Beat the Bonk

While British riders tend to snigger whenever it’s mentioned, the cycling phenomenon known as the bonk is no laughing matter. Deriving from the original meaning ‘to hit’, the bonk refers to that catastrophic moment when there’s suddenly nothing left in the tank; when the legs turn to jelly, and simply getting to the finish becomes a supreme effort of will.

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, although 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.

Stock up on carbs

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 a high 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.

“Per day, you need about 7-10g of carbohydrate per kilogram of body weight to keep glycogen fully topped up,” says Karen Reid, who runs the sports nutrition website “Carb overloading will result in weight gain.”

Fuel on the move

Clearly, pasta, rice and potatoes are not suitable for on-bike snacks so cyclists generally use a sports drink and supplements to replenish glycogen stores during the ride, especially as they also have hydration benefits. It’s estimated that Tour de France competitors get 50 per cent of their daily calorie intake from on-bike supplements, so it’s clear that a good carbo sports drink is essential. Picking the right one is trickier. Essentially it’s a balancing act between glycogen replenishment and rehydration, as the two offset each other.

Don't be afraid to experiment

The higher the carbohydrate levels of the drink, the more it disturbs rehydration and the harder it is to stomach. For that reason, most riders opt for an isotonic drink containing about six to seven per cent carbohydrate, which balances the need for glycogen replenishment with hydration. But this doesn’t mean you shouldn’t experiment with other levels if carbohydrate. On a cold day, rehydration may not be a priority, so if you can stomach a higher carbohydrate level, go for it, because it’ll ward off the bonk for longer. Trial and error should provide a sound basis for what works for you.

In the mix

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 rehydration, 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. 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 recent study found that caffeine can boost carbohydrate absorption rate by up to 26 per cent. “Two or three milligrams per kg of body weight will be enough.”

Put protein to the test

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 per cent of total energy demands. They also say that although carbohydrate remains the primary 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 recent studies have found this to be the case, but two have found that protein had no extra effect. The jury remains out.

Start sipping early

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. 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.

Discover more great features on every aspect of cycling and get ahead of the pack with a subscription to Cycling Plus magazine. Prudential RideLondon participants can click here for an exclusive offer.