The bonk is a funny name for a terrible feeling, when suddenly there’s nothing left in the tank, the legs turn to jelly, and getting to the finish or just back home becomes an altogether supreme effort of will.
But what is the bonk? The simple explanation for its occurrence is that long-endurance exercise depletes the body’s store of glycogen, which produces the energy required to maintain performance. When the glycogen depletes entirely, the body has no more fuel and instead burns fat, resulting in a surge of fatigue and a performance collapse. That’s the simple version anyway, but research shows it may be more complex than this nutritional description suggests, with genetics, mental factors and training all playing a role.
So in this examination of the bonk we start with the basics: the nutritional causes and how you can safeguard yourself against them; and then we move onto the more complex and controversial aspects.
A cyclist’s best protection against the bonk is to ensure that glycogen is fully topped up before starting and that it’s replenished throughout the ride. The former is generally achieved through carbohydrate loading — that’s ensuring all meals in the 48 hours prior to a big event or training ride contain an adequate level of carbohydrate.
But this certainly shouldn’t be taken to extremes. Many amateur athletes often use the carbo-loading excuse to pig out on pasta, potatoes and rice, believing it will protect them from the bonk. Unfortunately, it’s not that simple.
“Every day, you need about 7-10g of carbohydrate per kilogram of body weight to keep glycogen fully topped up,” says Dr. Karen Reid, who runs the sports nutrition website Performance Foods. “Overloading on carbs will result in weight gain.”
Clearly pasta, rice and potatoes are not suitable for on-bike snacks, so cyclists generally use a sports drink, gels, bars and other portable snacks to replenish glycogen stores during the ride. Drinking a good carbohydrate sports drink is essential, as it also helps with re-hydration as well as packing in some calories. Picking the right one is trickier.
Essentially it’s a balancing act between glycogen replenishment and re-hydration, as the two offset each other. The higher the carbohydrate levels of the drink, the more it disturbs re-hydration and the harder it is to stomach. For that reason, many riders opt for an isotonic drink containing about 6-7 percent carbohydrate, which balances the need for glycogen replenishment with hydration.
Glycogen theories
There is some evidence that mixing a few things into your sports drink may delay glycogen depletion further. Though it’s still controversial due to the fact that it’s a diuretic and may work against re-hydration, caffeine is now widely included in sports drinks. This is because studies have shown that ingesting it before and during exercise results in performance increases, particularly in bouts over two hours.“It’s crucial to know how much carbohydrate to consume during a ride to maximise your chances of avoiding the bonk”
While the exact mechanism remains unclear, one theory is that caffeine increases the release of free fatty acids into the blood early in exercise. This increases muscle fat oxidation and decreases carbohydrate oxidation, sparing muscle glycogen.
“You don’t need a lot of caffeine to get the maximum effect,” says Professor Asker Jeukendrup of Birmingham University, whose study found that caffeine can boost the carbohydrate absorption rate by as much as 26 percent. “Two or three milligrams per kilo of body weight is about right.”
Another controversial area is protein. Protein has long been used as a recovery food to repair muscle damage, but now some scientists believe it may work during exercise too, as extra protection against the bonk. The supporters of protein point out that during prolonged exercise it can contribute 5-10 percent of total energy demands.
They also say that although carbohydrate remains the principle fuel source, having a drink with a carbohydrate/protein ratio of about 4:1 means you’re topping up your protein stores too, and this additional energy source could make the difference between bonking and not. Three studies have found this to be the case, but two have found that protein had no extra effect. The jury remains out, but if you’re particularly prone to the bonk it could be worth giving protein a try.
Another common mistake is to assume that you don’t need to start topping up glycogen levels until well into a ride. Researchers from Maastricht University disproved this by examining 10 male subjects on a three-hour cycle ride. They found sports drinks reduced the glycogen used to maintain a certain pace all through exercise, not just at the end when the bonk threatened to debilitate. So it’s important to get sipping early.
Finally, it’s crucial to know exactly how much carbohydrate to consume during a ride to maximise your chances of avoiding the bonk. Helpfully, a separate study by the Gatorade Sports Science Institute found that the optimum amount was between 30 and 60g of carbohydrate per hour. Although it’s probably closer to the latter for most people, which, surprisingly, is almost a litre of isotonic drink per hour.
There are two training approaches to avoiding the bonk TT / Getty
Just a male thing? Recently, a team from Springfield College in Massachusetts studied the bonk in marathon running, where it’s commonly known as ‘the wall’. Rather than focusing on nutrition, their study looked at other factors such as expectation and gender. The results were intriguing. Of the 300 runners studied, 43 percent felt they’d bonked. The majority of these were men.
Indeed, the results showed that men were almost twice as likely to succumb. The researchers believe this is partly due to expectations: 28 percent of the men said they ‘expected’ to hit the wall compared to 20 percent of women, but competitiveness may also play a key role. This may be largely a function of the men being more likely to push the pace and, as a result, experience more characteristics of the wall than the women, the researchers conclude. Or could it be that men are just genetically more prone to bonk? Certainly there did seem to be a genetic role.
The researchers found no correlation with age or previous performance: the faster and more experienced athletes were just as likely to bonk. In fact, runners who had hit the wall before were the most likely to hit it again. Some athletes may simply have an elevated propensity for hitting the wall and even perfect preparation may not prevent it, the researchers say. Another school of thought links a propensity to bonk with pre-race nerves. This is based on the ‘Cusp Catastrophe Model’ formulated by L Hardy and J Fazey in a 1987 paper.
This paper states that when anxiety is low, for example in training, physiological arousal (increased heartbeat, sweaty palms, butterflies in the stomach) will interact with performance in the shape of an inverted ‘U’, that is, the more physiologically aroused an athlete is, the better their performance will be up to a certain point when it will tail off as gradually as it rose. But when anxiety is high, for example in a big competition, increased physiological arousal will be beneficial up to a certain point, at which time any further increase will be catastrophic. See ‘On the cusp’ below.
Bonk-busting training
There are two training approaches to avoiding the bonk, one is controversial, the other, on the whole, isn’t. The latter is simply to do as much base training as possible. This low-intensity work recruits more and more muscle fibres and ensures more blood capillaries wrap themselves tightly around these fibres, allowing the transfer of oxygen from the blood to the muscles to be as economic as possible. The more economic your system, the less glycogen you burn. Every muscle is made up of two types of fibre: fast-twitch and slow-twitch. Fast-twitch fibres are used predominantly in explosive events, such as weightlifting or sprinting, and slow-twitch are what you want primarily for endurance; you use a greater percentage of slow-twitch muscle fibres the more aerobically fit you are. Both types do require fuel but slow-twitch muscles are able to burn fat as well as carbohydrate/glycogen.“A more controversial school of thought advocates specific bonk training”
Joe Friel, author of several training books and coach to national champions and Olympians, recommends using your heart rate (HR) as a guide to knowing when you’ve recruited all the muscle fibres possible. Friel suggests cycling for two or three hours at a comfortable pace, say 75 percent of your maximum HR, and as long as the pace stays constant, so should your HR. If it drifts upwards throughout the ride, you should do base training a little longer.
As aerobic fitness improves, a greater percentage of the workload is done with slow-twitch muscle fibres, he says. This means that economy improves as more fat is used for fuel, sparing glycogen so the athlete can go longer without bonking. A more controversial school of thought advocates specific ‘bonk training’. This involves training early in the morning, before breakfast, so that your body’s store of glycogen is low. Proponents claim this forces the body to bonk early in exercise, causing the body to adapt to burning fat.
“There’s actually some research that supports this concept”, says Friel. “But I don’t use this with my athletes as it also means the quality of training decreases and there’s a longer post-workout recovery.” Though bonking during a race is catastrophic, you also want to ensure it doesn’t happen during training. If you don’t refuel properly, you may bonk during your next session, particularly if you train on a daily basis. Timing is crucial here, because for the four hours immediately following vigorous exercise, carbohydrate is converted into muscle glycogen at three times the normal rate.
Miss this window and your recovery will be much slower. Researchers from Loughborough University found low glycemic index (GI) meals work best. Athletes who ate tinned peaches, yoghurt, muesli and pasta (low GI) subsequently exercised for 12 minutes longer at the same speed than athletes on a high GI diet, featuring white bread, bananas and Lucozade (a British energy drink).
“The low GI diet burned more fat, sparing muscle glycogen for later in exercise,” says lead researcher Dr Emma Stevenson. “The athletes also reported feeling more satiated after low GI recovery meals even though they contained the same calories — so it should help with ongoing weight loss too.”