Will Glycogen-depleted Runs Improve Your Performance?


If you spend any amount of time talking about ultrarunning nutrition, you are almost certain to hear the term “fat adapted.” The general idea is that a “fat adapted” athlete will be very efficient at burning fat and thus have an immense supply of stored fuel, eliminating the need to ingest large quantities of carbs and hopefully eliminating many of the accompanying GI issues as well. While many runners striving for increased fat burning efficiency turn to a low carb high fat (LCHF) lifestyle, glycogen-depleted runs have also been used as a tool – either alone or as an adjunct to an LCHF diet – to teach the body to preferentially use fat as fuel. Will glycogen-depleted runs take your running to the next level? Let’s take a look at the two sides of this coin.

Heads (pro): Glycogen-depleted runs appear to promote certain physiologic adaptations that would support being a more fat-adapted runner. Specifically, people training with carb depletion show a boost in fat metabolizing enzymes, fat utilization and muscle glycogen storage. And since GI issues are one of the main causes of DNF’s at ultramarathons, anything that reduces these issues can increase the likelihood of finishing a race. With better fat utilization, athletes may be able to get away with consuming less on race day thus reducing some of the GI issues that occur when relying on ingestion of large quantities of carbohydrates. Plus a body that is more efficient at burning fat may have an easier time shedding excess pounds.

Tails (con): While glycogen-depleted runs may allow you to rely more on fat for energy, this does not equate to better performance. In fact, insufficient carbohydrate availability results in decreased power output, decreased intensity and increased oxygen consumption at any given pace. Because of these issues, athletes training primarily in a carbohydrate depleted state see less gains in their V02 max and overall fitness. Glycogen-depleted long runs are taxing on the body and add an extra layer of stress to high mileage training. And both calorie and carbohydrate deficiency are correlated with injury and ovetraining syndrome.

My two cents: While I often do my weekday morning runs in a fasted state, I am not a big fan of the true glycogen-depleted long run. One of the functions of any long run is to increase fat burning capabilities such that one doesn’t necessarily need to do a true glycogen-depleted run for many of these benefits. On the few occasions that I have tried this type of training, I have found that maintaining my usual pace makes for a very taxing and unfun run that is essentially on the verge of bonking the majority of the time. The other option is to ratchet the pace way down and get less bang for your buck when it comes to training benefit.

For those hoping to become better fat adapted, I think it makes a lot more sense to concentrate your efforts on your every day diet and non-running hours as that accounts for both the majority of your calories and the majority of your time. For those hoping to alleviate stomach issues, I would first recommend trying to “train the gut” to better handle calories while you run and to experiment with many different fuel sources.

For those still wanting to experiment with methods to increase fat burning, fasted state runs may offer a good compromise. The easiest way to do this is to run first thing in the morning before eating any breakfast. This will increase stored glycogen and teach your body to better utilize stored energy, including fat, but because your glycogen stores should be fairly well replenished after a night of sleep, you are not truly glycogen depleted. But don’t let these runs get too long: after 90-120 minutes most of your stored glycogen will have been used up and then you enter a glycogen-depleted state.

If you are still set on glycogen-depleted runs, the base building phase may be the best place to incorporate this type of run as a more mellow pace in early training is not likely to impact race day performance. It is also a good idea to keep this type of training to a small fraction of your weekly mileage (+/-20%) to minimize overall stress and risk of injury.


About Author

Pam Smith ran her first trail race in 1992 and started ultra-distance races in 2002, with more than 70 finishes. She was the 2013 Western States champion and holds the Angeles Crest course record. She has been on a total of seven national teams for both the 100km and 24-hour events. Pam is running her seventh Western States 100 in 2018 under the Active Joe sponsorship. She lives in Salem with her husband and two children and she works as a pathologist. More of Pam’s writing can be found on her blog, “The Turtle Path.”


  1. It takes several weeks to become fat adapted myself took over 7 to feel normal but after that you notice a big difference. What I’m getting at is one or 2 runs in a fasted state won’t prove anything too you!

  2. To me, this is a biggie….

    “And both calorie and carbohydrate deficiency are correlated with injury and ovetraining syndrome.”

  3. I’mn not à keto fan, but if you want to write an article on this subject, it would be a good idea to cite sources…

  4. Martin Fryer on

    Fasted state runs of 60 to 120 min at low intensity work perfectly well.

  5. Hey Pam I appreciate your knowledge and accomplishments. Amazing stuff! I’m an Exercise Physiologist and discovered Ultra events in 2013. I became fat adapted in 2016. I do 90 percent of my training fasted and feel at 58 years of age getting better! I finished the infamous C2C2C in Palm Springs yesterday, April 28, and took in less than 50 grams of carbs over 14 hours and felt amazing! I know it’s not for everyone but I believe the health benefits far outweigh the countless ultra athletes that are becoming pre diabetic and potential health problems! Would love your thoughts on this Pam! Thanks again for all you do!

    • Arnie, I am so glad that you shared that. I too am a LCHF (low carb high fat) ultrarunner, and I honestly get tired of the skepticism about this way of eating. I recently won the USATF 60-69 age group at the Caumsett 50km. I managed to break 4 hours and I didn’t ingest any calories during the race. I just drank water. My 5km splits were very even throughout, no bonking. There is a lot of misinformation and misunderstanding about this. I haven’t seen any evidence that carbohydrate deficiency (if there is such a thing) is correlated with injury or overtraining, unless of course you are carbohydrate dependent. While, being a fat adapted runner has nothing to do with calorie deficiency. As Kirk explained, the efficient use of our fat stores for energy (thus preserving glycogen stores) is a by product of carbohydrate restriction (and eating plenty of fat instead) day in and day out, and has little to do with fasted long runs. Zach Bitter’s website is a good source for information.