Frequent readers of my blog know just how much I like to use car metaphors to describe the human body’s function. So here’s another one: the aerobic system is the body’s main powertrain.

(The powertrain is the chain of systems within the car that power gets channeled through: from the engine, through the gearbox, down the main drive shaft, across the differential, and into the wheels. The drivetrain on the other hand is typically understood as the powertrain minus the engine.)

When most people think of increasing aerobic function, they think of increasing the capabilities of the body’s aerobic, Type I muscle fibers (also known as slow-twitch fibers). While muscle fibers are hugely important—they are the main power producers of the body—they are one subsystem of many that need to be working synergistically and at similar rates for the aerobic system as a whole to be able to express any kind of power.

It’s important for us to realize that when we are talking about developing the aerobic system, we are talking about much, much more than just the aerobic muscle fibers themselves. Quite literally, the whole powertrain from beginning (lungs) to end (muscle fibers) needs to work and develop together for it to be of any use.

The body, unlike the car, stays on all its life. The car can shut off if it runs out of fuel. But if the same thing happens to the body, it dies. So any system that is going to take on the responsibility of being the body’s main powertrain has to be able to provide a stable flow of energy over a very long term.

The best way to accomplish this is by burning a cheap, safe, light, efficient, and plentiful fuel source: fats. (As I’ve discussed before, burning carbs/sugar comes with a lot of strings attached: it’s dirty, heavy, scarce, inefficient, addictive, and dangerous. The only real advantage it has—and it is a BIG advantage—is that it produces energy at a much greater rate than fats.)

Being the system that provides stable, long-term energy means that you need to burn the stable, long-term fuel. Because of this, the aerobic system has to burn fats in particular as its primary fuel.

In other words, I use Phil Maffetone’s rendition of what the aerobic system is. This means that while I like statistics such as VO2Max (maximum volume of oxygen utilization per minute) as measures of aerobic power, I don’t believe they are a measure of the functionality of the aerobic system. Why? Because you can consume far more oxygen when you’re burning sugars than when you’re burning fats. And besides, we’ve defined the aerobic system as providing energy over the long-term. Therefore, aerobic functionality has to do far more with fat-burning, which occurs in a big way at moderate percentages (55-65%) of VO2Max, than with sugar-burning (which occurs in a big way at 65-100% of VO2Max).

(Note that very highly-trained endurance athletes are often an exception to these percentages. Why that’s the case is for another post.)

One of the reasons this system has the name “aerobic” is because fats cannot be burned outside of the presence of oxygen. So, bringing oxygen into the body and enabling its efficient transport throughout is absolutely essential to our capability to use fat as fuel. In fact, this is one of the most important differences between fat and carbs: carbs unlike fats, can be burned both aerobically and anaerobically.

This may seem like an advantage, but it’s somewhat of a disadvantage—in the same way that the disadvantage of cocaine is how powerful it is. You’re far better off experiencing the feeling of reward in the less powerful, sustainable, and old-fashioned way.

This is not to say that sugar has no place in our utilization of energy: at any given point in time when we’re at rest or doing light activity, we’re burning a small percentage of carbs. But when sugar stops being your auxiliary fuel (and becomes your go-to fuel), you’re in trouble.

By primarily using a fuel that is very powerful, it’s much easier to use only that fuel. Why would you use the other, less powerful fuel? (Sure, because it’s lighter, cleaner, safer, and more efficient.) But there’s also this: since carbs burn way quicker, the body can get lazy and forget about maintaining its fat breakdown and transpo systems, with little short-term negatives—but huuge long-term drawbacks.

By the time that the downsides of relying primarily on sugar begin to roll around, the body is hooked and the systems that burn and transport fat are in utter disrepair. The body can only store about 2,000 calories of carbs at a time (compared with some 120,000 calories of fats on the low end). When it prefers sugar over fats, it has to be eating all the time.

In layman’s terms, this is known as a “metabolic SNAFU.” (That acronym fits particularly well here, just because of how ubiquitous and “normal” this situation is.)

So what are these systems? Let’s trace the flows of oxygen and fats to find out.

Fats have to be broken down from fatty tissue, transported through the blood vessels, and burned by the mitochondria—the cell’s aerobic motors.

Oxygen comes into the body through the respiratory system, then gets transferred to the circulatory system, and finally permeates into the muscle cells where it is used as a reactant to convert the fats into energy.

But if we’re going to talk about flows of materials (oxygen and fats), it’s not enough to just discuss the parts that they flow through (and the systems that convert them into energy). We have to talk about the parts that regulate those flows, for the simple reason that if those regulatory systems stop working, chaos ensues. So these regulatory systems are as critical to the function of the aerobic system as, say, the car’s computer is to the function of its powertrain. It’s a part of it, pure and simple.

Let’s look at oxygen.

As any asthmatic or person with hay fever will tell you, those regulatory systems make a difference. The reason a lot of people start wheezing when they run too hard for too long is because the part of the nervous system responsible for increasing the body’s activity levels (known as the sympathetic nervous system, or SNS) gets too tired, and its function collapses. A crucial part of increasing activity levels is to open up, or dilate, all of the body’s ducts (a.k.a the airways and blood vessels) so that more stuff can flow through, at a faster rate. So, when the SNS becomes exhausted, its ability to keep the airways dilated goes away. Its opposing system—the parasympathetic nervous system, or PNS, whose job it is to shut the body down—takes over. One of its jobs is to constrict the airways—and so they close up (hence the wheezing).

Regulation of fat-burning functions in a very similar way. The system most directly responsible for regulating fats is the endocrine (a.k.a. hormonal) system, affecting primarily (and IMO most critically) whether or not, and at what rate, fats are broken down. This process is known as lipolysis: lipo = lipid (fat); lysis = breakdown.

Lipolysis is accomplished partially thanks to a hormone called leptin. In healthy humans, leptin exists in the bloodstream in a big way only when the body is at a reasonably low level of stress. So, one of the reasons that fat-burning starts going down at an exercise intensity even slightly over moderate—which is known in the biz as the AerT or aerobic threshold (go figure)—is because the increase in exercise intensity puts out stress hormones that inhibit the activity of leptin. As exercise intensity increases beyond the aerobic threshold, lipolysis begins to slow down.

So it doesn’t really matter if the muscles have a whole bunch of mitochondria that were developed by training at a high intensity (remember: a.k.a. stress), and burning lots of sugar in an aerobic way. If the body’s lipolytic systems haven’t been trained, it’s going to burn very, very few fats during exercise. So it doesn’t really matter what’s going on in the muscles. Muscles (even aerobic muscles) get really big really fast and their ability to consume fuel increases very quickly—but the rate of lipolysis takes much longer to improve.

The rate at which the body is capable of breaking down fats (rather than the rate at which it can burn them, or the rate at which oxygen can be supplied) is typically the bottleneck. And that’s why “fit” people all too often manage nary a shuffle when they start running under their aerobic threshold: they’re sugar-burning beasts, but under the AerT the hormones are optimized for burning fats, not sugar. Those powerful muscles they have? They’re being fed fats with a teaspoon.

And one of the reasons it feels so slow is because they’re exercising at a relatively small percentage of their oxygen intake and transpo capacity. Why? Because they’ve trained it primarily in concert with their sugar-burning system. Their fat-breakdown system needs to become waaay stronger before it’s going to break down fats at a rate that is challenging or even meaningful to their present oxygen transpo capabilities.

Understanding which systems comprise the aerobic system is far less important than grasping the point that the aerobic system really is the entire powertrain. It’s far less critical to know whether your car has a carburetor or a fuel injection system, than to know that you should consider how the entire powertrain (and the car as a whole) is going to behave when you decide to upgrade some particular component.

If you’re going to swap your L4 engine block from with a V8, you also have to swap out the fuel pump and a host of other systems (not to mention the entire chassis)—or you’re going to end up with a V8 engine getting fuel at a rate meant for an L4. The understanding that you need to go look at the whole picture, instead of just at the muscle fibers (or whatever)—will inevitably take your search in the right direction.

There’s a bunch of other parts of the aerobic system left to cover. In my next post I’ll talk more about how the fat-burning process goes down and why it’s impossible to burn more fats when the rate of sugar-burning goes up. I’ll also get more into why the body is wired to rely more on sugar as stress levels go up (hint: because sugar burns faster). In later entries I’ll talk about how the various other parts of the aerobic system interact with each other, and why aerobic function can really only be developed and optimized at relatively low levels of exercise intensity.