Glycogen v. Fat: Your long run fueling plan

This is part of a series of posts I’m doing focused on marathon training. To get “The .2” posts in your inbox, sign-up here!


Quick disclaimer: I’m a running coach, not a nutritionist. I’m making the following suggestions based on my experiences and the experiences of the runners I coach. I have no educational background in nutrition. With that out of the way, on to the show …


Your body has a HUGE amount of energy stored in it. So why do runners hit the wall in a marathon? Well, unfortunately most of your energy stores are in the form of fat, and your body primarily uses glycogen for fuel during high-intensity running.


Two ways to improve your performance


That gives us two ways to improve your performance in marathons with fueling: either replace glycogen during the run, or train your body to use more fat. To come up with an approach to fueling your long runs, let’s start at the end: marathon race day.


Your marathon fueling plan


During your marathon, you will want to be taking in energy in one form or another (gels, beans, sports drinks, etc.). Since every runner is different in what they need and what their stomachs can handle, there will be some trial and error in figuring out what works best for you. Here are a few ideas to help you start to put your plan together:


— Follow the package directions


Sounds crazy, I know. But if you’re trying a product, a good place to start is their recommendations on how frequently to use it during your run. (This is the kind of keen insight you come here for, right?)


Portman Nutrition Calculator


This is a very cool resource that allows you to input the duration and intensity of your exercise and get a recommendation on how often and how many calories you should be taking in. There’s a good chance the recommendation it makes will seem way high to you, but it will provide you with a good starting point to make your own personal adjustments.


— From the archives: Nutrition During Long Runs


Morgan Bettini, MS, RDN/LDN, RYT put together a handout for my blog back in January that gives general recommendations on calorie and fluid intake, as well as suggestions for “real food” alternatives to the traditional gels, blocks, beans, etc.


— Weird, bonus tip


Studies have shown that just swishing sports drink in your mouth and spitting it out can help improve performance. It’s worth a try if you have a particularly queasy stomach late in a marathon and can’t stand the thought of taking another cup of sports drink at the next water stop.


Cardinal rule of marathon running: nothing new on race day


One of the cardinal rules of marathon running is to never try anything new on race day. You want to be at the start line so confident in your fueling plan that you’re not even thinking about it — saving mental energy for more pressing matters, like those pesky 26+ miles in front of you. So you’ll need to practice your plan during your training.


Testing your fuel plan is going to be most effective if you do it in a real-world setting … meaning on really long runs. Any run over 16 miles should be a good gauge if your fueling plan is providing you the energy you need or if you’re hitting empty too soon.


Any run over 16 miles will be a good gauge, the real tests begin at 18+ miles


But the real tests will come in 18+ mile long runs (extra credit if it includes marathon pace work). Those are the runs that will not only tell you if your plan is keeping you well fueled, but also if your stomach can handle it!


Using more fat for fuel


Over the last 5 years, there have been several studies that have looked at whether training in a “glycogen depleted state” (for example: morning runs without eating first) can teach your body to increase the percentage of its energy it gets from fats versus carbohydrates.


Training without eating first can improve fat use


The short answer is: yes.


The medium answer is: but the studies didn’t show any increase in performance.


The long answer is: as with a lot of exercise science studies, it’s really hard to generalize from cyclists performing a 60 minute time trial to marathon runners.


Still, I don’t want to be a Luddite here. So let’s look at the best way to work this into your training if it’s something that you want to try.


NOTE: those recommendations are not necessarily indicated by the research, but are my best attempt at taking what the studies say and coming up with a real world plan to implement it.


— Don’t do it every run


The first thing to keep in mind is that you won’t want to try this approach for every run, and certainly not every long run. For one thing, you need to test out your fueling plan before the race.


Secondly, some of the training runs that you’ll do are important enough that you don’t want to risk bonking on those.


Finally, if you’re doing a speed workout with really high intensity running, you’ll be compromising your ability to complete the workout effectively by not getting some carbohydrates into your system ahead of time.


— Keep it short


With those caveats in mind, I think the best place to try this is on shorter long runs — distances that will take you between 90 and 120 minutes to complete. That is long enough that your body will be forced to seek out other energy sources, but not so long that you’ll really struggle to complete the run. And, that will still leave your longest long runs to practice your fueling plan.


I will say, too, that I don’t think you should do this on ALL your 90 to 120 minute runs. I would pick one run every 3-4 weeks if you want to try this approach.


Following these guidelines will let you get the best of both worlds in your training — and leave you ready to use fat and replace glycogen effectively on race day!


About Coach Carl
Coach Carl is a USA Track & Field Level 2 endurance coach who works with runners of all ability levels to reach their goals. He has been featured in Runner’s World, Women’s Health, Men’s Fitness, and Competitor. For information on his coaching services, click here.