There is a particular kind of hell that lives at kilometre 32 of a marathon you have undertrained for. The legs work. The lungs work. The mind does not. You stop. You sit on a kerb in Worli or Banjara Hills and wonder why the universe is cruel. It is not the universe. It is your liver. You ran out of carbs and your brain noticed.
Bonking is the most preventable disaster in marathon running. It is also the most common. The math is not mysterious, and the fix is not expensive. You just have to plan for the depletion before it happens.
What bonking actually is
Bonking is a sudden severe drop in performance caused by depletion of stored carbohydrate. It is not bad pacing. It is not bad shoes. It is biochemistry.
The carbohydrate budget
Your body stores about 400 to 500 grams of glycogen across muscles and liver — roughly 1,600 to 2,000 kilocalories. A marathon burns 2,200 to 3,500 kilocalories for most runners. The difference is what you must supply through fuel during the race. If you do not, the deficit shows up around kilometre 28 to 32 for most runners. You are running on a tank that empties before the finish line.
The brain dimension
The brain runs almost exclusively on glucose. When blood glucose drops, cognition degrades before muscle function does. You feel confused, weak, anxious. You misjudge your pace. You stop. Many runners describe the bonk as a wall in their head before it shows up in their legs. The Running Lab has runner accounts of this exact pattern from races like Tata Mumbai and Vedanta Delhi Half.
The fuel plan that prevents bonking
You can prevent the bonk completely. The protocol is well-supported by sports nutrition research.
Carbohydrate intake during the race
Take in 30 to 60 grams of carbohydrate per hour for races over 90 minutes. Elite-level intake can reach 90 grams per hour, but only after months of gut training. Most amateur marathoners do best at 45 to 60 grams per hour, which translates to one gel every 25 to 35 minutes. Pair every gel with two or three sips of water from an aid station. The fuel guide covers brand-by-brand carb content.
When to start fuelling
Take your first gel at the 30 to 45 minute mark, not at the first sign of fatigue. The carbohydrate takes 10 to 15 minutes to enter the bloodstream. If you wait until you feel low, you are already 15 minutes behind. Fuel on the clock, not on the feeling. Set a watch alert if you have to.
Carb loading before the race
In the 48 to 72 hours before a marathon, increase carbohydrate intake to 7 to 10 grams per kilogram of body weight. For a 70 kg runner, that is 490 to 700 grams of carbs per day. This raises muscle glycogen by 20 to 25 percent. The nutrition section has plate-by-plate examples that fit Indian kitchens.
The pacing dimension
Fuel is half the equation. Pace is the other half.
Why fast starts cause bonks
Running 10 to 15 seconds per kilometre faster than goal pace in the first 10 km can burn through 200 to 300 extra kilocalories of glycogen. That is a third of a gel's worth, gone, in the first hour. You will pay for it at kilometre 30. The energy you saved at the start is the energy you have at the finish.
The negative-split strategy
Aim to run the first half 30 to 60 seconds slower than goal pace, then the second half at goal pace or slightly faster. This preserves glycogen for when it counts. The STRIDD calculators will give you the right per-kilometre target. Stick to it for the first 10 km. Read the road, not your watch, after that.
How to train your gut
You cannot test your race-day fuelling plan on race day. The gut is a muscle. It needs training.
The long-run rehearsal
Every long run of 25 km or more is a fuelling rehearsal. Use the same gels, the same water, the same timing as race day. If a gel does not sit well, you find out on a Sunday in Hyderabad, not on race morning. Most gut symptoms in marathons are caused by switching brands or skipping practice. What you have trained, you can run.
Building tolerance
Start at 30 grams of carbs per hour in early training. Build to 60 over 8 to 12 weeks. Add fluid in the same way. By peak weeks, you should be tolerating your race-day plan in full. The STRIDD plan generator can build a fuelling progression into your block alongside the running. Skip the rehearsal and you are betting on luck.
The Indian context
Most race nutrition advice was written for cool European or American races. Indian races have their own variables.
Heat and humidity
In a January Tata Mumbai, the temperature climbs from 18 degrees at gun time to 26 degrees by kilometre 30. Sweat losses increase. Sodium losses increase. Plain water is not enough. Use an electrolyte mix or take salt tablets at 35 to 40 minute intervals. Dehydration accelerates the bonk by reducing blood flow to working muscles. The Tata Mumbai Marathon page has more on the course-specific conditions.
Brand availability
Train on brands you can buy. Unived, GU, and SiS are widely available in India. If a Belgian brand works for you but is not in stock the week before your race, the strategy fails. Test what you can sustain. Plan for what you can buy.
What to do if you feel a bonk coming
Sometimes the plan slips. Sometimes the heat is worse than expected. The early warning signs are clear if you know what to look for.
The signs at kilometre 25
Mild confusion. Empty feeling in the legs. Sudden drop in pace despite same effort. Foggy thinking. Cold hands in warm weather. If you feel any of these, take a gel immediately and walk for 60 seconds. The walk is not weakness. It is a triage decision that buys you the next 5 kilometres.
The rescue
One gel, 200 ml of water, 60 seconds of walking. Then jog. Build back to your pace slowly. If the symptoms return, repeat the protocol. You may slow your finish time. You will still finish. The alternative — pushing through — usually ends with a DNF or a hospital cot. The race is a long game. Live to finish it.
Putting it together
A defensible anti-bonk plan looks like this: 7 to 10 g/kg of carbs in the 48 hours before the race, a familiar breakfast 2 to 3 hours pre-start, 30 to 60 g of carbs per hour during the race starting at minute 30 to 45, water at every aid station, electrolytes built in, and pace discipline in the first 10 km.
The night before
Eat what you have eaten before long runs. Pasta, khichdi, rice with dal, biryani if you train on it. Same time. Same portion. Lay out your gels and bib. Hydrate steadily through the day, not all at dinner. The STRIDD plan generator can include race-week fuelling protocols alongside the running plan.
The morning of
Breakfast 2 to 3 hours before the gun. 60 to 100 grams of carbs — toast with jam, banana, oats, idli with sugar. Sip water. Caffeine 30 to 45 minutes before the start if you train on it. Walk the start corral. Mind on the plan, not the noise around you.
The bonk is preventable. The math is not magic. Build the fuel, build the pace, build the rehearsal. Then go run the marathon you trained for.