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Originating in the early 20th century, scientists formulated the ketogenic diet to mimic long duration fasting, helping users enter the metabolic state known as ketosis (1).
Since then, however, research has forked, initiating investigations into how the ketogenic diet influences body composition and athletic performance leaving us with questions of whether the ketogenic diet is advantageous or even advised for the athlete.
As an athlete, understanding what the ketogenic diet is and how it might influence your performance is integral in your decision to make the change.
THE KETOGENIC DIET IS NOT JUST LOW CARB
Before you even consider using the ketogenic diet for athletic performance, you need to gain an understanding of what the ketogenic diet is and what it is not. Since the emergence of the ketogenic diet in mainstream media, there are many ideas of what the ketogenic diet is, but really, there’s only one definition.
The ketogenic diet, above all else, is a fat-based diet. I say this because a diet can be low carbohydrate, yet not be a ketogenic diet.
In clinical settings, the ketogenic diet involves eating extremely low carb, relatively low protein and high fat. Many studies regarding the ketogenic diet and health experiment in the range of 60-75% of total calories from fat, around 20-30% from protein and a measly 5% or less from carbohydrates, but some even go as high as 90% of calories from fat (2, 3, 4).
The purpose of this diet is to switch the body's primary fuel source from sugar to fat and ketones (byproducts of excess fat metabolism), creating the metabolic state known as ketosis. If you're merely eating low carb, but not adjusting protein and fat intake, you may never shift metabolism into ketosis, thus not providing many of the benefits that a true ketogenic diet can provide (5).
Eating a balanced diet creates an environment in our bodies where the use of energy is dynamic. In some situations, our bodies prefer to use glucose (sugar), other times fat and ketones, and in some cases, both. The ketogenic diet, however, is an attempt to shift metabolism towards almost entirely utilizing fat and ketones instead of carbohydrates. Achieving this requires that carbohydrate and protein intake be relatively low with fat intake being relatively high.
As I'll touch on shortly, the ratios of fat, carbohydrates, and protein you consume on a ketogenic diet can be dynamic, meaning your body and situation may allow for a different ratio while you remain in the metabolic state of ketosis. But determining this according to your own needs as an athlete will take time and experimentation.
Understand that removing carbohydrates from your diet alone is not enough to switch your body's metabolism to ketosis, and unless you reach this metabolic state, you’re not using a ketogenic diet. Instead, you'll need to drastically restrict carbohydrates, consistently monitor protein and drastically increase dietary fat intake.
SPORT & THE BODY’S ENERGY SYSTEMS
For a regular person looking to improve body composition through the ketogenic diet, how the body uses produces and replenishes energy is not of much concern. As an athlete though, your performance in your sport relies heavily on how the body metabolizes and uses energy.
This reality is essential to understand when considering the ketogenic diet because this method of eating aims to change how your body uses and stores energy and depending on your sport, this could have serious implications.
For simplicities sake, we’ll stick to the body’s three primary energy systems: the phosphagen system, glycolysis (anaerobic) and the oxidative system (aerobic).
The phosphagen system is the first energy system to be called upon, regardless of sport. This system provides energy in the form of ATP rapidly but isn't very efficient. In fact, this system is the target of creatine supplementation.
In the muscle, creatine attaches to phosphate and then donates this phosphate during times of energy demand to recycle ADP (metabolized ATP) back into the usable form of energy, known as ATP. This happens rapidly, but cannot be sustained for long durations (6)
Glycolysis is a bit different, as it's primary purpose is to metabolize glucose (sugar) to yield more energy than the phosphagen system. Mostly, glycolysis is called upon during high-intensity activities that last longer than 10 seconds. For example, a hockey player that relies on intermittent high-intensity sprints will depend heavily on glycolysis to produce energy throughout a game (6).
The aerobic system, often referred to as the oxidative system, yields a lot of energy, but only does so during more extended duration activities at lower intensities. For example, a triathlete will rely heavily on the oxidative system to metabolize fat and carbohydrates for energy production over a longer time (6).
Mostly, the energy demands of your sport will determine which system gets called on preferentially. While all three systems will be active to some extent, the overall energy demands ultimately determine the one used most. Using the triathlete example once more, this athlete will undoubtedly call upon all three systems, but likely use the oxidative system primarily to ensure consistent and prolonged energy production.
The reason this is important to consider and understand is that the ketogenic diet plays a significant role in how your body stores and metabolizes energy. For instance, if you're a high-intensity athlete that relies heavily on glycolysis for energy production, using a diet that completely removes carbohydrates (glucose) might not be the best option, given the requirements of your sport.
Unfortunately, research regarding the use of the ketogenic diet and athletic performance is mixed and not entirely conclusive. Some experimentation with proper study designs have indicated improved performance, while others show quite the opposite (7, 8, 9).
These findings are important to consider because it means at the very least, you should consider how your body uses energy during your sport and begin to draw your conclusions. While research can indeed point us in the right direction, the energy demands of your body and sport should determine how you consume that energy.
KETOGENIC DIET STUDIES IN SPORTS
Fortunately, due to the rapid popularization of the ketogenic diet in mainstream media, scientists are looking into the diet not only for health and body composition but also to understand how it influences athletic performance.
These studies help to guide our decisions with regards to nutrition, but as mentioned earlier, many are not conclusive. Use of this information should only commence after consideration of your sport and energy demands.
KETO & CROSSFIT ATHLETESIn recent years, CrossFit has become a staple in the athletic environment and since Paleo, a low carbohydrate diet, is often associated with CrossFit, understanding the influence of the ketogenic diet could shed light on how these diets influence high-intensity activity.
STUDY 1 – THREE MONTHSIn one study, recreationally-trained CrossFit athletes were recruited and placed on one of two diets for 12 weeks. Group one was instructed to follow a ketogenic style of eating while group two ate as usual, (a more balanced approach).
Before and after the commencement of the study, participants underwent performance testing by completing a 1-rep max back squat, maximum repetition pushup test and a 400m run.
After completing baseline testing, the intervention commenced. During this time, blood was analyzed to ensure ketosis to know that athletes were following the diet. During this time, subjects continued participating in standard CrossFit classes.
The results of the study revealed there was little influence on performance when using a ketogenic diet. Notably, however, it was reported that control participants experienced improvements in their back squat one rep max and VO2 peak while those in the ketogenic diet group did not (10).
Summary: No benefit of using the ketogenic diet with control participants improving performance to a greater extent than keto participants.
STUDY 2 – SIX WEEKSIn the second study of a shorter duration, researchers recruited recreationally trained CrossFit athletes and placed them randomly into either a ketogenic diet group or control. During this time, subjects in the ketogenic diet group were instructed to eat as much as they desired, but to keep carbohydrate consumption to 50 grams or fewer per day. Blood ketone measurements were administered to ensure that subjects were in ketosis.
Before the commencement of the six-week intervention, participants underwent baseline performance testing with the following:
- 500-meter row
- 40 bodyweight squats
- 30 abdominal sit-ups
- 20 hand release push-ups
- 10 pull-ups
Upon completion of baseline performance testing, the subjects took part in the six-week intervention where they completed CrossFit workouts of the day, otherwise known as WODs, four times per week, for time. Upon completion of the six-week study, subjects then repeated the baseline performance metrics to understand if the ketogenic diet afforded growth, performance decline or neither.
Interestingly, the findings were similar to study one in that there was no significant benefit or decrement as a result of using the ketogenic diet. It was noted that the ketogenic diet subjects displayed significantly greater weight and body fat loss than control (11).
However, a closer look at the dietary practices of participants in the ketogenic diet group shows us they were on average, consuming 200-400 fewer calories by weeks four through six, compared to control, which could explain the difference in weight loss, rather than due to the ketogenic diet.
Summary: No performance benefit of the ketogenic diet. Keto participants did, however, eat fewer calories, resulting in more significant body composition changes.
Based on the limited evidence in CrossFit athletes, it appears that the ketogenic diet has little influence over performance during standard CrossFit workouts compared to a regular diet.
However, it’s important to note there weren’t any direct performance benefits afforded by the ketogenic diet, but in one case, use of the diet did result in worse performance than control, which should be considered.
Based on research alone, preference should ultimately be the deciding factor with regards to how you consume food as a CrossFit athlete. However, extrapolating based on the current data suggests that the ketogenic diet might not be the best choice for the CrossFit athlete since there was no significant performance benefit and the ketogenic diet can often be difficult to maintain.
USE OF THE KETOGENIC DIET IN OTHER SPORTS
While evidence regarding the use of the ketogenic diet for CrossFit athletes might be minimal, other sports are being considered for using the diet, such as long duration endurance athletes.
Longer duration endurance events rely heavily on aerobic metabolism. This means the body uses oxygen to metabolize carbohydrates and fats to yield as much energy per fat or carbohydrate molecule as possible. While this process takes longer than glycolysis or the phosphagen system, allowing the body to primarily use this energy system ensures extended energy production for prolonged muscle contraction (6).
Theoretically, switching towards a ketogenic diet could create an internal environment favorable for long duration events. By changing to a fat-based metabolism, this should allow for energy production from fat molecules at a higher intensity, which means the athlete will have higher energy yield for long durations of time potentially improving performance.
KETO & ULTRA-ENDURANCE ATHLETES
In one fascinating study, researchers recruited twenty male, professional ultra-endurance athletes. These athletes were then divided according to their typical eating patterns whether they eat high carb or a low carb, ketogenic diet.
After recruitment and being assigned their group, athletes reported to the laboratory to perform two exercise protocols to reflect the various demands these athletes might experience during activity.
Test one was a maximal aerobic capacity test. Subjects are placed on a treadmill, which sequentially increases incline and speed until the athlete is no longer able to continue. During this time, their breath is analyzed to understand how much oxygen is being consumed and utilized (indicative of fat metabolism) and also how much fat metabolism is contributing to energy usage.
Test two, however, was more closely related to normal performance activities these ultra-endurance athletes might encounter. During this testing, subjects completed a three-hour treadmill run at 65% of their VO2 max (moderate intensity) and then had blood analyzed immediately after.
Amazingly, when athletes were keto-adapted and following a lower carb approach, they displayed significantly greater fat metabolism at higher intensities of VO2 max when compared to athletes consuming a high carbohydrate diet. This means that at higher intensities, these athletes are preferentially burning fat when normal athletes would be metabolizing glucose.
And this is corroborated by data showing pretty incredible numbers. Researchers of this study report that 86% of all energy contribution for the keto-adapted athletes was from fat. This is compared to a measly 56% contribution from fat for athletes eating a normal diet (12).
DRAWING MORE CONCLUSIONS
Based on these findings, we can begin to understand how the ketogenic diet might be advantageous under some circumstances and not so much in others. For instance, the current evidence suggests that using a ketogenic diet for CrossFit athletes might not provide any benefit or could result in reduced performance relative to normal eating patterns (10).
However, for elite endurance athletes that rely on long duration performance, shifting the body's metabolism towards preferentially utilizing fat might be a good option (12).
It is, however, worth mentioning that some studies have shown significant performance decrements as a result of switching to keto, but as I'll touch on shortly, it's possible these decrements were a result of not being adapted to the keto diet, rather than the ketogenic diet alone (8, 13).
Summary: The ketogenic diet does not adversely affect the keto-adapted ultra-endurance athlete's performance, allowing for greater fat oxidation at higher intensities, which may prove useful for the longer duration, higher intensity events.
IMPORTANT CONSIDERATIONS AS AN ATHLETE
As an athlete, there are different considerations to be made when considering a ketogenic diet. Since this diet significantly changes typical eating patterns and how the body metabolizes food and energy, making the change incorrectly could ruin performance and body composition. Understanding these variables is essential so you can make the right decisions for your individual needs and performance.
KETO ADAPTATION IS PARAMOUNT
Research suggests that the ketogenic diet can be useful as long as your body is adapted to this unique style of eating. For instance, in the ultra-endurance study, these athletes were adapted to their specific dietary pattern, and as a result, performance measures were almost the same between those that regularly eat high carb and those that eat low carb (12, 13).
Alternatively, when subjects switch to keto and are forced to perform immediately, that's when we see performance decline or no benefit (10, 11).
This is why I also suggest only switching in the offseason so that you have ample time to acclimate to this new eating pattern before performance becomes a significant area of concern.
IT TAKES TIME TO REACH KETOSIS
If you switch to a fat-based diet, this is probably much different from your standard eating patterns. By removing carbohydrates almost entirely, significantly reducing protein and potentially increasing fat intake two or even threefold, you'll probably feel tired, fatigued and possibly even nauseous until you can reach ketosis consistently.
This period, which often lasts anywhere from a few days to multiple weeks, often results in symptoms similar to the flu, which can reduce performance and make you feel terrible. Take special care to account for this possibility (13).
YOU SHOULD NOT MAKE THE SWITCH IN-SEASON
It's possible that you'll feel sick and experience reduced performance when switching to the ketogenic diet. For this reason, you should only make the switch in your offseason. As an example, taking it easy in the gym or during practice for two weeks might not be a big deal in the offseason but could drastically affect performance during the season.
Not to mention, while some of the studies reviewed previously show little to no adverse influence on performance, one study did show significant performance decline as a result of making the switch, which should be considered (8).
STICKING TO KETO CAN BE HARD FOR AN ATHLETE
While many athletes use tactics that will improve performance regardless of other variables, the ketogenic diet can be challenging. Unfortunately, it's also not a diet that you can stop and return to since transitioning into the metabolic state of ketosis is volatile and often difficult to achieve.
Before jumping into the ketogenic diet, you need to consider how doing so will change the foods you can eat. Do you love to eat carbs? Do you enjoy eating pizza with friends and family or having a nice hamburger on a bun? When you’re on the ketogenic diet, carbohydrate intake will likely be anywhere between less than 5% to only 20% of total calories. Are you willing to make this sacrifice?
Interestingly, one study evaluating the use of the ketogenic diet in athletes also asked the participants of how they felt during the study. While some responses were positive such as lacking hunger and feeling better, many of them also responded with feedback like a tedious food regimen, lack of energy, irritability, and constipation (8).
While many people do report positive benefits afforded by the ketogenic diet, many people also report adverse side effects or difficulty sticking to the diet and you should consider these factors before making the switch.
REQUIREMENTS FOR KETOSIS ARE INDIVIDUALIZED
Many people miss the mark on reaching proper macronutrient ratios for the ketogenic diet. As mentioned earlier, traditional ketogenic diets involve roughly 60-75% of total calories being derived from fat and for some treatments, even up to 90% (2, 3, 4).
However, your situation may not be “traditional.”
Everyone’s metabolism is different. Some people tolerate extreme amounts of carbohydrates with no issue, while others have trouble eating one hundred grams a day. For reaching ketosis, it’s more of the same.
The reality is that some people will be able to enter ketosis with ease, even if they're eating relatively high amounts of carbohydrates. Other people can eat little to no carbs and still have difficulty reaching ketosis. If you add athletics on top of this, your requirements may be entirely different.
A good starting point is to make the transition into ketosis successfully, using the traditional model of 60-75% of total calories from fat. Once achieved, you can then begin to manipulate where you get your calories from, while continuously monitoring ketosis. This way you can adjust your intake to match your preferences more closely, while still ensuring that you're reaching the metabolic state of ketosis.
The ketogenic diet is easily one of the hottest topics regarding body composition, health, and athletic performance but that doesn't mean it's the best option available.
Research indicates that for the most part, using the ketogenic diet doesn’t significantly alter performance in either direction. Some people find benefit while others display reduced performance. However, it’s entirely plausible that these performance decrements are due to not being adapted to the ketogenic diet (10, 11, 12, 13).
Further, research with keto-adapted elite ultra-marathoners suggests that a ketogenic diet could result in a more significant contribution of energy production from fat, even at higher intensities, which could lead to improved performance over a long period (12).
If you do decide that a ketogenic diet is the right move for you, remember there are considerations to be made. There is a transitionary period where you can expect to feel sick and have reduced performance. As a result, I recommend that you only make the switch during your offseason.
Finally, understand that using the ketogenic diet successfully will require that you restrict certain food groups and macronutrients almost entirely. This reality can make adherence a bit difficult, and you should consider this variable before making the change.