In the modern approach to training and fitness, the idea that you should (or need to) train at a low intensity is utterly neglected. This neglect is a huge problem. It benefits the few, and harms the many. And even when this philosophy works, it only does so up to a point.

A recent article in Outside Magazine bit into this issue with great abandon. The Outside article discussed the extreme example: Overtraining Syndrome (OTS) in ultrarunners. Many elite ultrarunners have become seriously overtrained, finding that their legendary competitive and running ability evaporates almost overnight. And we see this sort of thing across the board: in crossfitters who get exertional rhabdo; in recreational runners that start too hard. But why does this happen?

Our present fitness culture has an extremely damaging “more is better” and “no pain, no gain” mentality. If your favorite sport is HIIT or CrossFit, you’re prompted to increase the intensity, to “feel the burn,” and to “not feel your legs after leg day.” You name it, it’s out there. If your favorite sport is running, everything around you tells you to collect miles like they were baseball cards—the more the better.

The problem is this: whether you’re an elite ultraunner or someone who is just looking to shed some pounds, the amount (or type) of training that society pushes you towards typically means a lot of stress. It’s not that you won’t get quick results with that high-intensity training program (or by going out and clocking as many miles as you can). It’s that in doing this, a majority of people cross a stress threshold beyond which it’s impossible to keep these gains. It happens to Joe Smith at the gym, and it happens to the ultrarunner.

But in order to understand why it happens (and why you can’t cheat your way around it) we have to discuss a critically important biological system known as the Hypothalamic-Pituitary-Adrenal, or HPA axis.

The HPA axis is the system that creates the autonomic stress response (ASR)—which kicks up the organism’s stress levels (think: alertness) in order to survive a challenge to its existence. Let’s put this in a real-world example: alertness alone isn’t enough for an antelope to escape a lioness. There are two more components to ASR: First, the antelope’s heart rate has to go through the roof in order to bring a high volume of blood to the muscles. Second, the antelope’s anaerobic energy system—which burns sugar without the presence of oxygen, kicks in.

There’s another energy system available to the antelope: the aerobic energy system. It burns a much more plentiful resource—fats—but it takes some time. The fats have to be broken down into sugar, transported through the bloodstream to the muscle fibers, and combined with oxygen inside the mitochondria, before they can be converted into energy. Typically, it takes the aerobic system 15 minutes to get to full burn. But the antelope doesn’t have 15 minutes. It doesn’t even have a few seconds for the initial gulp of oxygen to reach the muscles through the bloodstream. There’s a lioness charging towards it at 40 mph. It needs energy now.

Stress, a high heart rate, and the anaerobic system are hardwired together in every animal. This wiring has to be absolutely reliable. If it wasn’t—if, given certain conditions, you could get a high heart rate and stress but no spike in anaerobic activity—you will die. As far as your body is concerned, in a “real-world scenario” the price for not having these three things occur together every time, with utter certainty, is death.

That’s what your body is thinking—thanks to your HPA axis—every time you get too stressed. Your HPA axis has to assume that there’s an imminent threat to your life, and make all of your internal systems react accordingly. If not, you will die.

The anaerobic system takes over to ensure the immediate survival of the organism. It doesn’t just happen to burn the fuel we use in the short term (sugar). We are wired so that when our bodies are thinking and acting in the short-term (that is, prioritizing escape from a threat over long-term health) we use the anaerobic system.

On the other hand, when our bodies are behaving with the long-term in mind, we use the aerobic system. In the long-term, it doesn’t matter if all the energy isn’t available right now—we’re not running away from anything. On top of that, we have fats, which is a more reliable and plentiful energy source. Sure, it takes a little bit more time to get energy from fats than from sugar, but time is something we have.

But that’s not all: there are reasons to NOT use the anaerobic system in the long-term. Burning sugar without the presence of oxygen wears down the engine: it accumulates protons—hydrogen ions (H+)—which cause the body’s pH to fall, becoming more acidic. (The idea that lactate is the culprit of muscle acidification is a misconception: the presence of lactate predicts, rather than causes, proton-based acidosis in the body).

In the short-term, the antelope’s body doesn’t care about its pH balance. If it doesn’t move, NOW, that lioness will take it down. The temporary acidification of the body is a small price to pay for escape. If everything goes as planned, 45 seconds from now, the antelope will have a chance to calm down. Its stress levels will drop, it’s heart rate will slow down, and a powerful aerobic base will kick in and all the lactate will get churned through the muscle mitochondria and converted into more energy. The proton build-up that happened during the chase will be quickly negated. In that process, a final acidic by-product will come out in a form that the body is designed to quickly and competently expel: CO2.

As soon as the body’s short-term survival has been secured, and it starts thinking in the long-term, it uses its aerobic system.

But if you are under chronic stress, your body never gets a chance to think in the long-term. Remember that stress, an elevated heart rate, and anaerobic function cannot be untied. If you are under stress all the time (even if it’s work stress), you’ll have at least some anaerobic function. Your body will be burning more sugar and less fat. As you use the aerobic system less and less, it will grow less inclined (and less capable) of fueling your daily activities with fat. You’ll have to rely on dietary sugar to keep your energy levels up. You’ll burn even less fat. You’ll slowly and steadily gain weight. But your body will also have a higher proton concentration than it should. It’ll remain more acidic. You’ll wear it down, putting yourself at risk of chronic disease.

Just look at how this snowballs. The media (and your peers) are kind enough to pelt you with exercise programs that promise quick, short-term gains! You can see where this is going. You’re piling acute stress on top of chronic stress. Your problem wasn’t the excess fat itself: it was that your long-term energy system—the aerobic system—was compromised. And those quick, short-term gains that you’re promised? You might get them, but at the cost of keeping them.

Yet again, you’re using the short-term energy system. Yet again, you’re training your body to think in the short-term. The energy system that is responsible for your body’s long-term upkeep is incompetent. By definition, you’ll be unable to maintain that level of activity in the long-term. You’ll lose those short-term gains.

Period.

The problem isn’t that you’re flaky, or that you’re not an athletic person, or that you’re not determined. No amount of discipline or determination will be able to overcome the fundamental problem: that you trained for the short-term instead of the long-term.