The topic of low-carbohydrate, high-fat (LCHF) diets for athletes has become a subject of much discussion, challenging the traditional emphasis on high carbohydrate intakes for optimal performance. While carbohydrates have long been considered essential for athletes, particularly those engaged in moderate to high-intensity exercise, the potential benefits and risks of LCHF diets warrant careful examination.
Carbohydrates: The Traditional Fuel for Athletes
The conventional view of athletic nutrition emphasizes the importance of carbohydrates as the primary fuel source for muscles during exercise. This recommendation is based on the premise that muscles predominantly use glycogen (stored carbohydrate) as fuel during moderate to high intensity exercise and performance declines when liver/muscle glycogen stores become depleted. When carbohydrates are consumed, they are broken down into glucose, which is either used immediately to generate energy or stored in the muscles and liver as glycogen. Carbohydrate-rich diets help athletes replenish muscle glycogen stores, utilized during high-energy demand activities to rapidly produce adenosine triphosphate (ATP) for muscle contraction. Furthermore, carbohydrates are known to help with post-exercise recovery in athletes via the proposed accelerated replenishment of both liver and muscle glycogen. A high-carbohydrate diet (75-80% calories from carbohydrates) has been shown to correlate with increased aerobic performance during time-to-exhaustion trials and peak performance power on cycle ergometry testing.
The Rise of Low-Carb, High-Fat (LCHF) Diets
The importance of carbohydrates for athletes has recently been challenged by proponents of low carbohydrate, high fat (LCHF) diets, who suggest that high carb intakes are unnecessary for endurance athletes. Burning fat could potentially improve exercise performance through ‘sparing’ carbohydrate stores within the muscle/liver, which would avoid glycogen depletion and a subsequent decrease in exercise performance. LCHF diets aim to shift the body's primary fuel source from carbohydrates to fats. This metabolic shift, known as ketosis, occurs when carbohydrate intake is severely restricted, typically to less than 50 grams per day. In ketosis, the body breaks down fats into molecules called ketones, which then become the primary source of fuel for the body. This approach has gained popularity among some athletes, particularly in endurance sports, who believe that it can enhance fat oxidation and improve performance.
Potential Benefits of LCHF Diets for Athletes
Fat Adaptation and Glycogen Sparing
The primary theoretical benefit of LCHF diets for athletes is the enhanced ability to burn fat as fuel. By training the body to rely more on fat, athletes may be able to spare their limited glycogen stores, potentially delaying fatigue during prolonged exercise. A period of endurance training itself can increase the contribution of fat to total energy expenditure during exercise, irrespective of diet. The energy for ultra-endurance events, such as an Ironman Triathlon, could theoretically be achieved by an athlete who has trained their body to use fat as a fuel (fat adapted).
Body Composition and Power-to-Weight Ratio
Another potential advantage of LCHF diets is their effect on body composition. In terms of endurance athletes, a LCHF diet could theoretically improve performance by decreasing body fat. For example, if cardiovascular fitness remains the same, losing body fat would be beneficial to a marathon runner due to an improvement in power-weight ratio. The health benefits of low carb diets, such as weight loss, have been well documented, predominantly amongst overweight/obese individuals.
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Metabolic Benefits
Recent research indicates that a short-term low carbohydrate diet, or even training in a fasted state, can increase the rate of fat burning and improve the volume/function of the mitochondria (energy producing cells). Therefore, in recent times, some endurance athletes have adopted a strategy where training sessions are deliberately performed with low carbohydrate stores and carbohydrates are consumed prior to competition. This strategy has been termed ‘train-low, compete-high’.
Other Reported Benefits
Beyond performance, some athletes report additional benefits from LCHF diets, such as reduced inflammation, improved blood sugar control, and enhanced mental clarity. One study participant reported that the ketogenic diet helped with symptoms such as waking up multiple times a night, experiencing big energy shifts throughout the day, and noticeable swelling in the legs and ankles after big workouts and races.
Potential Risks and Drawbacks of LCHF Diets for Athletes
Reduced Glycogen Synthesis and Impaired Performance
Low-carb diets lead to reduced glycogen synthesis. A study in the American Journal of Clinical Nutrition confirms that athletes on a low-carb diet had significantly lower muscle glycogen stores than those on a high-carb diet. This reduction can lead to an early onset of fatigue and reduced endurance performance. Competitive endurance athletes should apply LCHF with caution if performance (winning a race, achieving a personal best) is the focus, as endurance sports often involve brief bouts of high and near maximal intensity exercise (e.g. breakaways and sprint finishes). A LCHF diet could be counterproductive to performance in sports that involve frequent periods of high-intensity exercise (e.g. football, tennis, rugby), as muscle glycogen is an important fuel during high intensity exercise.
Impaired Recovery
Carbohydrates play a crucial role in recovery after intense exercise. A low carbohydrate diet can impair recovery by interfering with the replenishment of glycogen stores. This significantly impairs the quality of the next workout. Replenishing carbohydrate stores and consuming high quality protein is also very important for muscle recovery.
Reduced Training Intensity and Increased Risk of Injury
Athletes on a low-carbohydrate diet show difficulties in maintaining high training intensities. Due to impaired recovery, earlier performance dips and lower energy levels, athletes on a low-carb diet were able to train significantly less and worse than their training partners during a training camp. Insufficient energy availability also increases the risk of injury due to impaired muscle regeneration. A study published in the Journal of Athletic Training shows that athletes who do not consume enough carbohydrates are at a greater risk of overuse injuries.
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Cognitive Function and Nutrient Deficiencies
A carbohydrate-rich diet is not only crucial for physical performance, but also for cognitive function. Low carb diets can lead to reduced concentration, which can be dangerous during races or long training sessions. Low carb diets can lead to an undersupply of other important nutrients. Many carbohydrate-rich foods provide important vitamins and minerals that are essential for overall health. A lack of these nutrients can lead to deficiencies that affect both performance and general well-being.
Hormonal Imbalance in Female Athletes
Female athletes have specific nutritional requirements that differ from those of their male counterparts. Hormonal fluctuations during the menstrual cycle can affect energy metabolism and nutrient requirements, making adequate carbohydrate intake even more important. The menstrual cycle influences energy utilization and glycogen storage. Data shows that women have a higher carbohydrate requirement during the luteal phase of the menstrual cycle, as insulin sensitivity and energy consumption change. Carbohydrates play a crucial role in maintaining bone density. Low energy availability (LEA), which often occurs with low-carb diets, can reduce bone density and increase the risk of stress fractures. This is particularly important for young athletes who have not yet reached their “peak bone mass”. The pressure on female athletes to maintain a certain body weight or appearance can lead to disordered eating habits. Low-carb diets, often seen as a quick fix for weight control, can promote unhealthy attitudes towards food.
Potential for High Cholesterol
Some protein sources - such as fatty cuts of meat, whole dairy products, and other high-fat foods - can raise cholesterol, increasing your chance of heart disease. The high-fat nature of the diet could also have negative impacts on heart health. The American Heart Association recommends limiting saturated fat intake to less than 6%. While high-fat diets can elevate LDL (“bad”) cholesterol, the type of saturated fat matters. The bottom line is that having more fats in your diet can lead to higher cholesterol.
Other Risks
The keto diet may not be appropriate for everyone, specifically people with kidney disease. Some people also experience dehydration on the keto diet because they’re eliminating glycogen, which holds water, from the bloodstream. Following a keto diet can also cause your blood sugar to drop, which can be dangerous for people living with diabetes.
Scientific Evidence: Mixed Results
The scientific evidence regarding the effectiveness of LCHF diets for athletes is mixed. Some studies have reported improvements in fat oxidation and metabolic markers, while others have shown no significant performance benefits or even negative effects.
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Studies Supporting LCHF
A more recent study (Volek et al. 2016) reported high rates of fat oxidation amongst elite ultra-endurance athletes following a LCHF diet (10% carbohydrate, 19% protein, 70% fat) diet for an average of 20 months. At the end of the study all dietary groups (1. high carb, 2. periodised carb and 3. ketogenic) improved aerobic capacity by 3-7%, indicating physiological markers of performance can improve following a LCHF diet. One study showed that after a four-day ketogenic diet, there was a statistically significant increase in anaerobic capacity as measured by the Wingate test. Another randomized controlled trial found a statistically significant increase in bench press after a 10-week ketogenic diet in 25 resistance-trained college-aged men. For lower body strength, a randomized, parallel arm, controlled prospective study tested the one-repetition maximum squat load of 21 strength-trained women after an eight-week LCD; a significant increase was found in both one-repetition maximum squat load and CMJ.
Studies Showing No Benefit or Negative Effects
During my fist discussion with the journalist I mentioned the seminal 1983 study by Phinney and colleagues, which is often presented as evidence that a LCHF diet can improve endurance performance. In this study, five trained cyclists consumed a ‘balanced diet’ for 1 week (1.75 g protein/kg/day, 66% CHO and 33% fat). This was followed by a 28 day ketogenic diet (<20 g/carbohydrate/day). Exercise capacity was assessed by the time it took the cyclists to reach fatigue when cycling at 63 % of their VO2max. The results showedthe time to fatigue was not statistically different between the balanced and ketogenic diet conditions (147 ± 13 vs. 151 ± 25 min). Supporters of LCHF for athletes often cite the (1983 and 2016) studies above to support the theory that a LCHF diet can improve endurance performance. The studies did not actually measure performance which is relevant to athletes - there are no gold medals for the highest rates of fat burning! The participants’ diets were not closely monitored throughout the studies, so It is uncertain whether the participants stringently adhered to the ketogenic diet, as short-term (3 day) dietary reporting has been shown to be unreliable. A low carb diet does not improve performance, was the obvious answer to the journalists original question. Every week the participants performed a 20-minute cycling time-trial (TT) to assess performance. The increased fat burning capacity, and lower body fat outcomes did not translate into improved TT performance. In a comparative study, 22 professional male soccer players significantly decreased the total distance covered in a 90-minute soccer game after a four-day LCD.
Systematic Review Findings
A systematic review of original studies was conducted for articles indexed in PubMed that utilized an LCD in athletes and reported athletic performance values. A total of 19 studies were included in the final synthesis. An LCD may aid in maintaining or increasing upper and lower body strength. However, this type of diet provides no consistent benefit and may even negatively impact some measures of aerobic and anaerobic performance. Taken together, no strong evidence indicates the benefit of an LCD on athletic performance.
Individualized Approach and Contextual Factors
The suitability of a LCHF diet for athletes depends on several factors, including the type of sport, training intensity, individual metabolic response, and personal goals.
Type of Sport
Endurance athletes, such as marathon runners and long-distance cyclists, might fare better on a ketogenic diet than players who use short bursts of energy. A LCHF diet could be suited to athletes wishing to decrease body fat and for individuals who perform most of their training at low-moderate intensities.
Training Intensity
Competitive endurance athletes should apply LCHF with caution if performance (winning a race, achieving a personal best) is the focus, as endurance sports often involve brief bouts of high and near maximal intensity exercise (e.g. breakaways and sprint finishes). However, it has been suggested that frequent training in a low carbohydrate state could reduce the capacity to burn carbohydrate fuels during high intensity exercise.
Time of Season
All of the above factors are important when considering a dietary strategy for athletes.
Practical Considerations for Athletes Considering LCHF Diets
Gradual Adaptation
If you’ve been following a low carb diet, don’t rush back into high carb eating overnight. Your gut and metabolism need time to readjust. Start with one meal per day. Add 25-50 grams of carbs every few days.
Monitoring and Adjustments
Athletes adopting a LCHF diet should closely monitor their performance, energy levels, and overall health. It’s important to work with a qualified nutritionist or sports dietitian to ensure adequate nutrient intake and to make adjustments as needed.
Emphasize Nutrient-Dense Foods
When following a LCHF diet, it’s crucial to prioritize nutrient-dense foods, such as lean proteins, healthy fats, non-starchy vegetables, and low-carb fruits.