The Keto Diet for Athletes: How Low Carb Should You Go?

Chris Albert
Written By: Chris Albert
August 2nd, 2018
Updated: June 13th, 2020
Categories: Articles Nutrition
19.5K Reads
The Keto Diet for Athletes: How Low Carb Should You Go?
A lot of controversy surrounds the traditional ketogenic dieting practice - especially when it comes to performance athletes. Can athletes eat a keto diet?

If you asked me five years ago whether the keto diet would ever catch the mainstream, I would have said never in a million years.

I’ve been doing one form of the keto diet or another for over 6 years now and with great benefit.

It’s helped me overcome ulcerative colitis, maintain a lean body, and it pulled me away from the 6 meal a day bodybuilding paradigm.

But the truth is that I’m a bit annoyed by the “Cult of Keto” that’s popped up around the dietary movement. We’re seeing a lot of overnight social media gurus without any real nutrition training doling out advice.

This leads to a lot of cookie cutter recommendations. For example, I read a lot of blogs telling people that if they go over 30 gram of carbs a day or over 0.6 grams of protein per pound of bodyweight, they’ll never get into ketosis.

That may cut it for a 50 year old overweight man whose pre-diabetic, but what about athletes?

The truth is that athletes, especially strength and combat athletes, require enough protein and some carbs if they are going to be able to perform. The goal here isn’t just to get into ketosis. It’s to leverage fat adaptation for increased strength, endurance, and recovery. This is why we see so many athletes crashing and burning when they follow popular low carb recommendations.

Related: 3 Things You Can Learn From Keto Dieters

But how many grams of protein and carbs can athletes sustain while still being able to benefit from fat adaptation?

To answer this question, we’ll explore one of the most important studies of the benefits of keto-adaptation for athletes over the past few years.

The Study

In 2016, Dr. Jeff Volek and his team published “Metabolic Characteristics of Keto-Adapted Ultra-Endurance Runners1.” This study is important for a few reasons:

  • It’s one of the few randomized control trials using live human subjects that compares low carb athletes with high carb athletes in athletic trials.
  • It’s the only study that compares carbohydrate and fat oxidation between low carb and high carb athletes during performance. We know what fuel sources these athletes were actually using while they were doing the trial.
  • It’s the only study that compares glycogen concentrations pre and post exercise between high carb and low carb athletes.

The population studied was 20 ultra endurance athletes who were active competitors in ultra marathons or triathlons.

Healthy Fat options for Keto Dieters

For 20 months prior to the trial, 10 of these athletes ate a high carb low fat (HCLF) diet consisting of approximately 60% carbs, 25% fats, and 15% protein, and 10 of these athletes ate a low carb high fat (LCHF) diet consisting of approximately 10% carbs, 70% fat, and 20% protein.

After following their habitual diets they were brought into the lab for a series of trials including a VO2 Max test and a submaximal 3 hour run done at 64% of their VO2 Max. Fat and carbohydrate oxidation rates were measured in both trials using indirect calorimetry.

Muscle glycogen retention was measured before, during, and after the run from muscle biopsies taken from the outer thigh. Blood tests were also taken before, during, and after the 3 hour run.

Here’s what they found:

  • During the VO2 Max test and the submaximal run, the LCHF athletes oxidized fat at twice the rate of the HCLF athletes.
  • During the VO2 Max test, the LCHF athletes had a peak fat oxidation rate at about 70% of their VO2 Max, while the HCLF athletes had a peak fat oxidation rate at about 55% of their VO2 Max. Previously, it was thought that a disadvantage of low carb diets was that athletes would not be able to perform high intensity exercise without switching to glucose. This study demonstrated that fat adapted athletes could burn fat even during intense exercise.
  • The most surprising finding was that the LCHF and the HCLF athletes had similar amounts of glycogen in their muscles before and after the three hour run despite the fact that the LCHF were limiting their dietary carbs habitually for an average of 20 months. This means that the LCHF athletes were actually more efficient at using and retaining glycogen than the carb fueled HCLF athletes.

So why should you, a strength or physique athlete, care about a study involving a bunch of endurance athletes?

You should care because better ability to oxidize fat while retaining glycogen means that fat adaptation is a potential way for you to be able to burn fat efficiently while building muscle, gaining strength, and improving athletic performance.

But How Can I Build Muscle on a Low Carb and Low Protein Diet?

Yeah, yeah, I can hear you now, “my cousin/friend tried a low carb diet for his show and he got weak, tired, flat and sick.”

Normally, when this happens, it’s because athletes are following extremely strict recommendations that aren’t meant for athletes.

If you’re using the keto diet as a therapy for cancer or epilepsy, then you probably need to follow the very high fat with very low protein and very low carb recommendations.

If you’re overweight, sedentary, and don’t have the ability to train with any intensity, then a more strict form of the keto diet may also be necessary.

But this kind of strictness may not be necessary for most athletes.

How can You Build Muscle on Keto Diet?

For example, in Volek’s study, the LCHF athletes weighed an average of 69 kilograms (152 pounds) and were eating an average of 226 grams of fat, 82 grams of carbs, and 139 grams of protein.

Carbohydrate wise, that’s way above the 30-50 gram recommendations that most keto “gurus” are telling people to eat.

Bear in mind, the guy who put this study together was Dr. Jeff Volek, who along with Stephen Phinney, literally wrote the book on low carb dieting for athletes.

In it they address carb flexibility in a single phrase “...some people may need to stay under 30 grams while others can consume as much as 100 grams per day of total carbs and still remain in nutritional ketosis2.”  I would add that, if you’re a healthy individual who’s lifting weights and doing intense exercise, that this number is probably going to be closer to 100 grams.

Related: Should You Eat a "Low Carb" or "High Carb" Diet?

Protein wise, these athletes were actually eating an average of 2.1 grams of protein per kilogram of bodyweight or close to 1 gram of protein per pound of bodyweight. That’s well above the scientifically recommended protein thresholds for building muscle3.

The point here is that, in practice, a properly formulated low carbohydrate diet for athletes is most certainly not a low protein diet. Of course, if your protein is ridiculously high at 2-3 grams of protein per pound of bodyweight, you definitely won’t be able to adapt. But if it is too low, your performance and ability to build muscle will plummet.

The Bottom Line

The very strict form of the keto diet is difficult to maintain for athletes who want to be strong and build muscle, but that does not mean that the benefits of being fat adapted are out of reach.

You may still need to go through a two week fat adaptation period where you’ll need to get your body used to utilizing your fat stores for fuel rather than glucose, and this period will probably need to be a bit strict. Plan this time wisely during a time where you’re deloading on your training, or maybe even during a vacation. Aim for 70% fat, 20% protein, and 10% carbs.

Once that two week adaptation phase is over, try to push the envelope a bit and aim for 65% fat, 25% protein, and 15% carbs on training days. Consume the bulk of your carbs either after your workout or toward the end of the day. On days when you aren’t training, keep your carbs closer to 10%.

Even if you aren’t producing enough ketones to be in ketosis, you’ll still likely get most of the benefits of fat adaptation, including increased fat oxidation, better glycogen retention, and increased leanness.

Regardless of what you do, don’t feed into the cultish hype when it comes to keto. Remember, the goal isn’t just to get into ketosis. You want to leverage fat adaptation for your athletic and physique goals. That means that you’re going to require some metabolic flexibility that may not fit into cookie cutter macros that a social media trainer learned from a post on Facebook.

  1. Volek et al. “Metabolic Characteristics of Keto Adapted Ultra-Endurance Runners” .
  2. Volek and Phinney. The Art and Science of Low Carbohydrate Performance.
  3. Morton et al. “A Systematic Review, Meta-Analysis, and Meta-Regression of the Effect of Protein Supplementation on Resistance-Training Induced Gains in Muscle Mass and Strength in Healthy Adults”