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.