The low carb diet and running may be popular, but is it best for your body and your performance? This post will break down the low carb diet for runners, what the really means, and how running on a low carb diet may impact performance. 

As an Amazon Associate, I may earn from qualifying purchases.

What is a Low Carb Diet?

A low carb diet generally refers to a dietary intake less than the recommended carbohydrate intake, which is about 45-65% of total calories. A low carb running diet could be anything below that.

Very low carb diets, such as the keto diet, limits carbohydrates to as low as 50g of carbohydrate per day. Pasta for running would not be an option on a low carb diet. 

Carbohydrates are one of the three macronutrients (protein and fat are the other two) that provide our body with calories (energy).

It is generally recognized that carbohydrates are the primary fuel source for the brain and the muscles. Therefore, an active individual needs more carbohydrates (fuel) than a sedentary individual.

Glycogen (stored carbohydrates) is the most important and most valuable fuel for moderate to high intensity exercise, such as long distance running.

Why Would A Runner Want To Do Low Carb Running?

Individuals typically follow a low carb diet in hopes of weight loss.

Since carbohydrates do help the body retain fluids, there may be a short-term drop weight reduction when starting a low-carb diet due to changes in the body’s fluid status. 

Additionally, one may try a low carb diet for certain medical conditions, such as diabetes, or blood sugar management.

I even find that athletes try to reduce carbs as part of their marathon taper nutrition plan, but we don’t really want to be doing that!

Doing low carb and running for weight loss may seem attractive, but how does it work. Low carb runners may be seeking a performance benefit, but is this what really happens?

There have been many studies done on athletes and the effects of low carb diets on performance.

There may be a slight benefit to low carb ultra running in that their bodies can rely more on fat for fuel so they don’t need to take in as much carbohydrates for longer distances.

Vegan ultra runners and low carb ultrarunners are two extreme ends of the spectrum. 

However, that’s not to diminish the importance of carbohydrates. It’s always a balance. Individual preference, gut tolerance and overall diet should be taken into consideration.

The research continues to show that carbs are the preferred fuel source for optimal athletic performance, meaning PR’s, high intensities, and quicker recovery.

How Many Carbs Do Runners Need?

Endurance athletes have long been using carbohydrates for optimal fueling and athletic performance.

Adequate intake of carbohydrates ensures adequate muscle and liver glycogen stores. Carbs and running go hand in hand.

In order to maximize glycogen (carb) stores, endurance athletes need 8-12 g/kg/day of carbohydrates. The low carb diet for runners is nowhere near these research-backed needs. 

To put that in perspective, a 150-lb. athlete who weighs 68.1kg would need 545-817g of carbohydrates per day. Here’s how you could reach those numbers with foods for carb loading. 

Note how that compares to the above mention of the keto diet at less than 50g of carbs per day!

Keto for runners, and even running and intermittent fasting, are generally not sustainable or recommended for long durations, performance benefits, immunity and injury prevention.

While eggs for runners are a great dietary protein source, pairing them with carbohydrates will give you more bang for your buck.

More on keto and running here. 

Carbohydrate recommendations for endurance athletes:

• Daily: 8-12 g/kg/day
• Pre-exercise: 10-12 g/kg/day + 1-4 g/kg (1-4 hrs. before event)
• During exercise: 60-70 g/hour or 90 g/hour if tolerated
• Post exercise: 8-10 g/kg/day first 24 hr.

Here’s how that translates into what to eat before a long run. 

The amount of glycogen stored in the muscles and liver has a direct effect on exercise performance. It’s been found that a high carb diet significantly increases endurance and will achieve a greater training effect.

Low muscle glycogen, on the other hand, will lead to early fatigue, reduced training intensity and sub optimal performance.

Furthermore, a low carb diet is likely to be missing key nutrients and micronutrients.

For example, magnesium for runners, is important for so many functions and is found in many carb-rich foods. 

Eating enough food is always king. If you need some ideas for real food fueling, check out this post on the best foods for ultra running. 

You can also use chews for running or gels to meet carb needs during long efforts. 

Carb Loading Strategies

Carbohydrate loading is a strategy commonly used among endurance athletes to increase muscle glycogen stores above normal levels. Carb loading for runners can help athletes exercise longer before the fatigue sets in.

Carb loading is good for endurance events lasting longer than 90 minutes or for events that involve several matches over a short period.

For events lasting greater than 90 minutes, carb loading in the 36-48 hours before the event may help improve performance by 2-3%.

Antony, Jenifer. (2017). Pre-Exercise Nutrition: Carbohydrates and Fluid Requirement for Training and Competition.

One example of how to implement carbohydrate loading: consume a moderate carb diet (5-7 g/kg body weight/day) for three days followed by a high carb intake (8-10 g/kg body weight/day) for the final three days before the competition/race.

Additionally, fueling with carbohydrates during endurance events also increases endurance performance.

For events lasting greater than 2.5 hours, higher carb intakes of 60-70 g/hour and up to 90 g/hour (if tolerated) are associated with improved performance.

You can see more about fueling for a marathon here and read about the best carbs for runners to enable proper fueling.

Should You Do Low Carb Marathon Training? 

Each individual is going to have different energy needs and a different response to training. When considering if you should do low carb marathon training, there are several things to consider, such as:

• What is your goal?
• Is your goal to finish the race, regardless of time?
• To achieve peak performance and possibly acquire a PR?

Once you are clear on your goals, that can help decide what strategies to use in training.

Next, think about previously experience. Have you tried running on a low carb diet before? If so, how did you feel? Was it sustainable?

 How was your energy level? Were you running or training at the time? If so, how were your performance times?

If you have not tried a low carb running diet before and are wanting to, it’s recommended to do so outside of peak training season. This is to allow your body to adapt and see how it feels so you can make necessary fueling and training changes prior to beginning a training season for a marathon.

Most runners who “train low” (more on that below) do so strategically, meaning not every run is low carb.

When it’s time to perform on race day, you would fuel with carbohydrates leading up to the race and on race day for optimal performance.

Marathon runners require higher carbohydrate intake prior to race day. Carbohydrate loading has long proven an efficient way to prepare the body with an excess glycogen availability to fuel the longer running times.

When athletes have been given a low carb, high carb, or moderate carb diet prior to exercise, the low carb diet athletes reached exhaustion faster than both the moderate carb and high carb athletes. For example, in one study of athletes cycling until exhaustion:

• High carb diet: 170 mins on bike
• Moderate carb diet: 115 mins on bike
• Low carb diet: 60 mins on bike

In summary, low carb training for a marathon runner is not recommended.

High carb diets have long been tested and continue to be recommended in endurance athletes.

Pros and Cons of Running on a Low Carb Diet

According to researchers, “training low” (low carb training) may improve oxidative enzymes, but an athlete’s tolerability to maintain their training load decreases and their quality of workouts and quality of overall training stress and adaptation declines.

 When athletes are in a “train low” period for a while, it can reduce the ability to generate max power in high-intensity situations.

Studies on low carb diets have found that athletes who were fatigued the fastest consumed the low carb diet, and failure to consume carbohydrates during pre and post training meals can cause muscular fatigue.

Testing in 8 off-road cyclists after 28 days of a low carb, high fat diet (15% Carb, 70% fat) found a significant decrease in max workload.

This reduction in max workload reinforces that fat adaptation may not benefit athletes desiring to function at very high intensities for prolonged periods of time.

Based on available data, there no data to support a low arb diet for runners for athletic performance and gains.  Carbs and running seem to go well together, why mess that up?

Want your running nutrition questions answered?
Fill out this form to be matched with one of our sports dietitians.

References:

• Antony, Jenifer. (2017). Pre-Exercise Nutrition: Carbohydrates and Fluid Requirement for Training and Competition. Conference: Sports Medicine : Applications and Recent Trends SMART 2017 At: V.O.Chidambaram College, Thoothukudi – 628008 Volume: OUTREACH Vol. X-Special Issue 1-2017 pg-134-138.
• Burke, L. M. (2007). Nutrition Strategies for the Marathon. Sports Medicine
• Costill DL, Flynn MG, Kirwan JP, Houmard JA, Mitchell JB, Thomas R, Park SH. Effects of repeated days of intensified training on muscle glycogen and swimming performance. Med Sci Sports Exerc. 1988 Jun;20(3):249-54. doi: 10.1249/00005768-198806000-00006. PMID: 3386503.
• Kaspar, M.B., Austin, K., Huecker, M. et al. Ketogenic Diet: from the Historical Records to Use in Elite Athletes. Curr Nutr Rep 8, 340–346 (2019). https://doi.org/10.1007/s13668-019-00294-0
• Snyder AC. Overtraining and glycogen depletion hypothesis. Medicine and Science in Sports and Exercise. 1998 Jul;30(7):1146-1150. DOI: 10.1097/00005768-199807000-00020. PMID: 9662687.
• Vitale K, Getzin A. Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations. Nutrients. 2019; 11(6):1289. https://doi.org/10.3390/nu11061289