Will a low carb diet improve athletic performance
Sport Nutrition

Can a Low Carb Diet Make You a Better Athlete?

Will a low carbohydrate diet improve your athletic performance? As athletes, many of us are willing to try just about anything if it means shaving a few minutes off our PRs. Whether it’s a new supplement, a new training program, or a new diet altogether if a technique promises performance enhancement, athletes are sure to be interested.

It comes as no surprise then that a low-carb diet has been gaining traction within the athletic community. Originally a diet that was heavily promoted for weight loss and diabetes management, it seems to have also caught on as a trend in the fitness world. Athletes wanting a lower body fat percentage, wanting to shave time off their PR, or even athletes who are simply interested in improving their overall health are giving it a try.

How exactly do we define a low-carb diet though?

If you ask 100 people you’re bound to get a wide variety of answers which definitely adds to the confusion and conflicting information when it comes to low-carb diets for athletes. Technically, a low carbohydrate diet would be any pattern of eating that provides 45% of calories or less from carbohydrates. This is because the “acceptable macronutrient distribution range” used in the development of dietary guidelines recommends a diet that contains 45-65% of calories from carbohydrates for the general population. Eating less than this 45% minimum would be considered “low-carb” compared to these dietary guidelines. A ‘ketogenic diet’ on the other hand is much much lower in carbohydrates, usually less than 10% of total calories. This is meant to induce “ketosis”, or “a ketogenic state” (but more on that later). Low-carb diets also vary in the way they treat dietary fibre, which is a carbohydrate that we are unable to digest, and therefore provides us with little to no calories.

Some low-carb diets focus on “net carbs” meaning they allow dieters to subtract the fibre from their total carb intake and base carbohydrate goals based off of this lower number, while others use “total carbs”, which includes the fibre with the calorie-containing starches/sugars. The remainder of calories from protein and fat also vary significantly between different types of low-carb and ketogenic diets which ultimately can have a huge impact on the results people experience, and also our athletic performance.

In a nutshell all ketogenic diets are low-carb, but not all low-carb diets are ketogenic.

It can get a little confusing, and at least partially explains why athletes have dramatically different experiences when they give these diets a try. The exact percent of calories you’re getting from carbohydrates, combined with your need for carbohydrates depending on your sport really makes all the difference. Another key factor is actually protein. A true “ketogenic diet” typically used for medical purposes is actually quite low in protein. This is because some amino acids (the building blocks that make up proteins) are actually “glucogenic”, meaning the body can use them to create glucose. Eating a very high protein diet can prevent the body from entering ketosis through this glucose creation. If you’re aiming for ketosis for medical reasons, too much protein is a problem. For athletes on the other hand, too little protein can seriously impair performance and body composition.

So, does a low-carb diet for athletic performance live up to the hype? Will eliminating carbohydrates to any degree help you run faster, jump higher, all while maintaining an effortlessly leaner figure?

How are carbohydrates used by athlete’s working muscles?

In order to understand the potential impact that a low-carbohydrate diet will have on athletic performance we need to understand the different types of fuel our bodies can use during exercise:

  • During low intensity activity (say, walking) we’re using mostly fat for fuel. This is because we’re in an aerobic state and fat is easily and completely broken down for energy.
  • When we’re exercising at a higher intensity, our body is able to shift gears and rely more on the carbohydrate stored in our muscles (glycogen) for energy instead.
  • The more we increase our exercise intensity (ie, the faster we run), the more carbohydrate we will need to sustain that activity.
  • We aren’t able to use fat as a primary fuel source during high-intensity activity because it requires too much oxygen to completely metabolize it. It’s carbohydrates or bust.

A byproduct of incomplete fat breakdown is something called ‘ketone bodies’. Ketones are where the “keto” diet gets it’s name from, because when we have high levels of ketones in the body we called it a “ketogenic state” or “being in ketosis”. These ketone bodies can be used as a backup fuel source during lower-intensity activity, similar to the fat molecules they come from.

Even when we limit the carbs in our diet, the body does and always will require a constant supply of glucose. Certain cells in our bodies (brain cells and red blood cells are two examples) require glucose as fuel. The brain can adapt to using ketone bodies over time (up to about 60% of it’s energy needs), but red blood cells have no alternative fuel choice. Because of this reality of being a human, the liver is able to create glucose on it’s own. (isn’t the liver amazing?) It’s able to do this by combining glycerol (taken from fat molecules) with amino acids to create new glucose molecules. Remember the amino acids I mentioned before that are glucogenic? This is where they come into play.

So, what does this mean for you as an athlete who’s curious about a low-carb diet? Basically it means that for most of us, a low-carb diet will not do anything to enhance our athletic performance, and for many athletes, a diet too low in carbs is actually more likely to impair performance. For speed events, team sports, or anything requiring bursts of speed/working at a high intensity for a long duration- running low on carbohydrates is going leave you feeling weak, fatigued, and slow. Your muscles simply run out of fuel!

Of course, like all things in life there are exceptions to the rule, so here’s a few areas that lower-carb eating may have some practical uses:

Ultra-endurance events, specifically races that would last more than 3 hours. So for many marathoners that have a finish time greater than 3 hours this could potentially apply to them too, and it has to do with the limitations of carbohydrate storage in our muscles and liver. Even with optimal carbohydrate loading, our muscles and liver can only store about 2000 calories of glycogen (plus or minus a few hundred depending on your body size). For endurance athletes racing much further or for longer than 3 hours, you’re going to need to bring along some extra fuel. Ultra-endurance athletes who are doing extremely long races would need to bring along a significant number of calories in order to stay fueled during their event.

Now, fortunately we also have body fat, which is essentially an unlimited fuel source. Obviously the more body fat you have the more potential fuel you’ve got, but for even for an athletic population who are quite lean, there is no shortage of calories there that can be used as fuel. Of course the intensity is really the issue here, since unfortunately at faster speeds we need carbohydrates no matter what. But, there is definitely some pros and cons that could be weighed in deciding if adapting to a low-carb diet is worthwhile.

The Pros and Cons of Low-Carb Eating for Athletes

On the pro side:

  • Being “keto adapted” means you could require less additional fuel during the race. This would allow you to carry less weight with you, meaning a lighter load and easier race. All you would need to ingest is fluids and electrolytes during your event.
  • Some athletes have difficulties with tolerating carbohydrates while training or racing, so not needing carbs on the run might save them a few trips to the bathroom.

On the con side:

  • If you are going to need bursts of speed throughout your event, there is a limited capacity to increase your intensity while racing low-carb. If you do have capacity to bring food along (say if you’re cycling or a triathlete), including carbohydrates in your nutrition strategy will definitely provide more opportunity for performance enhancement.
  • It takes time to train and become “keto adapted”, so if you have a race coming up this weekend and are used to consuming carbohydrates you’re probably not going to have success with this strategy
  • Central nervous system (CNS) fatigue (ie- brain fatigue) quickly leads to muscle fatigue. This means that when our brain begins to run low on carbohydrates we can start to feel that fatigue in our muscles as well. Ingesting a small amount of carbohydrate, or even just getting the CNS stimulation of the flavor of something sweet is enough to ‘perk up’ the brain and get a little pep back in our step.

Short Term Fat Loss 

Low-carb diets have been shown to be effective for short-term fat loss when used by athletes participating in physique or weight-class sports. While effective, this strategy should always be balanced with other factors such as the impact of a low-carb diet on athletic performance, the trade-off of potential muscle loss if training volumes are high, and the impact ‘dieting’ can have on an athlete’s body image and relationship with food. Basically, an overall balanced, well-planned nutrition strategy will pay more dividends long-term for any athlete vs having to ‘crash diet’ to lose weight quickly before a competition. Work with a sport nutrition coach to determine the best nutrition plan for you and your performance goals.

One final consideration

If you’re still thinking about a low-carb diet because of things you’ve heard surrounding insulin levels and fat storage, here’s an interesting research finding that shouldn’t be ignored: Consistently training “low” without adequate carbohydrate fueling has actually been shown to lead to changes in our muscle cell’s ability to utilize glucose after a training session, diminishing their ability to use carbohydrate as a fuel source in the future. So, avoiding carbohydrates and trying to exercise can actually have the opposite impact than you might be aiming for!

So, will a low carbohydrate diet improve your athletic performance?

While low-carb diets can be a useful approach for weight loss or diabetes management, when it comes to athletic performance eating low carb isn’t a winning strategy. Basically, if you’re racing to win or want to perform your best no matter the sport a low-carbohydrate diet is just not going to help you cross the finish line faster. Even if you’re just trying to beat your own PRs, cutting carbs out of your nutrition plan is very unlikely to pay off.

What will however, is a well planned, balanced nutrition strategy that takes into account your unique needs, goals, and training schedule. Rather than thinking about carbohydrates in your fueling plan as an “all or nothing” thing, tailor your intake for your needs! On heavier training days, or days when you’re doing two training sessions, be sure to get plenty of energy from carbs to fuel all that work! On your easier training days or rest days, eat a more moderate amount instead. Fuel your body adequately and you’ll definitely be on your way to becoming the best athlete you can be.

Do you want to learn how to optimize your athletic performance with simple, personalized nutrition strategies? Click here to get all the details for my one on one nutrition coaching programs, and Apply for a spot! 

About the Author: Stephanie Hnatiuk is a Registered Dietitian and Personal Trainer who specializes in helping athletes reach their peak potential with nutrition.

Here are a few additional references for further reading!


  • Burke, 2015. Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon? Sports Med. 2015 Nov;45 Suppl 1:S33-49
  • IAAF (Internation Association of Athletics Federation) Consensus Statement on Nutrition for Athletes (2019)
  • Maunder et al, 2018. Substrate Metabolism During Ironman Triathlon: Different Horses on the Same Courses, Sports Medicine, Volume 48, Issue 10, pp 2219–2226
  • McSwiney et al. Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes. 2018 Apr;81:25-34.
  • Purdom et al, 2018. Understanding the factors that effect maximal fat oxidation. Journal of the International Society of Sports Nutrition
  • Sansone et al, 2018.. Effects of Ketone Bodies on Endurance Exercise. Current Sports Medicine Reports: Volume 17 – Issue 12 – p 444–453
  • Scott et al, 2019. Keto-Adaptation and Endurance Exercise Capacity, Fatigue Recovery, and Exercise-Induced Muscle and Organ Damage Prevention: A Narrative Review. Ketones and Human Performance. J Spec Oper Med.;17(2):112-116.
  • Webster et al, 2018. A Carbohydrate Ingestion Intervention in an Elite Athlete Who Follows a Low-Carbohydrate High-Fat Diet. Volume:13 Issue: 7 Pages:957-960