"Hitting the wall' is when your muscles run low on glycogen (glucose) and must switch to a higher percentage of fat burning, which is inefficient compared to glucose metabolism. You can continue running, but it won't feel very good.
I found this fascinating article from Runner’s World breaking down the science and rules of the bonk and thought I’d share with all my running readers… I have highlighted bits for you I find particularly interesting. :-)
The Science Behind BonkingWhen your body stalls mid-run, it's called bonking. When scientists debate the causes, it's called a food fight. Here's everything you need to know. By Paul Scott
Chiang Kai-shek is said to have received news of his army's mutiny while still in his pajamas. Chances are you will be equally unprepared for the mutiny of your own body--in other words, for bonking. We're not talking about the mere cramping of a calf, or the everyday slowing caused by lactic acid build-up, or the deep muscle pain sometimes caused by downhill running. Marathoners used to call bonking "hitting the wall," but it's actually a bodily form of sedition. In some form or another, it becomes a collapse of the entire system: body and form, brains and soul.
Consider the muscle-glycogen bonk, where the brain works fine but the legs up and quit. Then there's the blood-glucose bonk, where the legs work fine but the brain up and quits. Let's not forget the everything bonk, a sorry stewpot of dehydration, training errors, gastric problems, and nutrition gaffes.
And then there's the little-purple-men bonk. "After about 20-K, I started to see little purple men running up and down the sides of these cliffs," says Mark Tarnopolsky, M.D., who wears hats as both a leading sports nutrition researcher and an endurance athlete. "I knew it was an hallucination, but I stopped in the middle of the race to look at them anyway," he says. "It was kind of crazy."
If you have run a distance race, chances are you have already become an aficionado of the bonk. You remember how your form held until you hit mile 18 and your feet turned into scuba fins. How your motivation held until you faced that last hill and became preoccupied with the idea of lying down on the pavement. Or, if you bonked thoroughly enough, how you began to see beings that belong in Dr. Seuss. And you thought sports nutrition was dull.
And now, the field is undergoing the scientific version of a food fight. The sanctity of carbohydrates has come under question. Endurance athletes are rediscovering protein. Products are making new claims, nutritionists are taking sides. And we haven't even gotten to the reasons why many runners act so weird about food in the first place. But in essence, the science of bonking comes down to 10 laws. If you learn them, you won't merely be on the cutting edge of sports-nutrition science, you may never bonk again.
First Law: Food is Chemistry
Start with the very spark of movement, wherein our muscle cells power their contractions through the continual breaking and reconnecting of a chain of molecules called ATP. Cementing their bonds anew requires stealing energy, in this case energy holding together bonds in other compounds--specifically, proteins, fats, and carbohydrates.
Even as they all sit on the same nutrition label, these three meta-nutrients are different beasts altogether: Proteins are amino acids, the alphabet beneath our very DNA and the material used to make flesh. Fats are also acids, in the form of oils, and as such help with the insulation of cells. Carbohydrates, on the other hand--literally hydrated carbons, shorthand for the carbon-and-hydrogen hexagons informally known as sugar--are different. Mammals have virtually no body parts made of carbs. While fats and proteins can ultimately be rendered into carbohydrates, the carbs you eat serve no purpose other than as booty, appropriated loot to be ransacked for its atomic mortar.
Second Law: Cinnabons=Plutonium
Upon entering your stomach, carbohydrates are broken down for transfer to the small intestine, where these sugars change into their blood-traveling form known as glucose and shoot on up into the liver. The liver extracts as much glucose as it can hold from this blood supply, which comes out to around 100 grams, or about the amount found in two Clif bars--enough to feed the brain for about four hours. Red blood cells burn some of what's left over. The remaining glucose molecules travel on through the bloodstream. This is what muscles burn. They take it preferentially from your bloodstream but also use glycogen stored locally. All told, your muscles hold 300 to 400 grams of glycogen. Your skeletal muscles, the ones that do the running, only hold about 100 grams. Still, this is enough fuel for a couple hours of fairly hard running, given the way the body eventually begins to burn primarily fat. (In other words, your skeletal muscles' maximum burn rate is twice that of your brain. Think about it.)When your liver and muscle stores max out at a combined 500 grams of carbs--pretty much the case for the typical American moseying back from lunch hour--the surplus triggers a hormone, insulin, to spike, which causes the sugar leftovers to turn into fat. Some carbs provoke bigger spikes, and more fat-packing, than others--they have what's known as a high-glycemic index. No matter where the extra calories come from, the average person totes enough fat to fuel a month's running at a pace slow enough for the oxygen necessary to burn it near-exclusively. But you couldn't, physiologically, and if you could, you would cross the finish line just after the cleaning crew. Which is why carbs were seen as the limiting factor in sport performance and the scientists' sports nutrient of choice from day one.As far back as the 1930s, researchers put athletes on a high-carbohydrate diet and compared them with people eating mostly protein. The carb-eaters had three times the endurance of the protein crowd. The Swedes got similar results when they put endurance athletes on high-fat, high-carb, and high-protein diets. Then researchers began to wonder: Can you make your muscles stockpile more than their usual share of glycogen in the days before an event?
Third Law: The Spaghetti Dinner Isn't Just a Cheap Way to Feed a Bunch of Cheap Runners
By the late 1960s, the most popular method of carbo-loading touted a seven-day pre-event cycle. You went on a low-carbohydrate diet during the first three days to deplete glycogen stores, and during the final four you got 70 percent of your food from carbohydrates (or 8.5 grams per kilogram of body weight, to be exact, though it's hard to see how a person could manage to so precisely calibrate every meal and still have a life). The method increased muscle glycogen by as much as 150 percent, a big boost to endurance. Researchers eventually refined this method by eliminating the carbo-drain phase, substituting it with a taper in exercise, and making carbs compose three quarters of the diet. Either way, carbo-loading staves off the classic muscle-glycogen bonk, in which the body seemingly runs out of available sugar and starts burning even a larger ratio of fat in the fuel--a process which, because it must first convert fats to sugars, entails 20-some metabolic steps compared with the 10 or so for burning glucose. It's like switching from high-test to coal. Fat takes its sweet time--even for runners like the Kenyans, who are the best of all of us at burning it--and you slow down.
Fourth Law: Your Brain is a Pig
So carbo-loading seems to aid your endurance by stuffing fuel into muscles. But let's not forget the brain. It burns only liver glycogen, and it's a glutton. As the fuel demands of muscles and brain draw down blood-sugar stores, your motivation, decision-making, and agility can go on the fritz. "You get what's called central fatigue," says Tarnopolsky. "It's the perception by your brain that you're tired, even though your muscles are fine."
In experiments that only cash-poor students would volunteer for, test subjects pushed below three millimoles of glucose per liter of blood (normal is 3.5 to 5.5) began to lose the ability to do calculations. They couldn't even read. In real life, according to those who've been there, you just stop caring. You lose your competitive edge. If your sugar level continues to drop, you can eventually hallucinate. Tarnopolsky credits late-stage central fatigue with his little purple men episode during a triathlon. When the brain is starved, neurons in the occipital cortex misrepresent incoming images. A tree could be perceived as a human. The brain could make up things that don't even exist: falling snowflakes become . . . purple men. "Some people incorporate them into their consciousness, like a dream state," he adds. Not a good state to be in when you're running. "Those are the people you see delirious, running off the edge of the road, collapsing," he says.
But there's a miracle cure, albeit a rather mundane one: the sports drink. Barring such a dire situation, as little as 50 grams of carbohydrates can bring your brain back to normal in 10 or 15 minutes.Common wisdom once had it that you should eat only slow-burning carbohydrates just before a race. Fast-burning (high-glycemic) carbs--those milled to a particulate size not seen in nature prior to the invention of the Ding Dong--are possibly the chief reason most Americans are overweight. In sedentary people they cause an insulin spike followed by
a blood-sugar crash. More recently, however, researchers have learned that the moderate activity and nervous energy of a runner before a race counteracts any insulin spike and renders all carbs equal. (Bagels, anyone?)
Fifth Law: A Long Run is No Time to Watch Your Weight
In the mid-1960s, the Human Performance Lab at Ball State University found that carbohydrate supplementation during exercise could not only keep the brain fed but also spare glycogen in muscles and improve endurance. And so they begat the sports beverage--water with salts and electrolytes (they keep up osmotic pressure among the cells to prevent dehydration), along with fructose and sucrose in as concentrated a form as the blood-depleted stomach can handle (generally a ratio sweeter than water but more watery than Coke). "The concept of consuming carbohydrate during exercise meant that you were raising blood-glucose levels, moving more glucose into the muscle cells, and the muscle cells were using the energy coming from the drink and sparing the muscle glycogen," says Robert Portman, Ph.D., a biochemist and president of the New Jersey-based PacificHealth Laboratories. "That's been the byword for sports nutrition since that time."
For 30 years, the equation changed hardly at all: Load up muscle glycogen the week before a race, load up liver glycogen the morning of, reload to spare muscle glycogen during the race. All was well. For a while anyway.
Sixth Law: Protein + Carbs = Kaboom!
Sports nutrition scientists naturally wondered whether they could find a way to speed up the body's own synthesis of glycogen, allowing us to draw down our stores more slowly and restock them faster. But how to get around our metabolic timetables? The conversion rate of each category of foods has been charted out for years; they seemed to be stumped. Unless.
Unless one dropped the quaint assumption that the body's only use of protein, carbohydrates, and fat was for each one's primary purpose. In 1987, researchers first began considering whether certain combinations of these nutrients actually interacted with one another in a helpful way. For instance, could protein make carbohydrates drive into their metabolic garages even faster? The answer was yes, says Portman, who has since gone on to carve out a growing niche of sports-nutrition research and commerce dedicated to the question. "They realized that protein strongly stimulates insulin release." Insulin speeds muscle cells' absorption of blood glucose by as much as 50 percent, so when you're burning stored carbohydrates at a break-neck pace, speeding up the entry of blood glucose is vital. Insulin also moves amino acids into muscle, blunts the release of the stress hormone cortisol, and stimulates blood flow to the muscle. "Carbohydrates were always the most obvious focus for insulin stimulation," says Portman. That's because the body tends to release insulin when glucose levels rise above a fixed rate--100 milligrams per 100 milliliters of blood, to be exact. "But here they noticed that when you added protein to carbohydrates, you got an additive effect."
Seventh Law: Timing is Everything
Portman joined forces with fellow nutrition researchers John Ivy, Ph.D., of the University of Texas and Ed Burke, Ph.D., of the University of Colorado. Ivy had discovered that you get maximum muscle glycogen replenishment if you eat within 30 minutes of exercising. "After 45 minutes, muscle sensitivity to insulin begins to decline. After about two hours, the muscle cell becomes actually insulin-resistant," Portman says. "Anybody who is savvy about nutrition and performance recognizes that timing is key. Your muscle responds to nutrients differently throughout the day and in response to exercise." Portman certainly is savvy: He and Ivy recently published a book called Nutrient Timing, dedicated to the implications of what Portman calls, sexily, "turning a catabolic process into an anabolic process." (Translation: You go from tearing down your muscles to building them up.)
The nutrition establishment counters that the body replenishes its glycogen on its own within 24 hours, which makes super-charged glycogen reloading a point academic to all but two-a-day workout fiends. But Portman points to a study in which a group (albeit seniors working with weights) who received a supplement immediately after a one-a-day workout enjoyed an eight-percent improvement in lean body mass over those who waited two hours each time. With this in mind, researchers focused on studying glycogen replenishment from postworkout recovery drinks, and in 1992 they hit pay dirt. "We were able to show that, postexercise, protein and carbohydrate had a very beneficial effect in terms of stimulating insulin release and stimulating glycogen replenishment," says Portman.
Then came the much more pressing question--whether it did the same during a race, when athletes need the glycogen-sparing effects of rapid replenishment. So Portman and his colleagues studied cyclists at varying rates of intensity. Some of them took a normal sports drink, some drank a four-to-one carbohydrate-protein solution, and the rest had plain water. Voilà, the protein-carb beverage enabled the cyclists to go an average of 27 minutes to the carb-only group's 20 minutes (the water group managed 14 minutes). The insulin levels were no higher, however. Which sort of meant they had no idea why protein worked better. (Later studies did assert the insulin connection, though.)Meanwhile, Portman had already done what any good American would do under the circumstances: He lined up venture capital. His sports drinks, marketed as Endurox R4 and Accelerade, use a patented four-to-one carbohydrate-protein ratio--enough protein to stimulate insulin, but not enough to make you sick. (Protein stimulates the release of a peptide enzyme that can cause gastric distress.) His product will gross $12 million in sales this year, and won the affection of cyclists, elite athletes, coaches such as Roy Benson, Frank Gagliano of the Nike Farm Team, runners Lornah Kiplagat, Ryan Shay, and his coach Joe Vigil.
Eighth Law: Good Science is Like a Bar Brawl
The scientific community had a classic response to the protein studies: Do they compare apples and oranges? Tarnopolsky and others charged that the Portman gang had compared two calorically unequal beverages; more calories means more insulin. Ivy and colleagues went on to deliver the protein effect with drinks of equal calories. But their critics, including Tarnopolsky, had the same results leaving the protein out of the picture.
Things haven't settled down much since then. In a response to a study I recently e-mailed to him, an unflappable Tarnopolsky gave his opponents' latest work the scientific equivalent of a wedgie: "THE AA + cho WAS NOT ISOENERGETIC TO THE CHO ALONE GROUP (AGAIN!)"
Well, it's hard to argue with that.
Ninth Law: Every Five Years, Good Advice Becomes Bad
At least your old sports drink does the trick. Or does it? "It is impossible to prove that muscle glycogen depletion alone limits prolonged exercise performance," writes University of Capetown physiologist Tim Noakes. See, while we know that loading carbs extends exercise, there's never been a way to reload our legs instantly, which is what you would need to do in a lab if you wanted proof that burned-out legs really caused your bonk.
Here we stumble into the inseparable relationship between our head and our legs. You may think your gams have run out of gas, but that information comes from your brain, which is hardly a disinterested party. To see whether the brain is a culprit in bonking, scientists made athletes work out until they thought they'd hit the wall. Then the researchers numbed their volunteers' central nervous system and artificially stimulated the muscles. (Please, Sir, may I have another!) "They continued to twitch, which meant that they had not in fact run out of glycogen," says Dan Benardot, Ph.D., researcher and author of Nutrition for Serious Athletes. Writing in the online journal Peak Performance, Noakes has pointed to a host of other evidence that glycogen depletion has had a bum rap. In one study, athletes were driven to the point of exhaustion after four hours. Their muscle glycogen concentrations and carbohydrate burn rates were the same as at three hours. "The tradition in the science is, you hit the wall when you run out of muscle glycogen," says Benardot. But he maintains that the carbs stored in the muscles and bloodstream, along with the energy coming from fat, should supply the 100 extra calories per mile that a runner needs and then some, provided he stays aerobic. "When you do the math, there should be plenty of glycogen left in those muscles," Benardot says.
The brain may have another opinion. "It's a very interesting phenomenon that we're only now coming to grips with--that mental fatigue will lead to the perception of muscular fatigue," says Benardot. He notes that the brain has a lot of processing to do during a run, monitoring blood volume and sweat rates, core temperature, blood sugar, and stress hormones. "The brain is juggling all of this information and can eventually make the decision: 'Whoa, things are not good here, I'm going to shut it down.'"
Interestingly, protein may play role in protecting against central fatigue as well. "Researchers in Oxford have found that branch chain amino acid [translation: protein] depletion leads to elevated levels of tryptophan in the brain," says Portman. And as anyone who has gotten sleepy after ingesting too much turkey dinner can attest, "tryptophan depresses the central nervous system."
Tenth Law: People Never Listen
Then there is the larger question of people, and how we may be bonking for far less metabolic, far more goober-headed reasons. Running to lose weight, for instance. As most of us know, it works better than just about any diet. But heavy training with less eating equals frequent bonking. Eberle says some people think, "'Wow, look at all the calories I'm going to burn during this long run. If I can burn 600 I don't want to have two gels and only end up burning 400 calories!'" In the world of sports nutrition, that's just not getting it. And there are the tactical mistakes. Some runners don't want to slow down and drink at the first and second water stations. "I was once on the ABC truck for the Olympic Marathon Trials, and there was only a small proportion of runners who were drinking anything early in the race" says Benardot. "That's a huge mistake. Perhaps the biggest one you can make." Even slight dehydration slows gastric emptying, the removal of food from your gut into your blood stream. You need to keep a constant flow into the system, he says. "A large bolus of fluid before a race will stimulate gastric emptying at the start. Then all you have to do is keep it up, drinking something every 10 minutes to keep the fluid flowing."
So if there were a law above all laws, it would be this: Never forget your bolus!