The long run is touted by many to be the centerpiece of training for marathoners and other endurance runners.
Most people think of the long run as a protracted effort that causes their body to produce the mental and physical adaptations needed for endurance races. But the ways in which people prepare, fuel, and run during these long efforts are often not the most optimal. And the reason is because long runs aren’t about running long per se—they are about training the particular systems of the body that enable us to run long.
This isn’t just wordplay: I often see well-intended runners filling their hydration belts with sugary foods and energy gels in preparation for a long run.
That’s a problem.
Let’s consider which of the body’s systems are designed to help us run a long distance. We need a very abundant fuel source, as well as an engine that can burn that kind of fuel for a long time. Sugars (a.k.a. carbohydrates) won’t be a good primary fuel source: they exist in relatively small quantities inside the body compared to fats. Furthermore, the Type II (fast-twitch) muscle fibers that utilize them fatigue quickly.
So we need to rely heavily on a more plentiful fuel: fats. In order to burn fats, we’ll need to use several systems: the hormones that help break down and transport fat, and the Type I (slow-twitch) muscle fibers that can burn them (as well as the lungs, heart, and blood vessels, which together allow oxygen to get to the muscle fibers and enable fat utilization).
Running for a long time is all about burning fats. But when a runner depends on sugar to fuel their long runs, as far as the metabolism is concerned, it’s not a long run.
Using sugars to fuel the long run means that (1) not only is the quickly-fatiguing sugar-burning engine being used for much longer than it’s designed for, but (2) it’s only being relied upon because the engine that is supposed to do the job isn’t powerful enough to produce the required activity levels.
The body is getting tired and worn down at an absurd rate. But that’s also only happening because it was already not capable enough to run that fast for that long.
As the body gets tired, it gets stressed. As it gets stressed, it use of oxygen declines, and it starts being forced to consume sugar anaerobically—without the presence of oxygen. This compounds the problem: the main by-product of anaerobic activity—lactate—suppresses the body’s ability to use fat for fuel.
What does this do to our definition of a “long run”?
I like to define the “long run” as a run that occurs (1) below a threshold of stress that allows for burning fat at a very high level, and (2) long enough that the various systems necessary for burning those fats (and for supporting and moving the body for the duration) become challenged enough to develop.
In my opinion, the ratio of fat to sugar utilization necessary for a run to qualify as a “long run” is 42% fats and 58% sugar, a Respiratory Quotient (RQ) of .87. This measure correlates with the aerobic threshold—the highest level of activity at which virtually all of the body’s energy is being processed in the presence of oxygen.
While the percentage of fat utilization at this point is already declining, after an RQ of .87 it begins to drop much more quickly. Since the lactate produced by anaerobic activity inhibits fat usage, the percentage of sugar used increases dramatically.
You can get an RQ test at any exercise lab, or even some doctor’s offices. But my favorite way of finding a ballpark measure of the aerobic threshold is Phil Maffetone’s 180-Formula. The 180-Formula gives you the heart rate at which you reach your aerobic threshold, which makes it very easy to keep track of your fat utilization while running.
Using sugars to support ourselves through a long-run is a self-defeating endeavor. We won’t create the adaptations we hope for. Because the body hasn’t adapted, we’re subjecting it to stresses it can’t really handle. It’s not going to grow that well, or that quickly, or in the direction we want it to, and it might break down on us a few times along the way.
Let’s keep our long runs easy enough.
UPDATE (10:46 AM, 12/14/15) : I’d previously written that total fat utilization was at its peak at an RQ of .87. A reader pointed out to me that this wasn’t the case.
UPDATE (11:35 AM, 12/14/15): I should mention that the criteria I discuss in this article are perhaps necessary but not sufficient to call something a “long run”: Commenter “Van” suggested that a better definition for “long run” is that which occurs at the heart rate which corresponds with the maximum rate of fat oxidation (rather than the maximum rate of oxygen use at which there is no anaerobic function). I’m not convinced at this point, but I’ll be sure to update again—or maybe write a follow-up post—if that changes.
running in systems 
I disagree with your assertion of 0.87 RQ. Imo long run pace should be kept below, well below the crossover point. You need to keep intensity at point where you are still using mostly fat. Besides, the Substrate usage profile differs wildly between different people even at their max aerobic heart rate. Some people are so carb dependent they are at 0.87 even on the couch, while some very good fat burners won’t reach that level until very high heart rates. It is best to ignore RQ when you are determining your long run pace/intensity.
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Van:
First you say that “you need to keep the intensity at the point where you are still using mostly fat” and then you say “it is best to ignore RQ when you are determining your long run pace/intensity” because some people are carb dependent. I don’t get it.
My main point in this post is that a “long run” should be minimally defined as a training run where it is overwhelmingly the long-term (aerobic) energy system that is being developed. I didn’t narrow it down to only or mostly fat-burning because at an RQ of .87, rate of fat-burning (but not percentage) tends to peak. So at an RQ of .85, you may be burning, say 90% as many fats as you are burning at an RQ of .87.
If you’d like to define “long-term energy system” usage as a higher percentage of fat oxidation than I did, then I’d love to hear your argument. But in any case, a “long run” should reflect a high level of fat-burning, regardless of whether we agree on the particular substrate utilization.
If some people are so carb dependent that they are still on the couch at .87, then there is NO WAY that they can be doing a long run without heading straight into the dangers I outlined in my post. RQ shouldn’t be ignored in these cases. Instead, measures should be taken to reduce carb dependency, so that they can then exercise at an aerobic heart rate.
And finally, since fat-burning can only happen oxidatively, any heart rate that reflects a mixture of fat and sugar burning that implies that very little lactate is being produced is essentially “aerobic.” Whether or not it is “high” or “low” relative to MAX HR is immaterial.
That said, I mentioned earlier that there are other reasons for why a very high fat-burner may not want to run all their long runs (or even a few) at that speed. Perhaps their training doesn’t require it, or it means too much stress on their heart due to a particular condition. Who knows.
Because of this I think that substrate utilization is everything. Since substrate utilization reflects energy system activity, it does in fact tell us whether someone is going primarily aerobic or not. After the AeT, and actually usually after .87, RQ rises steeply relative to heart rate because of a sharp increase in anaerobic activity—the rise in RQ reflects the need to metabolize lactate.
In other words, because substrate utilization reflects energy system usage, I would much rather define what an “aerobic heart rate” is in terms of substrate utilization (RQ) than some formula (particularly when formulas are presumably trying to get at energy system usage).
All I’m proposing is a cutoff point.
You want to propose another cutoff point? I’d be happy to discuss it. But I don’t think that ignoring RQ altogether is a good idea.
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OK, this is a bit of a long answer, but…
The RQ point of 0.87 (or any any other point) can occur at just about any heart rate depending on how good/bad a fat burner you are.
Maffetone’s definition of the MAHR is the heart rate at which your are maximizing your absolute amount of fat utilisation – it (almost) doesn’t care how much carbs you are also using at that point… all you are interested in the point (heart rate) where you are burning the absolute most calories from fat. To work this out for sure you have to know both (1) the total calories being expended, and (2) the RQ – which is why gaseous exchange test in a lab is required to know for sure. knowing just one or the other isn’t enough.
eg, if I am expending 600kcal/hr @ 140bpm and my RQ is 0.75, my fat usage is 83%, ie 500kcal/hr, or 55.5g/hr. If I bump up my intensity and my calorie expenditure goes to 800cal/hr @160bpm, but my RQ shifts to 0.82, I am now sourcing 60% ie 480kcal/hr or 53.3g from fat.. therefore 140bpm is a better estimation of my MAHR than 160.
Maffetone’s own backfitting of data has found that 180-age +/- health adjustments is a good approximation for that.
Even a super-fat burner like Zach Bitter has written about his testing results and stated that his MAHR is “somewhere between 140-150” which lines up very well with 180-age (which comes out even a bit on the high side).
“My fat metabolism peaked at 1.57 grams/minute. At this point in the test, my VO2 uptake was at 49.4. By dividing this number by my eventual VO2 Max of 66.1, I can calculate at what intensity I burn the most fat: 74.4%. At that intensity, I was burning 98% fat 2% carbohydrate (1.57 fat grams/minute and .07 carb grams/minute).”
– that’s a RQ of something like 0.705 at max fat burning heart rate – way off any sort of pre-conceived RQ “cutoff”.
http://zachbitter.com/blog/2014/04/takeaways-from-the-faster-study.html
Also, remember that carb oxidation does NOT mean “anaerobic” in a true physiological threshold sense. Your body can use both fat and carbohydrate oxidatively (ie aerobic), but only carbohydrate when anaerobic, ie the anaerobic substrate is also one of the aerobic substrates.
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Van:
I actually work for Phil Maffetone, and how the MAF HR relates to RQ is one of our big internal debates right now—in particular, whether we’re talking about maximum fat usage, or maximum oxidation with no anaerobic activity (of both substrates). We’re heavily tending toward the latter interpretation: “maximum oxidation in the absence of anaerobic activity” is much more reflective of MAF than “maximum fat oxidation.” That said, pertaining to what it means to “run long”—rather than about defining what “MAF HR” means—I tend to agree with everything you say, and perhaps I’ll reconsider my opinion of the particular threshold, given your point about absolute fat utilization.
Which means that I made a bad call by claiming that an RQ of .87 corresponds to maximum fat utilization. I got ahead of myself. Went back and updated that.
I’m well aware that carb oxidation is not anaerobic. My point is that a certain ratio of substrate utilization implies whether anaerobic activity is occurring, particularly since anaerobic activity (indicated by the production of lactate) inhibits the usage of one substrate—fats. So, when percentage of fat usage begins to drop precipitously, you can make a very safe inference that anaerobic activity begins then and there.
So RQ is actually a very good predictor of when anaerobic activity starts. Although the number produced by the 180-Formula may not be (and often isn’t) the same as the one produced by an RQ test, the maximum aerobic heart rate (defined not as the number produced by the formula, but as the number where there is maximum oxidation but little to no anaerobic activity) occurs at around 8.5/8.7.
To reiterate, I agree with much of what you’re saying. My article is primarily tailored to the lay athlete—the guy or gal that fills up with energy gels to go on a “long run.” It’s that person that I’m worried about—someone who is running at an intensity that far exceeds their aerobic capability, for a time or distance that they shouldn’t be operating anaerobically.
To everyone who is already doing their long runs without anaerobic activity, whether a “long run” should happen at maximum rate of fat oxidation or maximum rate of total oxidation in the absence of anaerobic activity is an important and useful distinction to make, but in my opinion a tad bit less pertinent.
What do you think?
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Hi Ivan, very good article.
I wonder to ask about cardiac drift phenomene? How to avoid this during long run.
Even if we do long run with MAF pace this still happen to lot of us (same heart rate slower speed).
Thank you so much.
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How would you fuel for a long run?
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Steve:
The short answer is this: for training runs I don’t fuel. But I’d like to answer your question with a blog post, for more insight. I’ll even include a recipe that I’d use for fueling during a race.
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