Bonking while running. Let's discuss.

Emil Zátopek, one of the greatest endurance athletes of all time, won gold medals for the 5K, 10K, and the marathon at the 1952 Summer Olympics. Zátopek knew pain, and when it came time to defend his titles, the charismatic Czech wasn’t embellishing when he addressed his competition four years later: “Men, today we die a little.” For as long as humans have moved, competitive runners have pushed the limits of human endurance and feared the consequences of going farther than the body will allow. What follows is the evidence that explains why.

Endurance Origins

Anthropologists believe that almost two million years ago hominids began to engage in persistence hunting—literally running prey to death over hours of pursuit. Collapsing in persistence hunting was not only risky to the hunter: it meant others suffered if the village couldn’t eat. Over-expenditure was also characterized in ancient Greek lore. In 490 BC, Pheidippides famously ran 40 kilometers barefoot to bring news from the Battle of Marathon to Athens. Legend tells us that Pheidippides cried: “Joy to you, we’ve won!” and fell dead on the spot. The overconfident hare in Aesop’s fable fell asleep and lost the race. While not a collapse due to exhaustion, the famous precept “hasten slowly” (festina lente) came from this story.

Today scientists know much more about the different aspects of collapse: from how we fuel our bodies, to our threshold of physical exertion, to our anatomical design. And athletes everywhere are still searching for the perfect balance—performing at their peak with nothing left at the performance’s end. The most famous researchers studying the anatomy of human endurance are University of Utah biologist Dennis Bramble and Harvard University anthropologist Daniel Lieberman. They uncovered how humans (slow relative to fast-sprinting predators) became able to survive on the savanna before stone tools, and found we’ve evolved in ways that aid distance running and prevent collapse.

Human Anatomy

For example, the human skull has several features that prevent overheating. As sweat evaporates from the scalp, forehead, and face, it cools blood draining from the head. Veins carrying cooled blood pass near the carotid arteries, helping to cool the blood that is flowing to the brain. Our tall, fur-free bodies—with lots of skin surface—promote cooling. The nuchal ligament runs from the back of the skull down to the seventh cervical vertebra and acts as a shock absorber, helping the arms and shoulders counterbalance the head. The upper body and lower body move independently, counteracting the twisting forces of the swinging legs. Other parts of our anatomy—such as long Achilles tendons, large vertebrae and disks, and large areas in the hip, knee, and ankle joints—distribute impact forces. “Have you ever looked at an ape?” Bramble asked. “They have no buns.” Human buttocks provide stability in running. Because people lean forward at the hip to speed up, the buttocks “keep you from pitching over on your nose each time a foot hits the ground.”

In other words, humans are designed to move, with multiple systems that help us push the limits—but these systems need fuel and energy to work at maximum capacity. To gain energy for endurance, humans eat and metabolize food. How much is enough? The University of Missouri estimates that male athletes generally need about 23 calories per pound of body weight daily: roughly 3,900 calories for a 170-pound man. Female athletes can get by on 20 calories daily for each pound of body weight, or about 2,600 calories a day for a 130-pound woman. A North Carolina State University study says that male and female athletes building muscle mass need 24 to 27 calories per pound of body weight. The food we eat provides this fuel. The American Academy of Orthopaedic Surgeons says that most athletes should get 60 to 70 percent of their calories from carbohydrates, and many scientists say that depletion of carbohydrate stores during exercise is a leading cause of collapse.

What an athlete might call “hitting the wall,” “bonking,” or “dying” . . . this phenomenon starts as the body runs low on glycogen, a long, sugar-based molecule produced and stored in the liver (and also saved in our muscle cells). When we eat more carbohydrates than our bodies need, the surplus is stored as glycogen. Then when blood sugar levels fall, the glycogen breaks down to release glucose back into the blood. The catch is that for most of us, our metabolism controls how quickly we process this glycogen (among other food molecules)—be it when we exercise, or when we settle in to watch football after Thanksgiving dinner.

The key is in our genes. Karsten Suhre, a professor of physiology and biophysics at Weill Cornell Medicine noted: “Differences in hair color are apparent to the observer at first glance. However, in the case of metabolism, it takes much more effort to identify the role which the respective gene variant plays in the metabolism of a person.” How we use those big butts and long legs—they appear in genes involved in the breakdown of fats—isn’t the same in any two people. The resulting differences in metabolic capacity can affect a person’s susceptibility to diseases such as diabetes and hyperactivity, or even their ability to train and hit their peak athletically.

The easiest way most people calculate their mass relative to how they feel is by measuring their weight. But since glycogen molecules have three to four parts of water for each part of glucose, “water weight” (which is lost as we metabolize calories, particularly when athletes limit their intake of carbohydrates) can often be a misleading metric when exercising, as the body replaces those glycogen stores with fats, and some of that “water weight” returns. This explains why the body prefers burning glycogen to converting fat directly into energy; it’s easier and takes less resources. So a large portion of the energy used during exercise comes from glycogen. The lore about a marathon being “half over at 20 miles” is based on how long the average person’s glycogen stores will last. The body can store about 2,000 calories in glycogen at a time—which may not be enough for endurance athletes who burn that many calories in a couple of hours.

Regardless of when glycogen stores run low, the result is the same: the body recognizes the danger of collapse and slows down to conserve energy. At this point, athletes can still run, but their pace will flag unless they increase their effort. Eventually glycogen stores can get so low that the body shuts down completely and even jogging becomes impossible. Congratulations . . . you’ve bonked! Athletes who bonk shuffle, stagger, and lurch. They feel dizzy or light-headed (brains need glycogen too). Some feel nauseated. And the athletes who literally crawl to the finish line? Those are extreme cases.

But there are a few tried and tested ways to prevent bonking. One is to train by exercising at a predetermined fatigue level. Another is to fuel appropriately for the type of exercise. Unfortunately, it’s just as bad to over-fuel as it is to under-fuel. The body can process only about 30 to 60 grams of carbohydrates per hour. Eat more and nothing gets absorbed. A diet two-thirds carbohydrates, like the one encouraged by the American Academy of Orthopaedic Surgeons, has been encouraged for decades. But now other scientists are advocating deeper research into energy consumption, wondering if we can push past previous limits.

Bucking Conventional Wisdom

Tim Noakes, author of Lore of Running and professor at the University of Cape Town, suggests a nutritional U-turn. Turning decades of conventional wisdom on its head, Noakes says we shouldn’t load carbs, but protein and fat instead. Noakes began to question the role of carbohydrates after examining US dietary guidelines, which promoted a food pyramid built on six to 11 daily servings of carbohydrates, and from testing on himself. Clearly, the high-carb, low-fat diets of years past haven’t worked, Noakes thought. Instead, the very diet that was supposed to be making people healthier appeared to contribute to obesity and chronic lifestyle diseases.

Noakes wondered if humans were meant to metabolize refined carbohydrates efficiently. “Cereals and grains have been a staple of the human diet for only the past 20,000 years, whereas we began to eat meat perhaps 2.5 million years ago,” Noakes said. “I believe that the intense discomfort you feel near the end of ultramarathon races is not due to muscle glycogen depletion but is more likely due to muscle damage and your brain trying to tell you to please stop, as you are going to destroy your muscles if you continue.”

Noakes discovered that the brain-signaling molecule interleukin-6 alerted the brain when muscle damage approached, and saw that as interleukin-6 levels rose during exercise, athletes’ performance plummeted. So, in an experiment, runners were injected with interleukin-6 before a 10K time trial and ended up running a full minute slower than they did when given a placebo. He and other scientists also tested protein consumption during exercise and learned that when athletes consumed protein with carbohydrates and fluids during exercise, they had less muscle damage and more endurance than on carbohydrates and fluids alone. In one experiment, muscle damage was 83 percent lower and endurance 29 percent greater.

So why does adding protein to a sports drink reduce exercise-related muscle damage? The protein may be used preferentially for energy during extended exercise, and it may also raise amino acid levels in the blood, which reduces muscle protein breakdown. Some substances, including vitamins C and E, can reduce muscle damage during running without enhancing performance. Caffeine, on the other hand, does the opposite: it enhances performance without reducing muscle damage . . . by tricking the brain. Endurance fatigue alters the balance of important neurotransmitters in the brain: when dopamine levels decrease and serotonin levels increase, you start to feel miserable. Caffeine slows this shift and consequently delays fatigue. So all told, it’s not just muscles and blood sugar that signal the impending bonk—the bonk is in your brain, too.

Control Your Mind, Control Your Body

Giving in to exhaustion is related to self control. Conventional wisdom held that willpower—the capacity to exert self-control—was a limited resource depleted by exertion. In 2010, a group of researchers at Stanford University put this to the test. They found their subjects’ belief in personal willpower could moderate “ego-depletion effects.” In other words, people who thought their capacity for self-control was “unlimited” didn’t show diminished self-control after a depleting experience. The findings say “reduced self-control after a depleting task or during demanding periods may reflect people’s beliefs about the availability of willpower rather than true resource depletion.” How people viewed their ability to persevere usually indicated whether they could.

To counter fatigue and harness this willpower, all humans had to do was push their limits. Distance running coaching legend Mark Wetmore summed it up well:

“In football, you might get your bell rung, but you go in with the expectation that you might get hurt, and you hope to win and come out unscathed. As a distance runner, you know you’re going to get your bell rung. Distance runners are experts at pain, discomfort, and fear. You’re not coming away feeling good. It’s a matter of how much pain you can deal with on those days. It’s not a strategy. It’s just a callusing of the mind and body to deal with discomfort. Any serious runner bounces back. That’s the nature of their game. Taking pain.

Contributing writer Andrew Hutchinson is the author of The Complete History of Cross Country Running.