I probably get asked this question more than almost any other by runners I know. It's a relatively simple concept on the surface, but it always seems to confuse even the most seasoned runners. In fact, I could probably write a whole book on fueling for a long run, but I'm going to settle for a three part series on this topic. This is Part 1, where we're going to chat about why fueling your run is necessary.
Stop me if this sounds familiar. You've been running for a while, and you just don't see the point of fueling on your long days. You made it through a marathon without any fuel at all! You routinely run 15 or 20 miles on a weekend and don't feel like it's hurting you to not eat. In fact, you sometimes take a gel with you and honestly, you haven't noticed any difference.
Here's the deal. The body has enough glycogen (its energy stores) to maintain your aerobic activity for about 75 minutes depending on your effort level. After that, you're likely to start slowing down, feeling fatigue in your legs, and maybe even suffering the dreaded "bonk" - this is called "hitting the wall" in a marathon. If you're running at a very easy effort, you might be able to make it a little further, but eventually you're going to start feeling pretty rough. You might also make it through the run only to feel like you need to nap the rest of the day.
You Shouldn't Feel Destroyed After Long Runs!
If you feel like you need to be scraped off the pavement every run, I have good news - you don't need to feel that way! What you need is fuel. Fueling helps you hold harder efforts, keeps your body in a state where it can adapt more efficiently, and keeps you from draining your stored energy so you can get back to your life when your workout ends.
Fueling well enables you to run strong throughout races (hello, negative splits) and ensures that you won't feel like crap after every single long run. Of course, if you need a nap or a day of rest, that's one thing - but you shouldn't feel like you're useless for the remainder of the day after a long workout.
Are You Consuming Enough?
Now, let's chat about how much you need, because this is one area a lot of people get wrong. One gel for a two-hour run is not going to cut it. If you've never noticed a difference in your runs when you're fueling vs not, it could be that you're not consuming enough. And you probably need more than you think!
Start off with about 30-45 gm of carbohydrates per hour of activity (that's 1-2 gels or a pack of gummies every hour), starting about 30-45 minutes into the run. Over time you can work your way up to about 60 gm carbs/hour (that's about 3 gels) and even up to 90 gm/hour if your run is longer than 2.5 hours. If you're not used to fueling, you definitely want to start on the low end to avoid any GI issues (we'll talk more about this in Part 2 of this series!).
You don't have to fuel for runs shorter than 75 minutes, but when you get up into the 90+ minute range, you need to bring some carbs on board. Notice that pace/distance is not relevant - what matters is time on your feet. A 2:45 marathoner and a 5:45 marathoner both need fuel, but the runner who is racing for longer needs more.
Isn't That A Lot of Calories?
Now, if you're thinking "that's so much food! I'll never burn any calories!", let me assure you that this is not the case. Not only is it impossible for you to consume enough energy to equal the amount you are using on a run (due to limitations in how much you can absorb at one time), but this way of thinking is fundamentally flawed.
Long runs promote the metabolic adaptation that makes it easier to hold harder paces for longer periods of time. Think things like increased vascularization so your body can deliver blood more effectively, more efficient oxygen use, higher numbers of mitochondria in your cells, improved ability to clear lactate (making it easier for you to run faster for longer), and more efficient use of energy stores in your body. You want your training to reap these benefits, because they translate to more fitness and faster paces (on race day and likely during your regular runs, too).
In order to gain the most benefit from these types of runs, you need your body to be as capable of maintaining form, pace, and distance successfully. To do that? You guessed it - like a Ferrari, you need fuel in the tank to have your best performance. Fueling helps you feel better during the run itself, which is good for promoting those metabolic adaptations. Running without fuel reduces the time to fatigue and increases perceived rate of exertion - i.e., it makes running feel hard. And when running feels hard, people typically slow down. Fueling during the run also makes it easier for your body to recover after that hard effort. And when you run well, and you recover well - you get faster.
But What About Fat Adaptation?
Ah, fat adaptation. Beloved of low-carb enthusiasts everywhere, it's the idea that you can train your body to burn fat (which we humans store a lot of) instead of carbs (which we store in more limited quantities). This is thought to stave off the dreaded "bonk" that happens when you don't fuel properly and your legs turn to jello after about 20 miles of running. And there is some evidence that training depleted (without taking in fuel before or during your workout) can result in improved fat oxidation, which theoretically could improve your ability to run further before you have to dip into your glycogen stores.
Fat-adapted training sounds good in theory. However, in practice, there's no strong evidence that fasted training improves performance. A study of elite race walkers from 2017 found that although fat oxidation rates and VO2max were increased after low carb, high fat training, their performance was impaired. Women in particular do not seem to benefit from fasted training and perform better in a fed state. Frequently training depleted also means you run the risk of not eating enough to meet your energy needs overall, which can have serious implications for injury risk and recovery during a training cycle. Meanwhile, we know that training with fuel will lead to improved fitness. It's not that it never works at all to train fasted, it's just that it might be more complicated and
So is it worth it to train fasted? In my opinion, there are better ways to build efficiency. We know that the most efficient runners are the ones that are really freaking fast - so the rest of us would probably benefit more from training to be fast, rather than trying to hack our way to better race results.
In part 2 of this series, we'll dive into gut issues, a common barrier to fueling on the run.
Have you ever tried fasted long runs? What was your experience?
Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition, J Sports Sci. 2011;29:sup1,S17-S27, Published 2011 June 9. DOI: 10.1080/02640414.2011.585473
Knuiman, P., Hopman, M.T.E., Mensink, M. Glycogen availability and skeletal muscle adaptations with endurance and resistance exercise. Nutr Metab (Lond). 12, 59 (2015). https://doi.org/10.1186/s12986-015-0055-9
Stannard SR, Buckley AJ, Edge JA, Thompson MW. Adaptations to skeletal muscle with endurance exercise training in the acutely fed versus overnight-fasted state. J Sci Med Sport. 2010;13(4):465-469. Published 2010 May 10. doi:10.1016/j.jsams.2010.03.002
Burke LM, Ross ML, Garvican-Lewis LA, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2017;595(9):2785-2807. doi:10.1113/JP273230
Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014;11(1):54. Published 2014 Nov 18. doi:10.1186/s12970-014-0054-7