endurance events by supporting a higher fat oxidation rate. They provide more fat as a fuel source, sparing muscle glycogen, and reducing the need to supplement with carbohydrate during exercise.
LCHF diets continue to spark debate and curiosity, with steadfast research scientists and elite athletes alike sharing about the transformative potential of LCHF diets for achieving superior performance in ultra endurance events.
Let’s dive into the science behind these diets with high-performance dietitian Rebecca Dent.
WHAT IS A LCHF DIET?
A LCHF ketogenic diet is defined by a carbohydrate intake of less than 20–50 grams/day, moderate protein intake and a high fat intake with fat making up 75-80% of daily energy needs (4).
KEY ELEMENTS
Low Carbohydrate Intake: Carbohydrate intake is limited to a minimal amount, typically less than 50 grams per day (this equates to ~ 2 x slices of bread, 1 x banana + 1 x apple, 1-1.5 cups of cereal). This restriction forces the body to enter a state of ketosis, where it relies on fat for fuel.
Moderate Protein Intake: Protein intake is moderate (1.2-1.5 g protein per kg of body weight) and tailored to individual needs. Consuming too much protein may lead to gluconeogenesis, a process where excess protein is converted into glucose, potentially hindering ketosis.
High Fat Intake: The majority of daily calories come from healthy fats (55-70% fat or 120-130g fat within daily intake) such as avocados, nuts, seeds, olive oil, coconut oil, and fatty fish. This shift in macronutrient composition encourages the body to burn fat for energy.
Nutritional Ketosis: Ketosis occurs when the liver produces ketones from fatty acids, which become the main energy source for the body and brain. Achieving and maintaining a state of nutritional ketosis is the primary objective for those following this dietary strategy.
WHAT DOES THE RESEARCH SAY?
Consuming a LCHF diet has been shown to increase fat oxidation during exercise and at rest (5). Studies have demonstrated a substantial increase in fat oxidation in well-trained athletes following a LCHF ketogenic diet, with a greater percentage of fat being used as a fuel source at low intensity exercise, i.e. Zone 1 & 2 (6, 7, 8, 9).
One of the earliest research investigations into the impact of a LCHF diet on athletic performance was conducted in 1983 (10). The study involved five well-trained cyclists who were fed an energy-balanced diet for one week, followed by four weeks of an energy balanced ketogenic diet (containing less than 20 grams of carbohydrate per day). The subjects continued their normal training throughout the study.
An endurance exercise test was carried out in a fasted state after week one and following the 4-week LCHF diet and performed at an exercise intensity of approximately 64% VO2max. Overall, the study showed that chronic ketosis without caloric restriction resulted in a preservation of submaximal exercise capability with reduced carbohydrate oxidation and increased fat oxidation during exercise.
Although the study found that aerobic endurance exercise of well-trained cyclists was not compromised by four weeks of ketosis, this was an underpowered study with just five subjects.
Another study in 2017 (11) investigated the long-term performance implications of a Low Carbohydrate Ketogenic Diet (LCKD) in well-trained endurance athletes. Twenty male endurance-trained athletes self-selected into a high-carbohydrate (HC) group or a LCKD group. The LCKD group adhered to a diet with a macronutrient distribution of 6% carbohydrates, 17% protein, and 77% fat. Both groups underwent a 12-week period of keto-adaptation and exercise training.
Post-intervention testing involved a 100 km time trial, with the HC group consuming 30–60 g/h of carbohydrates during the trial, while the LCKD group consumed water and electrolytes. Keto-adaptation and exercise training led to enhanced body composition in both groups. Fat oxidation during the 100 km time trial was significantly greater in the LCKD group compared to the HC group. There was no significant difference in the performance of the 100 km time trial between the HC and LCKD groups.
In another long-term study involving twenty elite ultra-marathoners and ironman distance triathletes, participants were divided into two groups: one group consumed a traditional high-carbohydrate (HC) diet, and the other group consumed a low-carbohydrate (LC) diet for an average of 20 months. On average, the low-carbohydrate (LC) group exhibited a 2.3-fold higher peak fat oxidation compared to the high-carbohydrate (HC) group. There was no difference in aerobic capacity between subjects (12).
It’s undisputed that this nutrition approach does increase fat oxidation and rates up to 1.9g/min have been reported (12). An extensive review (13) of the collective evidence of the influence of a LCHF diet highlights that endurance performance can be sustained at moderate intensities.
However, individual responses vary, and adherence to such a restrictive dietary regimen poses challenges. The diverse range of studied populations and different study approaches make it hard to draw clear and reliable conclusions from these investigations, particularly when subject numbers are limited.
Crucially, there is no conclusive evidence supporting consistent performance enhancement, even at moderate exercise intensities (13).
There is no conclusive evidence supporting consistent performance enhancement, even at moderate exercise intensities, for LCHF diets.
In an attempt to determine a performance effect following a LCHF diet, a study was carried out that involved elite male and female race walkers who underwent supervised training and repeat testing on different diets (14). The following diets were: Diet 1 – A high carbohydrate availability (HCHO), Diet 2 – periodized carbohydrate availability (PCHO), Diet 3 – a ketogenic low-carbohydrate, high-fat (LCHF) diet.
After the intervention, all groups showed an increase in aerobic fitness. The LCHF diet significantly increased whole-body fat oxidation. In terms of performance, the HCHO group showed the greatest improvement, with a 4.8% or 134-second improvement in the 10,000 m race time. The PCHO group also showed a trend towards faster performance, with a 2.2% or 61-second improvement. However, the LCHF group experienced a significant impairment in performance, with a 3.3% or 86-second slower race time.
Nutrition coaching
The study found that a ketogenic low-carbohydrate, high-fat (LCHF) diet reduced exercise economy and impaired performance in elite race walkers, despite increasing the capacity for fat oxidation during intense exercise. This was observed in both male and female athletes.
Reduced exercise economy means that more oxygen is consumed at a given work rate, which reduces the ability to sustain work rates at higher intensity. A LCHF diet has been shown to reduce the muscles’ ability to use carbohydrate as a fuel source, meaning that athletes lose their top gear and the ability to kick in and perform high-intensity exercise (13). In other words, there is a potential price to pay when consuming primarily fats in your diet.
The same study (14) looked at short-term effects of following a non-ketogenic LCHF diet for five days in well-trained athletes with replenishment of carbohydrate prior to exercise performance. Fat adaptation took place within 5 days showing an increased ability to use fat as a fuel source. Even when carbohydrate was introduced prior to exercise performance, despite this causing a reduction in fat oxidation rates, fat oxidation rates remained higher than prior to the fat adaptation strategy. Despite this fat adaptation, exercise capacity and performance were not enhanced.
Studies of LCHF diets tend to look at fat oxidation rates in well-trained elite athletes. Such strategies for adjusting carbohydrate intake (either periodizing carbohydrate or following LCHF) are sought to enhance the training effect of elite athletes looking for the extra performance gain.
For uphill athletes, it would be far more beneficial to focus on consistent training which will naturally achieve higher rates of fat oxidation along with the required physiological adaptations to strive towards full fitness potential. Following a LCHF diet will not fast-track the adaptation process of aerobic training. Sorry—there aren’t any shortcuts!
Following a LCHF diet will not fast-track the adaptation process of aerobic training.
Long training sessions (e.g. long runs and weighted hikes) are usually longer than two hours. As highlighted in our article on fasted training, this time spent training will deplete your muscle carbohydrate stores and your intake of carbohydrate during this session will not be sufficient to meet energy demands. This will naturally lead to higher fat oxidation rates in most athletes (15).
Since it’s unlikely that athletes eat enough carbohydrate to meet the energy expenditure demands of their training sessions, big mountain days or ultra endurance races, their bodies will naturally tap into fat as a fuel source and potentially influence fat oxidation rates without the need to follow any restrictive dietary approach.
LCHF DIET and MOUNTAINEERing
There’s been a surge in popularity of consuming a LCHF diet in the sport of mountaineering.
A recent pilot study looking at the effect of a 4-week energy balanced ketogenic diet on physical performance at very high altitude (16) shows that a LCHF diet may benefit performance at altitudes of 1500-3000m, however caution needs to be taken when interpreting these results. This was a pilot study and sample size was small with just six participants, and improvement in performance (measured by VO2max) was likely due to weight loss incurred by following the LCHF diet as opposed to the diet itself.
This paper also supports other research showing a LCHF diet worsened hypoxemia (meaning this exacerbated low levels of oxygen in the blood), and that following a LCHF diet above altitudes of 3000m will be detrimental to health and performance.
The sports nutrition consensus widely acknowledges that consuming carbohydrate around exercise benefits endurance performance (18). The intake of carbohydrates at altitude has been shown to benefit oxygen availability and there is an increase in glucose utilization at altitude. Carbohydrate is actually the preferential fuel source at altitude (17).
Considering that appetite is reduced at extreme altitude, a LCHF diet will limit an athlete to one type of food option, leaving very little variety. In base camps, the food provided is based on carbohydrate-rich options such as rice and lentils. Adding fats into your day in the mountains will be helpful to top off the energy supply and help to prevent weight loss and offer a flavor/taste/texture change.
THE IMPACT ON HEALTH
THE COST TO HEALTH
A LCHF diet is not advised for female athletes since it tends to result in low carbohydrate availability, low muscle and body carbohydrate stores, which pose a greater challenge to meeting daily energy demands for training and health (19).
Emerging research shows that low carbohydrate availability elicits a concern to athlete health independent of low energy availability (19). Carbohydrates help regulate the menstrual cycle, and they support the immune system and bone health. Low carbohydrate availability that is caused by limited carbohydrate before/during/after exercise can lead to poor bone health (bone injuries/stress fractures, low bone mineral density), menstrual dysfunction, impaired immunity, and a lowered resting metabolic rate (19).
Just a short-term period (six days) of carbohydrate restriction from following a LCHF diet has been shown to impair bone formation as well as suppress immune response and iron regulation in male athletes (20).
OTHER NEGATIVE IMPACTS
- Low carbohydrate availability
- Impaired training quality during high-intensity efforts
- Increased challenge to meet daily energy demands that supports training and health
- Compromised ability to use carbohydrates as a fuel source
- Reduced endurance performance opportunities
Although these recommendations might be different to what you have been following, it might be a great opportunity to try fueling a little differently, particularly if your training has felt flat, stagnant, or you feel more tired than you should based on training load.
We shared some key nutrition recommendations to support your training in our previous nutrition article. These will help you to fuel well before, during, and after your training sessions and allow you to optimize the hard work you are putting in!
Further listening: Nutrition 101 podcast episode with Rebecca Dent.
CONCLUSION
We’ve witnessed the breaking of world records in ultra-endurance events by some talented athletes who advocate a LCHF diet. It’s crucial to note that these athletes are highly trained, backed by extensive training histories, and supported by sports science support teams.
For example, Outside online magazine reported that Camille Herron employed a strategy of consuming less than 30g of carbohydrates per hour during the Spartathlon, but she follows a normal diet outside of races.
These achievements are a result of years of training, race experience and are highly individual and strategic. These results are not solely down to dietary choices as sensationalized by the media. It’s essential to recognize that a restrictive dietary approach like the LCHF diet will pose challenges in optimizing training and performance on the trails and in the mountains. We may just buy into the hype of the very few who do it, simply because fueling with carbohydrates becomes a much less interesting approach.
For Uphill Athletes with only a few months or a couple of years of training behind them, adopting a low-carbohydrate diet or restricting carbohydrate intake to less than 30g per hour won’t serve as a transformative performance factor.
For Uphill Athletes with only a few months or a couple of years of training behind them, adopting a low-carbohydrate diet or restricting carbohydrate intake to less than 30g per hour won't serve as a transformative performance factor.
Training and fitness gains are not enhanced by reducing carbohydrate intake; it takes years of consistent, dedicated training and nutrition practices that support the energy demands of training to see benefits show up in your endurance performance. Just like Zone 2 is important as a foundation to your overall fitness, so is fueling for your daily and training needs in order to optimize both your health and your performance.
Reviewed by Chantelle Robitaille, MSc.
SOURCES AND FURTHER READING
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Hetlelid KJ,Plews DJ, Herold E, et al. Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners. BMJ Open Sport Exerc Med 2015;0:e000047. doi:10.1136/bmjsem-2015- 000047.
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Purdom et al. Understanding the factors that affect maximal fat oxidation. Journal of the International Society of Sports Nutrition (2018) 15:3 DOI 10.1186/s12970-018-0207-1
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Jeffrey F Horowitz and Samuel Klein (2000) Lipid metabolism during endurance exercise. Am J Clin Nutr 2000;72(suppl):558S–63S.
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Volek, J.S., & Phinney, S.D. (2012). The art and science of low carbohy- drate performance. Beyond Obesity LLC.
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Volek JS, Noakes T & Phinney SD (2015). Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci 15, 13–20.
Phinney SD, Bistrian BR, Evans WJ, Gervino E & Blackburn GL (1983). The human metabolic response to chronic ketosis without caloric restrictions: Preservation of submaximal exercise capacity with reduced carbohydrate oxidation. Metabolism 32, 769–776.
Burke et al (2016) Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol 595.9 (2017) pp 2785–2807.
Volek JS, Freidenreich DJ, Saenz C, Kunces LJ, Creighton BC, Bartley JM, Davitt PM, Munoz CX, Anderson JM, Maresh CM, Lee EC, Schuenke MD, Aerni G, Kraemer WJ & Phinney SD (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism 65, 100–110.
Webster CC, Noakes TD, Chacko SK, Swart J, Kohn TA & Smith JA (2016). Gluconeogenesis during endurance exercise in cyclists habituated to a long-term low carbohydrate high-fat diet. J Physiol 594, 4389–4405.
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