Avoiding the Bonk: Why Cycling Nutrition Matters

What Actually Causes the Bonk

The bonk is what happens when you run your carbohydrate stores down faster than you're replacing them, usually a combination of depleted muscle and liver glycogen and, in the more severe cases, dropping blood glucose. Once liver glycogen runs low, your body struggles to keep blood sugar stable, and that's when the lightheadedness, irritability, and sudden inability to hold power show up. It's not just fatigue. It's your fuel supply running out faster than your body can compensate [1].

The fix isn't complicated, but it does require planning ahead rather than reacting once you're already in trouble. Fueling well before, during, and after a ride is what keeps you out of that hole in the first place.

Fueling Before a Ride

For anything over about 90 minutes, the goal of your pre-ride meal is to top off glycogen stores in the hours beforehand, not to eat something heavy right before you clip in. A meal built around carbohydrate, with moderate protein and modest fat, eaten roughly 2 to 4 hours before the ride, gives your body time to digest and get that fuel into storage. Something like oatmeal with fruit, rice and eggs, or a bagel with a banana works fine. If you're riding first thing in the morning and don't have time for a full meal, a smaller carbohydrate-heavy snack 30 to 60 minutes out is a reasonable substitute, though it won't fully replace an overnight glycogen deficit from a hard session the day before.

Fueling During a Ride

This is where most bonks actually get created or prevented. For rides under about an hour, you generally don't need to eat anything, water is enough. Past that, your body starts drawing meaningfully on glycogen, and you need to start replacing carbohydrate at a rate your gut can actually absorb.

For efforts of roughly 1 to 2.5 hours, aim for somewhere around 30 to 60 grams of carbohydrate per hour. For longer or harder efforts, particularly anything approaching 3 hours or more, well-trained athletes can push that up toward 90 grams per hour, but only if it's a mix of glucose and fructose (multiple transportable carbohydrates), since a single sugar source caps out how much your gut can absorb per hour [2]. That's the science behind why most sports drinks and gels list both glucose and fructose on the label rather than just one.

In practice, that's a gel every 30 to 45 minutes, a carbohydrate drink mix sipped consistently, or a mix of real food and product, whatever you can tolerate at race intensity without gut distress. The specific source matters less than hitting the target consistently, and figuring out what your gut tolerates is something you want to test in training, not on race day.

Hydration

Fluid losses compound the problem, since even mild dehydration reduces plasma volume and makes it harder for your cardiovascular system to keep up with demand [3]. There's no single number that works for everyone here, sweat rates vary a lot between riders and conditions, but a reasonable starting point is 500 to 750 ml per hour in moderate conditions, adjusted up in heat and down in cooler weather. If you're riding longer than about 90 minutes or sweating heavily, adding electrolytes, particularly sodium, helps maintain fluid balance and reduces cramping risk [4].

After the Ride

Once you're off the bike, the priority is replacing glycogen and getting some protein in for muscle repair. I've written in more detail about exactly how much that timing matters, and it's less urgent than most people assume unless you're training again within a few hours.

What This Looks Like in Practice

Most of the athletes I coach who bonk aren't undertrained, they're underfueled. The fix is rarely more training. It's usually eating a real pre-ride meal, hitting a consistent carbohydrate target during the ride instead of winging it, and not waiting until you're already flat to start eating. Get those three things right and the bonk mostly stops being a problem you have to think about.

References

[1] Costill, D. L., & Hargreaves, M. (1992). Carbohydrate nutrition and fatigue. Sports Medicine, 13(2), 86-92.

[2] Jeukendrup, A. E. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25-S33.

[3] Coyle, E. F. (2004). Fluid and fuel intake during exercise. Journal of Sports Sciences, 22(1), 39-55.

[4] Sawka, M. N., et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377-390.