Gels (and carbs) are all the rage these days.
And for good reason. Taking fuel during a marathon is helpful. There’s plenty of scholarly and anecdotal evidence for that.
But sometimes – maybe often – I find that people are hyper focused on fueling strategies and gels, and as a result when they diagnose problems they neglect really obvious (and important) things like training volume and pacing.
This exact scenario played out in a couple of threads on r/AdvancedRunning this month, and it got me thinking about the issue again. So I went digging into some scholarly research, and I found a really interesting article that hit this topic right on the head.
I need some time to re-read and digest that article fully. But in the meantime, I wanted to take a little time to unpack the particular scenario from Reddit and offer a few thoughts based on this research.
The Problem: Why Didn’t My Marathon Go to Plan?
The original problem was posed in a thread posted by a young woman who ran a race and was disappointed with the results.
In short, she (25F) had run a race in 3:43 off minimal training. She committed to a higher level of training, ran a 1:37 half marathon, and then came up short (3:42) at a full marathon despite targeting 3:35. She was (understandably) disappointed in the results.
Somebody asked about fueling, and she replied that she took a gel at 45 minutes and another one every 30 minutes after. So that’s likely 6 gels. A decent amount, but multiple commenters thought it wasn’t enough. Somebody suggested taking up to 8 gels throughout the race, and someone else suggested targeting 60-90 grams of carbs per hour.
About a week later, somebody brought this back up in the weekly Q&A thread. They thought the original runner’s fueling plan seemed reasonable and then asked for advice on how to fuel for a 3:30 marathon. And again, multiple people suggested 60-90 or 60-100 grams per hour.
This common advice originated from some research showing that it was possible to ingest this much – and that it might be helpful. And it’s now being parroted around the internet by well intentioned runners. But in a lot of cases, I think it’s somewhere between unnecessary and besides the point.
How Much Energy Do You Need to Run a Marathon?
Let’s start with a fundamental and simple math problem.
How much energy do you actually need to run a marathon?
I think it’s a common perception that this increases with time – so slower marathoners are often advised to take just as many gels per hour and take on as much fuel as possible. But that’s simply not true.
The amount of energy expended during a marathon is a simple function of weight and distance. This paper offers a simple way to estimate this: 1 kilocalorie per kilogram of body weight per kilometer of distance. Here, a kilocalorie is what we call a “Calorie” in food.
Time does not factor into this at all. I weigh ~78kg, and whether I jog a marathon in 5 hours or race it in 3 hours, it’ll take me approximately 3,300 calories of energy to finish.
Depending on my pace, some of those calories may come from fat (at the rate of 9 calories per gram). But, and especially as you get closer to your hypothetical marathon pace, a majority of that energy will come from carbs (at the rate of 4 calories per gram). But at a pace you can sustain for three hours (or more), this will not be 100%, and some energy will still be coming from fat.
Since the amount of energy required to run the race is a function of distance (and weight) and not time, the same person would require a similar amount of fuel whether they ran a race in 2 hours, 3 hours, or 6 hours.
For the elites, the math challenge becomes how do I take the necessary number of gels – let’s say 6 – in two hours. Now, you’re butting up against the body’s capacity to actually digest and utilize the fuel.
But if you’re taking three hours, you have a little more time. At 3.5 hours, you can comfortably get in 6 gels by starting at 45 minutes and taking another one every 30 minutes. And at 4-5 hours, you could maybe push that to 45 minutes between gels.
How Much of That Energy Comes From Carbs?
It’s not quite that simple, though. There’s one other factor at play – the relative intensity at which you are running.
The more intense your pace, the more that energy comes from carbs. The easier that pace, the more can come from fat.
As a general rule, this is fairly common knowledge. I think. It’s been plastered on every treadmill and exercise bike that I’ve ever seen in a gym.
But as a practical fact, I don’t think many runners actually think about or understand it. We’ve internalized the idea that marathons = fuel, that the very idea that you can run a marathon without fuel is verboten.
If you’re running at ~80% of your VO2 max, about 2/3 of the energy required comes from carbs. For me, that 3,300 calories requires ~2,200 calories in carbs.
But if the intensity is easy – say 60% of VO2 max – the breakdown is closer to 60-40, with only 40% of the energy coming from carbs. So instead of 2,200 calories in carbs, I’d only need ~1,300-1,400 calories from carbs.
A person generally has between 300-700g of muscle glycogen in their body, and each gram is worth 4 calories. Use the middle value – 500g – and you can expect to have ~2,000 calories available. Not quite enough to race at 80% of VO2 max, but more than enough to jog at a steady state.
A Model That Makes It Make Sense
The same study cited above takes all of the math and science behind this and creates a really useful model for understanding the problem.
There are essentially four variables at play:
- Weight
- VO2 Max
- Pace
- Muscle Mass
Weight determines how much energy is actually required to finish the race.
Pace, relative to VO2 Max, determines how much of that energy is derived from fat and how much is derived from carbs.
Muscle mass, relative to body weight, determines how much muscle glycogen a person has available.
All that math works out to a pretty simple maxim. If you’re running at a target marathon pace that’s appropriate for your VO2 max (based on any number of calculators out in the world), you’re going to need ~30 calories per kg of body weight.
This falls in line with the typical amount of muscle glycogen a person has available if they’ve properly carb loaded for a race. Taking on some fuel during the race is a good idea. It’s some added insurance you don’t completely bonk, and it’s probably the case that fatigue sets in slowly as fuel reserves drop (but before they totally bottom out).
But if you’ve got enough fuel already (or close to it) and you take on an additional ~600 calories (6 gels) during the race … fueling is probably not the number one issue.
How Does This Relate Back to Our Original Runner?
Now let’s go back to our original runner.
We don’t know all of the details, but based on her half marathon time, the projected finish time of 3:35 seems reasonable. If her VO2 max is ~45, she’d need ~30 calories per kilo to finish the race at that intensity.
Assuming she’s somewhere between 55 and 65kg, that works out to ~1,600 to 2,000 calories. With typical muscle mass and good carb loading (which she indicated in the thread), she should be right around the edge of what she could accomplish without any fuel.
Taking six gels throughout the race is making an extra 25-35% of that fuel available. That’s plenty. Trying to cram in a seventh or eighth gel is not going to magically make her go faster. She’s got the fuel that she needs, and now it’s about whether her body can use it efficiently and effectively.
So What’s the Issue?
Some other commenters pulled at a few other threads – specifically the pace early on (which was hot) and not recovering enough from a tune-up half marathon 5 weeks out.
Based on the full thread, I’d point to those two issues – especially pace – well before I looked at any fueling issues. Frankly, if you’re getting 6 gels in during a race, fuel is unlikely to be your issue.
It’s much more likely that there’s a pacing issue or a training issue. In this case, the training seemed fine, and running a 1:37 half is an indicator of good fitness.
But aiming to run 3:35 and then starting out at ~8:00 or below for the first few miles is a recipe for disaster. I’ve been there, and it happens to the best of us.
But it’s just another reminder that the marathon is a race that rewards patience – and it punishes greed.
I’m going to return to this scholarly article again in the future, because I think it offers some really interesting insights into fueling. But I definitely need to read it over again to fully wrap my head around it. In the meantime, I thought it was interesting and worth sharing with some initial thoughts.