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.