There are few sensations in endurance sport quite like a proper bonk.
One minute you're riding comfortably, holding your target power and feeling good. Then something changes. Your legs suddenly feel empty, your pace drops, concentration disappears, and even the thought of maintaining your previous effort seems ridiculous.
You haven't suddenly become unfit. You've run into the wall. Bonking is one of the most common mistakes I see in endurance athletes, particularly when they underestimate how much carbohydrate they need during longer training sessions and races.
The good news is that bonking is largely preventable. Once you understand what causes it, what it feels like, and how to fuel properly, you can dramatically reduce the chances of experiencing it.
The symptoms of bonking during exercise can appear quite suddenly, although there is often a gradual decline beforehand.
A sudden drop in energy
Heavy or empty-feeling legs
Difficulty maintaining your normal pace or power
Feeling unusually weak
Poor concentration
Irritability or feeling mentally foggy
Dizziness or light-headedness
Strong hunger or cravings for carbohydrates
A feeling that you simply cannot continue at the same intensity
For a triathlete, this can be particularly frustrating because your cardiovascular system may still feel capable of working, but your ability to produce the required effort has disappeared.
It's important to understand that bonking isn't simply being tired. Endurance training creates fatigue for many reasons. A bonk is specifically associated with inadequate carbohydrate availability and depleted glycogen stores, although dehydration, heat, and other factors can make the experience considerably worse.
Your body stores carbohydrate primarily as glycogen in your muscles and liver. During endurance exercise, you use glycogen alongside fat to provide energy.
The harder you exercise, the greater your reliance on carbohydrate tends to become. If you start a long session with inadequate carbohydrate stores and don't consume enough during the workout, those stores can eventually become depleted.
That's when performance can fall off a cliff.
This is why bonking in a triathlon is particularly common during longer events. You may swim without consuming anything, complete a demanding bike leg, and then discover halfway through the run that your energy supply is disappearing rapidly. The mistake often happened hours earlier.
Remember, whether you’re cycling or running, longer endurance sessions place significant demands on your energy stores.
Starting a long ride or race under-fuelled makes it much harder to maintain performance. You don't need to eat an enormous meal immediately before exercising, but you should arrive at longer sessions adequately fuelled.
This is probably the biggest mistake. Many athletes simply don't consume enough carbohydrate because they don't feel hungry during training. Unfortunately, waiting until you're hungry or exhausted is often too late. Fuel early and consistently rather than trying to rescue yourself once your energy has already disappeared.
Intensity matters. If you ride or run significantly harder than planned, your carbohydrate consumption increases, and glycogen can be depleted much faster. This is one reason pacing and nutrition are closely connected.
Hot conditions increase physiological strain and can make an already under-fuelled athlete feel considerably worse. Dehydration doesn't cause a classic bonk on its own, but it can compound fatigue, reduce performance, and make it harder to maintain your planned intensity.
The best strategy for preventing bonking is remarkably simple: don't wait until you're in trouble. For longer sessions, think about nutrition before you start.
Start fuelled: Eat an appropriate carbohydrate-rich meal or snack before longer training sessions.
Fuel early: Don't wait until you're exhausted before taking your first carbohydrate.
Fuel consistently: Spread your carbohydrate intake throughout the session rather than consuming everything at once.
Practice your race nutrition: Your long training sessions are where you should test exactly what you'll consume on race day.
Match your intake to the session: A short, easy session doesn't require as much fuel as a three-hour ride followed by a run.
This is particularly important for triathletes because nutrition needs to be practiced across all three disciplines. The goal isn’t to make every training session brutally hard. Effective endurance training is built around managing intensity and accumulating quality work without excessive fatigue.
If you think you're bonking during a ride or run, the first step is to reduce the intensity. Don't try to push through it simply because your training plan says you should.
Take in a readily digestible source of carbohydrate and drink some fluid. Gels, sports drinks, or other carbohydrate sources that you have already practiced using are generally more practical than trying to eat a large meal while exercising.
Give your body time to respond. Once your energy starts returning, resist the temptation to immediately go back to maximum effort. If you're training, it may be smarter to continue at an easier intensity or finish the session.
If symptoms are severe, unusual, or don't improve with rest, carbohydrates, and fluids, stop exercising and seek appropriate medical assistance. Not every episode of dizziness or weakness during exercise is a bonk.
Once you’ve stopped, refueled, and rehydrated, your focus should shift to proper post-workout recovery.
Once you've finished, your priority is recovery nutrition. A meal containing carbohydrate and protein can help replenish glycogen and support muscle recovery.
For athletes looking to support recovery after demanding training sessions, SLS3 compressio n recovery systems can be incorporated into a broader recovery strategy.
Rice with lean protein
Oats with fruit and yogurt
Potatoes with eggs or another protein source
A carbohydrate-rich recovery drink followed by a meal
You don't need to overcomplicate it. The bigger lesson is figuring out why you bonked in the first place.
Did you start under-fuelled?
Did you underestimate the duration?
Did you ride harder than planned?
Did you forget to take your nutrition?
Fix the cause rather than simply treating the symptom.
A bonk during training is frustrating, but a bonk during a race can completely derail your performance. That’s why your long training sessions should be treated as nutrition rehearsals.
Use these sessions to test everything you plan to rely on during competition: your breakfast, gels, drinks, and, most importantly, how frequently you need to consume them. Your goal is to find a nutrition strategy that provides enough energy without causing stomach discomfort or digestive issues.
The last thing you want is to discover on race day that your stomach doesn’t tolerate the nutrition you’ve chosen.
It’s also important not to confuse a race-day bonk with a lack of fitness. An athlete can be extremely fit and still completely derail their performance through poor pacing and inadequate nutrition.
Fitness enables you to go fast. Fuel lets you keep going.
Bonking isn't a badge of honor, and it isn't something you need to experience to prove you're training hard. For most triathletes, it's a fuel-management problem that can be prevented.
Start your sessions properly fuelled, take in carbohydrate before you're desperate for it, pace yourself sensibly, and practice your nutrition during training. Do that consistently and you'll spend far more time finishing strong than wondering where all your energy disappeared.
The goal isn't simply to train hard. It's to have enough energy to execute the training and the race, the way you intended.
