Training & Nutrition

Ask the Expert: What Should I Do If I Bonk While Cycling?

How not to bonk, and what to do if it happens...

Dizziness, a sudden, all-consuming loss of energy, a desire to get off the bike, sit down and have a nap – bonking is a cyclist’s nightmare.

We spoke to nutrition scientist, athlete and pioneer of Osmo Nutrition – Dr Stacy Sims – to find out exactly what’s going on when you bonk, how to avoid it, and what to do if you do bonk when cycling.

There are two kinds of ‘bonking’, but most don’t know this. “One type of bonking is from a drop in blood volume (loss of body water) and is perceived as flat, dead legs, a drop in power with an elevated heart rate. Most people think this is a need for calories, but in fact, they are behind the 8-ball in hydration.”

How to manage your hydration levels on the bike

If you find yourself struggling with this, the outlook isn’t great for the rest of your ride, Dr Sims explained: “This takes hours to come back from- no, guzzling water doesn’t help (water will just sit in the stomach), and typical sports drinks can exacerbate the dehydration.”

“The only real way to mitigate this kind of bonking is to avoid it by drinking a low carbohydrate-electrolyte drink throughout your ride (don’t go for “liquid calories”- eg calories in the bottle- this is more fueling and does not address hydration needs).”

However, there is a way to limit the effects: “If you haven’t been able to avoid this, to get home, drink water with a dash of salt (1/16th tsp in 20oz water) to pull fluid into the body- but remember, it takes hours to recover from body water losses.”

The other type of bonking occurs when you’ve kept up on your hydration, but failed to eat enough: “The second kind of bonking is from low fuel – this is often felt as a “lightheaded” or “spacey” feeling with a bit of tunnel or blurred vision.”

“Slowing way down helps stop the symptoms – as you drop intensity, the body can use mainly fat. The fastest way to fix this is to eat something! A glucose tablet helps almost immediately, but then eating something more substantial (a bar or other food, not a gel!) will help longer term.”

‘Get out of jail’ bars and drinks dashed with salt will help, but if you find you are struggling with regular bonking on rides – you need to pay more attention to your nutrition – Dr Sims said: “The way to mitigate both is to start early with food and fluid- take a ‘sip sip nibble nibble’ approach – try to avoid having to ‘catch up’ by eating a full bar and drinking 1/2 a water bottle when you think – ‘oh no, I forgot to eat/drink!’”

To recap

If you feel you have flat, dead legs, are losing power (or just significantly slowing down), and your heart rate is elevated – drink more. This takes time to recover from, but you can speed it up with water and a dash of salt.
If you feel lightheaded, spacey, or suffer blurred vision, you need to eat more – you can drop the intensity so your body uses fat, not carbohydrates, and take a glucose tablet for near immediate relief. Eat something solid as soon as you can.
Long-term – eat and drink at regular intervals during your rides

Osmo makes sports nutrition designed to work with women’s bodies, accounting for changes in hormones throughout the month – find out more here.