The high-fat, low-carbohydrate diet for potentially improved endurance performance has received a lot of attention in the media lately. Diet is an important consideration in athletic performance. Having a consistent, well-balanced diet as an athlete has been shown to correlate with increased success in sports performance. Currently, there are a variety of diets to consider, ranging from a vegan diet, ketogenic diet, high-protein diet, intermittent fasting, etc., all of which have attained mainstream popularity. However, in spite of all the popular press coverage and internet blogging, the peer-reviewed published scientific data to fully support this diet, and subsequently apply it widely for all levels of endurance athletes, is sparse. And the scientific literature that does exist is equivocal in nature. Additionally, no published studies to date have provided conclusive evidence that a high-fat, low-carbohydrate diet results in an actual improvement in endurance performance. This review aims to examine the evidence for the impact of an LCD on athletes’ aerobic, anaerobic, and upper and lower body strength performance.
The Science Behind Low-Carb Diets
The one clear conclusion from the scientific literature is that endurance athletes adhering to high-fat, low-carbohydrate diets become highly proficient at utilizing fat as a fuel source. Make no mistake, if your diet consists of almost exclusively fat that is what your body will adapt to and burn as your primary fuel source. Your body will become efficient at burning fat as a primary fuel source for all activities.
As mentioned, in order for your body to burn primarily fat as a fuel source, you have to enter a metabolic state known as nutritional ketosis, or simply ketosis. To have your body fully enter nutritional ketosis takes approximately three to four weeks. And during this time period you need to reduce your daily carbohydrate intake down to approximately 50 grams per day, or even lower for some people, perhaps as low as 35 grams per day. Keep in mind that one medium banana has approximately 25 grams of carbohydrate. Accordingly, your total carbohydrate intake for the day while trying to enter nutritional ketosis could consist of two bananas, or some other combination of carbohydrates, but no more than 50 grams. To accurately determine if your body has fully entered what is also known as a ketogenic state you must measure ketone levels in the blood. This is accomplished in a similar manner to how a diabetic measures blood glucose, via a finger stick to obtain a drop of blood and the use of a small hand held electronic monitoring device.
When you have entered nutritional ketosis your body will have fully depleted your glycogen storage (glycogen is the storage form of glucose). In a more traditional dietary state, when carbohydrate is abundant, the only fuel your brain and motor nerves (the nerves that connect to your skeletal muscles and allow contractions) can utilize to obtain ATP is glucose. Your body will not choose to utilize ketones for fuel, which are the byproduct of fat metabolism; they are produced when your body burns fat as fuel. However, when dietary carbohydrate intake is extremely low your body adapts, and is able to use the ketones that are produced as a byproduct of fat breakdown for fuel. Accordingly, once you have fully entered nutritional ketosis your daily dietary intake will need to remain very high fat and low carbohydrate.
Both dietary fat and carbohydrate are able to supply your skeletal muscles with ATP, the fuel for all body functions. There is no question that fat can be burned as fuel, but the issue related to running performance is you only get the ATP from fat breakdown half as fast as you get it from carbohydrate breakdown. Therefore, high intensity exercise becomes very difficult. When you "hit the wall" or "bonk" it is the result of your body being completely out of carbohydrate (glycogen storage is fully depleted), and you have to rely on fat as fuel. You are still able to run, but it feels extremely labored and hard. This is due to your fuel supply (ATP) only being delivered half as fast to your working skeletal muscles when you have to rely on fat as the way you obtain the ATP.
Read also: Safety of Low-Carb Diets During Lactation
Potential Benefits for Ultra-Endurance Athletes
Where there appears to be advantages to an extremely high fat diet, and being in a state of nutritional ketosis, is for athletes competing in ultra-endurance events. Running events that are at a minimum of 50 miles, and the longer the better. The published data indicate that athletes competing in ultra-marathon races that are 100 miles plus seem to benefit the most. In ultra-endurance races the intensity of exercise, the speed you are moving, is quite slow compared with a half or full marathon. And there are often several times in ultra-endurance running events where you stop running completely for nutritional or medical aid stations. These sorts of ultra-endurance events mimic the early hunter-gatherer continual pursuit hunting. Because the intensity of exercise in an ultra-endurance race is low (relatively speaking) burning fat as a fuel is not an issue. Obtaining your ATP only half as fast is still fast enough.
Risks and Considerations
Athletes focus on the carbohydrate and protein macronutrient dietary composition rather than fats, as fats do not play direct roles in glycogen storage or muscle hypertrophy. A low-carbohydrate diet (LCD) may help individuals decrease body fat and lose weight by greatly limiting carbohydrates (to <10% caloric intake or <50 g carbohydrates daily) and increasing fat intake; this may also be referred to as a ketogenic diet as it induces a state of ketosis. However, this diet will greatly restrict the two most important macronutrients for athletes and replace them with fats, which may be detrimental to athletic performance. Furthermore, competitive athletes often do not aim for weight loss, as many already have a low body fat percentage. Male athletes have a body fat percentage as low as 5% for bodybuilders. In comparison, female athletes have body fat percentages as low as 10% in elite runners, such that the ketogenic diet is unappealing to a variety of athletes. In addition, athletes utilizing a high-fat diet can be at risk of increased total cholesterol levels.
It is also important I believe to consider what living in a state of nutritional ketosis means from a long-term sustainable lifestyle and daily dietary perspective. Once your body is in a state of nutritional ketosis maintaining it requires that you consume very little carbohydrate every day. Typically carbohydrate intake can be no more than 15% of daily total calories, or less for some people. If you remove nearly all the vegetables and fruits from your diet your body will not naturally obtain a large number of essential vitamins, minerals, bioflavonoids, phytochemicals, and carotenoids that are found in these foods. All of those nutrients are important for optimum health. Simply put, I do not believe long-term nutritional ketosis is a sustainable lifestyle or diet for most people.
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. 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. 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.
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.
Read also: Best keto-friendly chips
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”. This is because it cannot be built up again later. If the build-up of bone mass is hindered by low-carb diets during the developmental years, the damage remains for life. 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. A study published in the Journal of Eating Disorders emphasizes that restrictive diets can increase the risk of developing eating disorders, which can significantly affect both physical and mental health.
Research Findings on Low-Carb Diets and Athletic Performance
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.
Aerobic Exercise Performance
Studies of aerobic exercise performance tests for participants on ketogenic diets of varying lengths are summarized in Table 1. In a randomized controlled trial with 16 semi-professional soccer players that followed a 30-day LCD, the Yo-Yo intermittent recovery test (of maximum aerobic fitness) indicated significantly increased aerobic performance on day 30 compared to the pre-dietary intervention baseline. However, a randomized sequence crossover trial with 14 recreational male athletes showed no change in their exercise efficiency or economy after a 14-day LCD. Furthermore, there were no significant changes in 10-km race times for 21 professional male walkers after a 21-day LCD clinical trial. However, a repeat of this clinical trial with 26 professional male and female race walkers showed a significant decrease in 10-km race times after a 5.5-week LCD.
In a two-arm randomized controlled trial, seven competitive male runners that followed a six-week ketogenic diet had no significant change in their 5-km time trial at days 4 and 42, nor did they have a difference in time to exhaustion after six weeks of consuming an LCD. In another randomized cross-over trial, 14 recreational male athletes had no significant change in work output on a 90-minute cycle ergometry test after two weeks on a low-carbohydrate, high-fat diet. In a randomized, repeated measures crossover study, eight trained male endurance athletes were tested in a run-to-exhaustion time trial following a 31-day ketogenic diet. When the respiratory exchange rate (RER) was >1.0, indicative of anaerobic conditions and a greater dependence on carbohydrate metabolism, there was a statistically significant decrease in the run time to exhaustion. However, for an RER <1.0, there was no statistically significant change. In addition, the athletes who completed the ketogenic diet had impaired exercise efficiency, which was more pronounced at >70% VO2max. In a randomized crossover trial, eight recreational male swimmers had no significant change in swimming economy at 50%, 60%, and 70% VO2max after a three-day LCD. In a randomized controlled trial, six well-trained competitive cyclists found a significant decrease in time-to-exhaustion cycling trials at 10°C and 30°C after a three-day LCD. 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.
Anaerobic Exercise Performance
Results of studies of ketogenic diets of varying lengths on anaerobic exercise performance tests are summarized in Table 2. In a randomized sequence crossover trial of a four-day ketogenic diet with exercise-trained participants, there was a statistically significant increase in anaerobic capacity as measured by the Wingate test. However, in studies where the ketogenic diet was longer, anaerobic performance either stayed the same or decreased. In a clinical trial with a four-week LCD, there was a notable drop in anaerobic performance in basketball players, with significantly decreased performance on the Wingate test. This contrasts with a randomized clinical trial using a 10-week LCD with trained college-aged men, in which researchers found that subjects’ Wingate performance test results did not significantly change.
Read also: Best Keto Tortillas
Strength Performance
Studies of upper and lower body strength after a ketogenic diet are summarized in Table 3. A one-repetition maximum weighted bench press was used to measure strength in the upper body. In contrast, a one-repetition maximum weighted squat or counter-movement jump (CMJ) test was utilized to measure lower body strength. To assess upper body strength, a randomized, parallel arm, controlled prospective study tested the one-repetition bench press maximum load of 21 strength-trained women before and after an eight-week LCD; no significant change was observed. In another two-arm randomized clinical trial, 20 experienced resistance-trained men underwent an eight-week ketogenic diet; again, no substantial change in their one-repetition bench press maximum load. However, a different two-arm randomized clinical trial found a statistically significant increase in bench press after a 10-week ketogenic diet in 25 resistance-trained college-aged men. Another randomized controlled parallel study, which utilized an eight-week ketogenic diet and included 19 male athletes, also observed that the participants significantly increased their bench press.
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. Similarly, a two-arm randomized clinical trial found a statistically significant increase in one repetition maximum squat load after a 10-week ketogenic diet in 25 trained college-aged men. Another randomized controlled parallel study had similar results, in which 19 male athletes significantly increased their one-repetition maximum squat load after an eight-week ketogenic diet.
To assess the effects of a six-week ketogenic diet on total upper and lower body strength, a single-arm within-subjects clinical trial examined the effects on total workout repetitions per week and maximum volume load, the multiplication of weight used by the set and number of repetitions, in 13 resistance-trained individuals. There were statistically significant increases of 11.4% in weekly repetitions and 7.6% in volume load. In a randomized crossover study, 14 trained powerlifters self-selected a lift of choice and underwent a three-month ketogenic diet. There was no significant change in their self-selected lift’s one-repetition maximum load before and after a three-month ketogenic diet.
Practical Recommendations
Let there be no question, and I cannot emphasize this point strongly enough. Most people could, and should, eat less carbohydrate every day. But what they need to cut out is the “junk” carbohydrate. Completely eliminate from your diet the processed and packaged high carbohydrate foods many Americans consume. Further, strive to completely eliminate all of the added sugars in prepared and processed foods. By added sugars I am referring to table sugar, brown sugar, all varieties of syrups, honey, confectioners glaze, dried cane extract, dextrose, high-fructose corn syrup, agave nectar, molasses, and all other calorie containing sweeteners found in prepared and processed foods and beverages. Do not be fooled into thinking that because something is sweetened with, for example, honey or agave that it is “natural and healthy”. It is not, and those sweeteners still count as added sugars in your daily caloric intake.
Replace all the carbohydrate you eliminate with nutrient dense, healthy fat choices and sources of high quality protein. Sources of high quality protein can be vegetarian and vegan or from animal sources, and many protein sources also include healthy fats. Vegetarian and vegan sources of high quality protein include quinoa, many varieties of beans (e.g., black, pinto, red, navy, kidney, lentils, lima, and great northern), chickpeas, non-GMO tofu and tempeh, hemp, chia seeds, seitan, and non-dairy milks (e.g., almond, soy, rice, and coconut).