Low-carb performance in the gym and on the road: can it work? A rational view.

Recently, on a popular Italian blog which supports the so-called “Paleo Diet,” I had the chance to read a series of articles about Ketogenic and Low-Carb diets. From the author’s point of view, they can’t work in the athletic performance, both in the endurance and the anaerobic/glycolytic context.

On top of that, always relating to the ketogenic diet, on the same blog is promoted the idea that energy balance doesn’t matter; therefore anyone can lose body fat while obtaining “anti-aging” effects without applying a voluntary caloric restriction “because of low insulin levels,” which should be enough to mobilize adipose tissue (ignoring the first principle of the thermodynamics).

In few words: “Ketogenic diet? Not OK for athletes, but advisable for those sedentary people who wants to lose body fat and lower the markers associated with cardiovascular disease”.

As a moderator of Ketogains -one of the largest international groups aimed at the study of nutritional ketosis applied to athletic performance and general well-being, by following an evidence-based approach- I want to explicit my thoughts concerning the arguments mentioned above:

A ketogenic diet is a nutritional approach which brings the dieter to restrict dietary carbohydrates enough to determine the production of ketone bodies. This happens when the hepatic glycogen store empties enough to start the production of BHB (Beta-Hydroxybutyrate) and bringing it over 0,5 mmol/dl. The vast majority of (sedentary) people can enter in a state of nutritional ketosis in 24-48 hours, by restricting net carbs assumption below 30g daily. 


Recent studies show that such metabolic state leads to better cardiovascular health, besides determining lower general insulin levels during the day, leading to better glycemic control and normalizing blood pressure in those affected by hypertension.  

The ketogenic diet is used yet in the clinic context for the treatment of neurodegenerative diseases because of the neuroprotective proprieties of BHB -ketone body found in the blood, used as an alternative energy source to glucose-.

Eminent clinicians as Dr. Ph.D. Dominic D’agostino, Dr. MD Peter Attia, Dr. Ted Naiman, researchers Dr. Phinney &  Dr. Volek and Dr. Ken Ford are still involved in the study of ketogenic diet and the various possible therapeutic applications.

Sources: (1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15).

We all agree at this point, except who isn’t properly instructed to read a bunch of articles about scientific evidence.

About the athletic performance theme, many ask themselves if this kind of diet is suitable for satisfying the physiologic functions of the organism in different athletic contexts.

To properly understand how to adopt an effective low-carb ketogenic approach aimed to support the athletic performance, it’s necessary to understand how the organism reacts to different kinds of effort.

-If you already know the metabolic pathways theme, feel free to skip this part-

The human body utilizes one “high-energy” molecule to manage every kind of metabolic necessity: ATP, which has three phosphate groups (heads). Energy is obtained by the breaking of one of those three bonds.  

During resting, ATP is stored in little quantities in the cells. Stored ATP is enough to deal with around 5 seconds of effort (the first two or three reps of a set of heavy squats). After that, the ADP byproduct obtained by the breaking of the third phosphate bond get readily reconverted in ATP by getting back a phosphate group from Creatine Phosphate (another high energy molecule found in cells which “donates” his phosphate head). This metabolic pathway (ATP/CP) can sustain 10 to 20 seconds of strenuous physical activity. After that period, the cell must switch over the anaerobic glycolysis, using glycogen and glucose for producing ATP, releasing lactic acid as a byproduct.

The accumulation of lactic acid in the muscles determine muscle fatigue, forcing the athlete to stop or to diminish the intensity of the effort. This kind of anaerobic pathway can sustain two minutes of strenuous physical activity at maximum.

Over this limit and diminished the intensity of the effort, the body relies on the oxidative phosphorylation (aerobic pathway) for continuing the ATP re-synthesis trough the Krebs cycle, the electron transport chain and the beta-oxidation of fatty acids.

So it is easy to understand why a standard ketogenic approach isn’t the best dietary style for those “elite” athletes, practicing highly glycolytic activities in the anaerobic context. Anyway, the ketogenic diet per se can sustain aerobic activity pretty well.  

This kind of physical effort indeed is determined in a range between 55% and 85% VO2 Max, which in the median person is equal to 65-85% Max Heart Rate.  

-VO2 Mx is the maximum volume of oxygen consumed for muscular contraction in a precise timeframe. It is measured in ml/kg/min.-  

Over 70% VO2 max -in those people who are following a standard carbohydrate rich diet- the main fuel for aerobic metabolism is muscle glycogen. Nearer you get to the upper limit of the aerobic range, more the organism will rely on muscle glycogen, stored in the cellular granules, rather than fats. Until to get entirely dependent on glycogen once the anaerobic threshold is reached.

Anyway in trained subject maximum sustainable VO2 for extended timeframes get higher, while achieving lower heart rates at rest. This is due to the superior efficiency of the cardiovascular system for what concerns oxygen transport, bringing to a higher limit of muscle glycogen consumption in terms of intensity. Therefore a well-trained cyclist or a marathoner can utilize more the beta oxidation during higher aerobic efforts when compared to an amateur, conserving glycogen more efficiently for the sprints or the most intense phases of the race. (Holloszy JO, Coyle EF. 1984)

A dietary context scarce with carbohydrates (read low-carb), as already explicated, brings lower insulin levels. This makes the adipose substrate more accessible for the energy metabolism. Therefore those people who follow a low-carb diet ends up -after an adaptation period- to utilize in a higher way triglycerides and ketone bodies for the energy metabolism of the cell, becoming less glucose-dependant.

For these subjects, this has an inevitable impact on the percentage of fats and glycogen utilized during and an endurance event. As proved by this study made in 2015 at the University of Ohio, led by researchers Phinney & Volek (Metabolic characteristics of keto-adapted ultra-endurance runners 2016), where a group of 20 people composed of ultra-marathoners and triathletes has performed an endurance test for 180 minutes at 63% VO2max, to determine the metabolic response, following an ingestion of a pre-workout containing 340 Kcal.

Of those 20 athletes, 10 were following an high-carbohydrate diet (59% carbs, 14% protein and 25% fats), while the other 10 had gone through 20 months of low-carb (10% carb, 19% protein, 70% fats). During the test, the low-carb group showed a major peak of fatty acids oxidation (1,54g per minute versus 0,67g for the high-carb group). The mentioned peak has been registered at a higher VO2Max than the high-carb group (70,3% versus 54%).

Since the two groups were at the same level of athletic preparation is legit to assume that the low-carb group is less glycogen dependent during the races, augmenting the working margin before it is necessary to quickly replenish” the muscle glycogen during a very long endurance event.

This study appears to confirm what was suggested by the recent sportive chronicles, mentioning low-carb ultra-runners Timothy Olson and, more recently, Zach Bitter, breaking the world records in their respective disciplines.


It would be false, however, to state that the mentioned athletes follow a standard ketogenic approach during the events. This kind of races can make the athletes who participate in burning as much as 10000 Kcal. It doesn’t matter if the athlete is keto-adapted or not, part of the “fuel” will be tapped from the glycogen muscle stores which can account only as little as 2000 Kcal. The keto-adapted athletes, anyway, can get the advantage of tapping more efficiently from the adipose substrate, therefore not necessitating high carb-loads pre-race, and the “sugary gels” consumption for preventing the bonking effect is as little as 1/4 compared to the median consumption of the high-carb runners, much more glycogen dependent than their low-carb colleagues. They just empty their glycogen stores faster!

Therefore it is legit to hypothesize that the low-carb ketogenic approach, by improving insulin sensitivity and the capability to use fats for the energy metabolism, is a valid instrument in the arsenal of the endurance athlete (which maybe is inclined to develop insulin-resistance), for using carbohydrates in a more efficient way during the races.

This doesn’t mean that an athlete who is not inclined to develop insulin-resistance can’t adopt an high-carb approach for improving his performance.

The best diet isn’t the one that eliminates dietary carbohydrates “just because.” The focus should be put on the maintenance of a high insulin sensitivity, aiming at the highest carbohydrates consumption depending on the context and the individual.

For what concerns the anaerobic disciplines, a fast and closed-minded conclusion could be that the ketogenic diet isn’t right for sustain this kind of physical activities, being the beta oxidation non-existent in the anaerobic context and that it is crucial to replenish muscle glycogen and the end of every workout session, by ingurgitating a generous quantity of dietary carbohydrates.

The truth is that a study of 1998 showed that a total body workout composed of 9 exercises of 3 sets each, executed at 80% 1RM (One-repetition maximum) causes a depletion of muscle glycogen equal to 1/3 to the total (-36% for each muscle group).  (B.D. Roy, M.A. Tarnopolsky)

Another study of 1993 (Glycogen resynthesis in skeletal muscle following resistive exercise) shows that after performing 6 sets of leg extension at 70% 1RM until muscular failure and without assuming any post-workout (only water), it is possible to replenish 75% of the muscle glycogen trough the gluconeogenesis processes.

The Cori cycle, indeed, re-synthesize muscle glycogen by recycling the lactic acid (converted into lactate) in the liver.  This pathway is ecori1nough to deal with the glycogen necessities of the organism in the absence of dietary carbohydrate. On top of that, the gluconeogenesis processes utilize even the glycerol backbone of fatty acids, broken by the triglycerides combustion (even if in small quantities). In the end, part of the dietary proteins gets always decomposed and converted into glucose independently from the quantitative assumed through the diet.

The only context in which is useful to consider an exogen form of carbohydrate in order to expedite the muscle glycogen replenishment process is when two distinct highly glycolytic workout sessions are performed in a timeframe of 8 hours. For example by practicing a double resistance training during the same day (morning and afternoon), exercising the same muscle groups.

Out of this context, taking carbohydrates in the post-workout is not necessary. Source (Alan Albert Aragon, Brad Jon Schoenfeld 2013).

Anyway, for a mere concept of performance optimization, an athlete that wants to obtain the benefits deriving from a  state of nutritional ketosis, without renouncing to his performance in the gym -after an adaptation period- can practice the so-called TKD (Targeted Ketogenic Diet).

The TKD approach advice to assume a modest amount of high GI carbohydrates (dextrose/glucose) 15-20 minutes before the workout session. The higher insulin sensitivity makes the keto-adapted athlete to use those carbs in an efficient way to enhance his performance in the gym or during an exhausting soccer match (where there is a continuous switch between high-intensity and low-intensity effort), suspending the state of nutritional ketosis temporarily.

Anyway, the keto-adapted athlete in spite of the high-carb one can use promptly even the ketones accumulated in the blood during the resting period with the carbohydrates, reducing the need of glycogen for performing those activities.

A well executed TKD protocol makes possible to re-establish nutritional ketosis in an hour from the end of the workout session when the insulin levels (which however would raise even without pre-WO carbohydrates) return to the baseline.

It is important to enlight to those who workout in a “casual” fashion, both in the aerobic and anaerobic context that for dealing with the Sunday’s soccer match or the regional competition, even those who workout in the gym for 3/4 times per week that isn’t necessary to practice “carb-loads” of every kind or very high-carbohydrate diets. The vast majority of you (readers) would not be able to utilize in an efficient way such an abundance of carbohydrates, and some of you could even experience the symptoms of insulin resistance in the long run.

It is important on the other hand to consider that in the ketogenic context, due to the lower insulin levels, it’s fundamental to integrate electrolytes (magnesium, potassium, and SODIUM) in a higher way than those who are following a high carb diet, because of the minor water retention.


Quite often those who can’t perform well while following a low-carb diet found themselves to experience the symptoms of a sodium deficit while thinking that the fatigue is caused by an “excessive” dietary carbohydrate restriction when that’s not the case. The athletic trainer Lyle McDonald, among the pioneer of the ketogenic diet applied in the sportive context, suggest assuming 5000mg of sodium on a daily basis to everyone who follows a ketogenic diet while practicing sportive activity.

The ketogenic diet is not limited by the application of an arbitrary “ketogenic-ratio.” The ketone production, as already explained, is subjected to the depletion of hepatic glycogen, not the muscle glycogen. The macronutrient percentages assumed by those who follows a ketogenic diet can vary a lot between the different contexts.

An individual who wants to lose body fat by following a ketogenic diet will be subjected to the law of thermodynamics without exception. Therefore he will have to determine a caloric deficit. The fact that eating non-refined foods on a low carb diet brings quite often to faster and more evident results is derived from this simple explanation: yes, the insulin sensitivity is restored in a certain measure, but eating whole foods brings to higher TEF (thermic effect of food). On top of that “paleo” or whole food is less calorie dense at the same volume when compared with processed food.  Therefore is way simpler to adhere to the individual TDEE (total daily energy expenditure) without going in a caloric surplus, by stimulating in a higher way the stomach mechanoreceptors, determining higher satiety. This combination causes the subject to be in a caloric deficit without being aware.

However, an individual who wants to get under 10% body fat (men) or 20% (women), will have to monitor the caloric intake carefully even if following this kind of diet.

In conclusion, the topics in this post/article should offer constructive suggestions and encourage educated experimentation. The cited scientific evidence can’t be ignored. Trashing the low-carb branch of thought about sportive performance completely denote only a degree of mental closure that doesn’t fit anyone who wants to give a contribute to the compass of nutrition sciences.

Be wary by anyone who proposes easy solutions “just cuz we always have done this way.”