Fuel yourself properly to avoid energy crashes during the bike leg of a triathlon.


Coaching editor Phil Mosley shows you how.

When  it comes to cycling, the word “bonk” has its own unique meaning. It’s a common term used to describe that feeling when you suddenly run out of energy during a long ride. It’s a bit like hitting the wall in marathon running. It can come so swiftly and significantly that it leaves you barely able to turn the pedals. One minute you’re keeping up with your friends on a tough, long ride. The next minute they’re disappearing up the hills and you’re left behind, powerless to chase them. And it may not be because they’re fitter than you.

The bonk happens when your body runs out of its number one fuel: glycogen. This is the energy you store primarily from eating carbohydrate, and it’s hidden away in your body ready to fuel your muscle contractions among other things. If you’re well fuelled before you start cycling, you should have enough glycogen to last you approximately 90 minutes, depending on your fitness, body size, and exercise intensity.

As it runs out, your body will progressively switch over to using fat as a fuel instead. Not that there’s anything wrong with burning fat – it can keep you going for days – it’s just that you’ll only be able to cycle about half as fast, like a hybrid car using its electric battery in slow traffic to save petrol.


During a three-hour bike ride, you’ll work your way through anything between 1,800 and 3,000 calories. To put this in perspective, that’s the equivalent of an entire day’s worth of food for most people. So riding hard on an empty stomach, and then not refuelling during the ride, will leave you with a massive calorie deficit. Under such conditions, your body has little chance of performing well. Not only will your performance suffer, but you’ll lose concentration. Not a good idea on a busy road.


The best way to avoid bonking is to consume somewhere between 100 and 250 calories every 30 minutes (more if you’re racing). It’s amazing how many people know this, but forget or ignore it. It doesn’t have to be a commercial energy product – if you prefer jam sandwiches they will work too. You don’t even have to calculate everything to the nearest gram. Experiment a bit with what works best for you, or set yourself a countdown alarm if you think you’ll forget to eat. Just remember that the harder you ride, the more calories you need to replace.

Don’t worry about what anyone else is or isn’t eating or drinking. Stick to what you know is right, and you’ll ride stronger and recover quicker. If you’re riding for less than 90 minutes, you may not need to eat at all, providing you were well fuelled when you left. And if all else fails and you still bonk, stop at a shop and buy yourself a Coke and a flapjack. That way you’ll make it home in one piece, ready to train again the next day.

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