I heard you decided to try a low carb diet in hopes of increasing your ability to burn fat for fuel. You heard that athletes can perform just as well on a low carb diet and can also “go forever” burning massive amounts of fat. Sounds like a panacea for endurance athletes right?
You made the commitment and have spent months restricting your carb intake, eating healthy fats, and even incorporating more fasted training into your routine. You are finally seeing your hard work pay off. Your fasted workouts are getting easier, you no longer get “hangry” in between your meals, you even lost a few pounds in the process.
Despite all these positive benefits, there is one thing that is concerning for you….
….your performance has dropped off a cliff!
Aerobic workouts feel great, you feel like you have a never ending supply of energy. But when you go out to do a hard workout where you push yourself, your performance is nowhere close to what it used to be or even in the same “area code”.
You head to your favorite low carb Facebook group to ask what is going on. To your surprise people have no shortage of answers….
“You need to give your low carb diet more time.”
“You need more electrolytes.”
“Try some fatty coffee before your workout”
“Try eating nothing before your workout”
“You are eating too much protein, it’s turning into glucose and keeping you in carb burning mode!”
“50-100 grams of carbs is too much, you need to eat 20g or lower.”
“Do a 36 hour fast!”
You try various things, but none of them seem to work and it has been 6 months since you have gone low carb. What gives?
Turns out your body is doing exactly what you are telling it to do.
Our bodies have survived for 100’s of thousands of years to be as efficient as possible. This applies to optimizing metabolic functions based on the inputs it receives from our diets. Since you are not consuming many carbohydrates, your body starts to down regulate, i.e. switch off, the metabolic functions used to access the glycogen stored in your body and upregulate the ones used to oxidize fat. As a consequence you can try and ingest all the carbohydrates you want, they body won’t have the ability to access the glucose because it has “forgotten” how to. You have entered what I like to call “the low carb no man’s land”.
(cue ominous music)
If you want to understand “the why”, aka the science, behind your lack of performance, keep reading, if not skip down the practical stuff below.
Ok you have been warned….
PDH, The Glycogen Gatekeeper
Your body’s ability to utilize glucose is controlled by an enzyme called PDH. In other words PDH is the gatekeeper to your body’s ability to access stored glycogen.
Studies have found that after just six days on a low carb diet (5% carbohydrate, 75% fat, and 20% protein) PDH levels start to drop. After the 6 days on the low carb diet, researches had participants eat a higher carbohydrate meal (88% carbohydrate, 5% fat, 7% protein), and while participants initially saw PDH levels rise back to pre-low carb diet levels, at the 3 hour mark post meal PDH levels were back down to the same low carb levels.
Perhaps the best research in terms of PDH activity and endurance athletes was a study by Stellingwerff, et al . In this study, participants followed a low carb high fat diet (fat 67%, carb 18%, protein 15%) for 5 days while training. On the 6th day they rested and did a carbohydrate refeed (carbs 70%, fat 15%, protein 15%). Then on day 7 they performed an exercise test. The test involved 20 minutes of steady state cycling at 70% of their V02 Max then resting for 1 minute before doing an all out sprint at 150% of peak power output (PPO) for one minute. They then rested for 5 minutes and did a time trial at 90% of their V02 Max for about 10 minutes. Participants then had a 2 week washout period and performed the same testing protocol on a high carb low fat diet (carbs 70%, fat 15%, protein 15%).
The results showed lower PDH activity at rest (56% lower), during the 20 minutes of cycling (29% lower), and during the 1 minute sprint. (There was no data on PDH activity for the time trial.) It is also interesting to note that this was post carbohydrate refeed on day 6. So despite an entire day of higher carbohydrate eating participants still had lower PDH activity.
In addition to tracking PDH activity in the athletes, researchers also quantified the amount of glucose in the muscles of the participants while performing the exercise tests on both diets. Before starting the exercise tests participants had similar levels of glucose in their muscles on both diets. However, after finishing the exercise tests the participants had higher levels of glucose still in their muscles on the low carb diet than on the high carb diet.
These two data points, lower PDH activity and higher glycogen levels while on a low carb diet, make a strong case for why low carb athletes can sometimes lose that “high gear” even after reintroducing carbohydrates into their diet before their hard workout or race.
Getting Your Mojo Back
Does this mean low carbohydrate diets are bad for athletes?
No, not at all.
In fact, many athletes toeing the starting line could benefit from a low carb diet from the perspective of their overall health. If your goals have nothing to do with performance, then it is completely fine, and perhaps beneficial, to be on a low carb diet long term.
Once performance becomes a priority though, we know carbohydrates can be a huge benefit by increasing performance when you need to drop the hammer. If your diet is preventing you from benefiting from this important fuel source, you are leaving something on the table.
The question you are probably asking yourself is if it is possible to get the benefits of a low carbohydrate diet and still eat carbs?
Many low carb athletes think that as soon as they eat one strawberry that they suddenly lost their fat adaptation and they are once again a carb burner. This just simply isn’t true. Remember your body is going to adapt to the signals you are sending it, so if you are putting it in situations where the majority of your diet is fat (or no food at all) and also having periods where you are consuming higher carbs than it is going to keep the metabolic machinery around to utilize both. This is called metabolic flexibility and is the ideal state for those athletes that want all the benefits of being able to burn fat for fuel but also want the performance benefits associated with carbohydrates.
The best approach to building metabolic flexibility involves periodizing your nutrition at both a macro and micro level based on your background and your current level of metabolic flexibility.
The Macro Approach To Achieving Metabolic Flexibility
For example, if you are like most athletes and are heavily reliant on carbohydrates, you would probably benefit from a longer period of time on a low carb diet. The ideal time to implement this type of diet would be during your offseason, when performance is not a priority and workout volume is generally lower. A few weeks to a couple months on this diet should help you immensely. As the offseason comes to an end and workout volume goes up, you can add back in carbohydrates and give yourself some time to correct any PDH down regulation that may have occurred.
If you are someone who has been on a low carbohydrate diet for a while and has lost some of your top end performance, you want to spend some time slowly titrating up your carbohydrate intake. What you don’t want to do is start taking in a ton of carbohydrate immediately, your body is just not used to that. Again, the perfect time to try to make these changes is in the offseason, giving your body time to adapt, without any potential negative impact to your races. What you will want to do is add somewhere in the range of 20-40 grams of carbohydrates into your diet every 4-8 weeks or so. Continue adding in carbohydrates until you start to see your performance return.
The Micro Approach To Metabolic Flexibility
In season it is best to do some periodization at a micro level, by that I mean on a day to day basis. We know carbohydrates are beneficial when performance is important so it is best to target carbohydrate intake when they will best be put to use. On days where you will be doing glycolytic work like power and/or speed work, training sessions of 90+ minutes (even if you are aerobic), and strength training sessions try eating more carbohydrates that day.
Now I know that saying, eat “more carbohydrates” is vague and that is because the amount is going to be individualized on where you are coming from. Try starting out with 50-75g more than normal and see how you feel and perform. You can adjust up and/or down from there based on how you respond.
On easier days, you can lower the carbs as they are not needed for these workouts. These would include workouts where you are entirely aerobic for less than 90 minutes and things like walks, yoga, and easy bike rides. By keeping carbs lower your body will be more likely to use fat as its primary fuel during these workouts.
You can also try doing these easier workouts fasted to ensure you are using mostly fat and keeping those fat burning pathways active.
By activating both your carb and fat burning metabolic pathways throughout the week you body will keep everything working optimally so everything operates efficiently.
If you have noticed that your performance has declined since switching your diet all hope is not lost. Your body is amazing at quickly adapting to its environment, so if we give it the correct signal and tell it that carbs are available, you can quickly gain back the ability to access your glycogen stores and your performance will return.
Have you been trying to get yourself out of the low carb no man’s land for a while and haven’t been able to? I can help. Reach out to me on Facebook Messenger or Instagram. If you want to receive more content related to optimizing your fat and carb burning metabolism delivered to you weekly for free, be sure to sign up for my newsletter.
- (n.d.). Metabolic characteristics of keto-adapted ultra-endurance …. Retrieved May 30, 2020, from https://www.ncbi.nlm.nih.gov/pubmed/26892521 ↑
- (2009, July 22). Carbohydrate refeeding after a high-fat diet rapidly … – NCBI. Retrieved May 30, 2020, from https://www.ncbi.nlm.nih.gov/pubmed/19625693 ↑
- (2005, September 27). Decreased PDH activation and glycogenolysis during … – NCBI. Retrieved May 22, 2020, from https://www.ncbi.nlm.nih.gov/pubmed/16188909 ↑
- (n.d.). Carbohydrate Feeding During Prolonged Strenuous Exercise …. Retrieved May 31, 2020, from https://pubmed.ncbi.nlm.nih.gov/6350247 ↑
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