The science behind depleted runs: are they for you?

On several occasions I've incorporated depleted long runs into my marathon training. A depleted long run can be a combination of going into the run in a depleted or fasted state, not taking fuel or limiting fuel during the run or any combination of the two. Usually they work best as an early morning run where you come off "fasting" through the night and head straight out of your run. Each time I've done them it has been early in the training cycle, usually the first or second long run over 16 miles. I've always considered them beneficial both physically and mentally as it helps me to push through when my body feels depleted and tired. My personal experience has been a positive one, but depleted runs may not be for everyone or even for me in my next training cycle. I've had quite a few questions asked about these depleted runs and while I try to be as knowledgable as possible when it comes to running specific nutrition, I am no expert. Which is why I reached out to one. I recently had the chance to chat with Chrissy Carroll, RD, ACSM CPT and author of Snacking In Sneakers. Chrissy was able to lend her expertise as a Registered Dietitian to a few questions I had regarding depleted long runs.

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So, first off what happens to the body when you don’t take fuel on runs longer than 90 minutes?

When you’re out on a run, your body is always using a combination of carbohydrate and fat as sources of energy. The specific percentages vary based on a few factors, including the intensity and duration of the exercise session as well as the athlete’s fitness level and training routine – but there’s still always a mixture of these two used. While we have lots of fat available to burn in our body, we are much more limited in the amount of carbohydrate that’s stored in our body (which is stored in our muscles as glygocen). That’s why we typically take in carbohydrate (in the form of gels, bananas, sports drinks, or whatever works for you) during a run. When you don’t take in any fuel during a run lasting longer than 90 minutes, you might start to struggle and feel tired more easily tired since you aren’t giving your body any more carbohydrate. Even though you still have plenty of fat available to burn, your body needs/wants the mixture of fuels. (1)

Is a depleted run really ‘teaching’ the body to burn fat as fuel? Is there a better way to phrase what’s going on?

I think that’s probably the easiest way to understand what’s going on, but the key is to realize your teaching the body to use more fat as fuel – not to only use fat as fuel. Essentially your body goes into kind of a conservation mode, and adapts by using more fat as fuel and less carbohydrate to try to ‘stretch out’ the amount of carbohydrate you have available. The kicker though is that while studies do show that depleted exercise may improve the ability to burn fat as fuel, many do not show an improvement in performance (3, 4)

How might the depleted run be beneficial to marathon training?

If you can burn a greater percentage of fat versus carbohydrate, you are in theory creating a more efficient way of fueling your exercise. But again, whether that translates to actually racing better is still up in the air. A few studies show that the adaptations may help with performance but many don’t show that (7, 8).

When could the depleted run be detrimental to marathon training?

I’ll be honest – I’m not a huge fan of depleted runs for most people, for three reasons:

1) you may feel more tired during your run and perform worse because of that.

2) training in a depleted state may also cause more protein breakdown to be used as fuel, which is something we generally don’t want. (2)

3) because regular carbohydrate intake is important in immune function, recovery, and regulating cortisol (a stress hormone), a consistently depleted state might lead to symptoms of overtraining (5, 6)

Who should (or should not) consider incorporating depleted runs into their marathon training?

I think a promising use for depleted runs could be with athletes that consistently have GI issues when taking in fuel during a run. I don’t mean a new runner who got an upset stomach while trying out a gel, because just like you train your muscles to run – you train your gut too, and you need to practice running while taking in fuel. But if you’re a runner that consistently has GI issues towards the last half of endurance runs, then trying some depleted runs that make your body more efficient at utilizing fat may be worthwhile because you might be able to lower your fuel use on the course. Sports nutrition is so individualized though that this isn’t a fool proof solution and doesn’t work for everyone.

Along the same lines, if you’re a competitive athlete and you like experimenting with different training and nutrition protocols, then depleted runs might be of value to you to try out. Like I mentioned above, the science doesn’t necessarily support improved performance in many studies – but it might vary based on the individual.

For the most part though, if you’re a recreational runner that’s out there to have fun or if you’re new to running – I’d stay away from depleted runs. I guess I just think why make running more challenging than it needs to be?!

What would be the safest way to incorporate a depleted run into your training?

I’d recommend working with a dietitian or coach who can help guide you based on individual goals if you decide to try this, and keep your expectations realistic – it might not benefit you at all, and may make training more difficult.

If you do decide to give it a shot, though, you can try an occasional long run without fuel and you can also mix in a weekly short run that is done after an overnight fast (i.e. no breakfast). Keep your training plan balanced. If you do all your long runs depleted, you’d likely end up on the course with an upset stomach because your body isn’t used to taking in fuel during a run, which is not a pleasant experience of course.

In addition, when you do depleted runs, you may get that ‘hitting the wall’ feeling early and your run may feel tough. Be sure that your subsequent day’s run(s) are focused on recovery, not intensity, and that you are fueling properly in those subsequent days. Also, avoid depleted runs all together if you’re already feeling run down, feel like you might be getting sick, or have any signs/symptoms of overtraining.

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References:

1) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3784189/

2) http://www.ncbi.nlm.nih.gov/pubmed/9831734/

3) http://www.ncbi.nlm.nih.gov/pubmed/20351596/

4) http://www.ncbi.nlm.nih.gov/pubmed/18772325/

5) http://www.ncbi.nlm.nih.gov/pubmed/12617688/

6) http://www.ncbi.nlm.nih.gov/pubmed/14660506/

7) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4687103/

8) http://www.ncbi.nlm.nih.gov/pubmed/20840562

Want more from Chrissy? You can check out her blog Snacking in Sneakers HERE and also find her on these social media platforms:

Twitter - https://twitter.com/chrissytherd