How to Avoid the Bonk
The complete fueling guide for long runs and marathons. Real gram targets, a minute-by-minute checklist, carb loading science, UK gel brand comparison, and what to do if you bonk mid-race.
The Direct Answer
To avoid the bonk on a long run, eat 30 to 60 grams of carbohydrates per hour starting around the 45 minute mark and never wait until you feel low. The bonk is glycogen depletion, not willpower. Once your liver and muscle glycogen stores run out, no amount of pushing fixes it. You will move through treacle until you take on simple sugars.
The bonk, also called hitting the wall or the hunger knock, is the sudden severe fatigue and brain fog that occurs when your blood glucose falls and your muscles exhaust their stored glycogen. It typically strikes marathon runners between miles 18 and 22, roughly 2 to 2.5 hours into a race. The three warning signs are legs that stop responding to your commands, a strange foggy slowness in your thinking, and a cold sweat with mild shakiness that comes on without warning. If you notice any of these, you are already bonking and need simple carbohydrates within the next two to three minutes.
What the Bonk Actually Is
Your body stores carbohydrates as glycogen in two places. Your liver holds roughly 80 to 100 grams, which it uses to maintain blood glucose levels for your brain and organs. Your muscles hold between 350 and 500 grams depending on your size, training status, and how well you carb loaded beforehand. Combined, that gives you enough stored energy for roughly 90 to 120 minutes of running at moderate-to-hard effort.
During a marathon run at race pace, you burn through glycogen at a rate of about 3 to 4 grams per minute. The maths is brutal. Even with a perfect carb load and full stores at the start, you will exhaust those stores somewhere around mile 18 to 22 for a 3 to 4 hour marathon runner. Fat burning continues but fat oxidation produces ATP far more slowly than glycogen, which is why pace collapses so suddenly when stores are empty. Your body can keep moving, but not at race pace.
The glycogen maths
The 3 Warning Signs
These signs appear in roughly this order. By sign three you are fully bonking. The goal is to act before sign one becomes sign two.
Leaden Legs
Your legs stop responding to your mental commands the way they normally would. Pace drops by 30 to 60 seconds per kilometre without any conscious decision to slow down. The muscles feel heavy, dense, and unresponsive. This is glycogen running low in the fast-twitch fibres your body uses to maintain pace.
Brain Fog and Confusion
Your brain runs almost exclusively on glucose. When blood sugar drops, your thinking slows. Runners describe tunnel vision, difficulty concentrating, losing count of laps, forgetting their goal pace, and a strange dreamlike quality to the environment around them. This is a serious signal. The brain is fighting for the last available glucose.
Cold Sweat and Shakiness
A sudden cold sweat with mild trembling in the hands or arms indicates hypoglycaemia. Your body releases adrenaline as a last-ditch attempt to mobilise more glucose from the liver. The adrenaline response causes the cold sweat, pale skin, and shakiness. If you reach this stage you are already in a full bonk and you need simple sugars immediately.
Why the Wall Hits at Mile 18 to 20
Mile 18 is not a random number. For a runner finishing a marathon in 3.5 to 4.5 hours at a moderate aerobic effort, the two-hour mark falls somewhere between mile 14 and mile 18. That is when unreplenished glycogen stores start to become critically low. Add the cumulative muscular fatigue of 17 or 18 miles and a slight pace increase in the second half of many races, and the bonk arrives like a door slamming shut.
Several factors make mile 18 particularly brutal. First, the muscle damage accumulated over two-plus hours is significant, reducing mechanical efficiency and increasing energy cost per kilometre. Second, dehydration, which typically worsens through the race, increases perceived effort and heart rate, pushing the body toward higher glycogen use. Third, race-day adrenaline often causes runners to start faster than planned, burning glycogen at a higher rate in the first half and arriving at mile 18 with lower stores than expected.
The solution is not to slow down in the second half, though that helps. The solution is to start fueling at 30 minutes and keep fueling every 25 to 30 minutes without exception. The gels you take at mile 6, 10, 13, and 16 are the gels that protect you at mile 20.
Fueling Decision Tree by Run Length
The right fueling strategy depends on how long you are going to be out. Use this as your starting point, then adjust based on temperature and intensity.
Under 60 minutes
0g per hourNo in-run fueling needed
Your body carries enough stored glycogen for approximately 90 minutes of moderate running. A run under 60 minutes will not deplete those stores to a dangerous level. Focus on good pre-run hydration and a light snack 1 to 2 hours before if running in the morning on an empty stomach.
60 to 90 minutes
20 to 30g total1 gel at 45 min or 200ml sports drink
You are approaching the zone where glycogen can become a limiting factor, especially if pace is high. One gel (roughly 22 to 25 grams of carbohydrate) at the 45-minute mark keeps blood glucose stable and gives you a buffer for the final 15 to 45 minutes. Alternatively, 200ml of a 6 to 8 percent carbohydrate drink at the same point works well.
90 minutes to 2.5 hours
30 to 60g per hour30 to 60g carbs per hour, starting at the 30 minute mark
This is the standard half marathon to easy marathon territory. Start fueling at 30 minutes, not when you feel tired. Take a gel every 30 minutes. Wash each gel down with 150 to 200ml of water. Aim for 45 grams per hour at minimum. If you are racing at the higher end of your ability, push toward 60 grams per hour using two gels per hour plus a sports drink.
2.5 hours and beyond
60 to 90g per hour60 to 90g carbs per hour using mixed carb sources
At this duration you need to maximise absorption. The gut can absorb up to 60 grams per hour from glucose alone but can handle 90 grams per hour when glucose and fructose are combined, because they use separate gut transport proteins. Use a gel or drink that contains a 2:1 ratio of maltodextrin to fructose, brands like Maurten, SiS Beta Fuel, or Tailwind. Gut training in the weeks before your event is essential at this intake level.
Carb Loading: The 3 Days Before
Carb loading is not eating as much pasta as possible the night before. It is a structured three-day protocol designed to saturate your muscle glycogen stores above their resting level. Research supports an intake of 8 to 10 grams of carbohydrate per kilogram of body weight per day across the two to three days before a marathon or long race.
Rice, pasta, white bread, oats, bananas, sports drinks, boiled potatoes without skin
High-fibre vegetables, pulses, heavy sauces, alcohol
Reduce training volume significantly on this day. Your muscles will absorb glycogen more effectively when you are resting and eating.
Same as above. White rice is easier to eat in large amounts than pasta. Add fruit juice and rice cakes as snacks between meals.
Anything new to your diet. Spicy food. High-fat sauces that sit heavily in the stomach.
This is the peak loading day for most protocols. You may feel bloated and heavy. That is normal. Each gram of stored glycogen holds approximately 3 grams of water.
Classic pasta dinner but not an enormous serving. Rice is a better choice for some runners. Keep the meal plain and familiar.
Rich sauces, creamy dishes, new restaurant food, alcohol, high-fibre foods
Eat your main meal by 6pm so digestion is well underway before bed. A small high-carb snack before sleep (banana, small bowl of cereal) tops up liver glycogen.
White toast with jam or honey, porridge made with water, white rice with a small amount of banana, energy bar
High-fat food (eggs and bacon), high-fibre food (bran cereal, wholegrain toast), dairy if it causes you any gut sensitivity
Eat 2 to 3 hours before the gun if possible. If the race starts early, eat what you can at least 90 minutes before and take a small gel 20 minutes before the start.
Minute-by-Minute Long Run Fueling Checklist
This checklist covers a 3-hour long run or marathon effort. Adapt timing to your own pace. The principle is the same at any speed: start early, fuel consistently, never wait to feel depleted.
Eat a carbohydrate-rich dinner
1g to 1.5g carbs per kg body weight. Pasta, rice, or potatoes. Keep fat and fibre low to avoid gut trouble.
Race morning breakfast
75 to 100g of carbohydrates. White toast with jam, porridge with honey, or a banana and white bread. Avoid high-fat foods that slow gastric emptying.
Optional top-up gel or banana
20 to 25g of carbohydrates if you feel low or the run exceeds 90 minutes. Not essential if breakfast was solid.
Sip 250ml of water or electrolyte drink
Arrive at the start slightly hydrated. Over-drinking is as problematic as under-drinking so do not force fluids.
Start moving, save fuel for later
Begin at a controlled effort. Going out too fast burns glycogen faster and guarantees a bonk later.
First gel or 200ml sports drink
22 to 25g carbs. Take with 150ml water. Never take a gel without water as the concentrated sugar can pull fluid into the gut.
200ml water or electrolyte drink
Sip to stay on top of hydration. Dehydration raises perceived effort and accelerates glycogen use.
Second gel or chews, 200ml water
22 to 25g carbs. You are now one hour in. If you feel fine this is a good sign the fueling is working. Do not skip it.
Electrolyte check
If you are sweating heavily, add sodium. A high-sodium gel or a salt capsule prevents cramping and maintains fluid balance.
Third gel or drink mix, 200ml water
22 to 25g carbs. You have now taken 66 to 75g total. This is the level that protects most runners through a half marathon finish.
Small sip, assess GI comfort
If stomach feels settled, consider increasing to a slightly bigger dose at minute 120. If gassy or uncomfortable, stick to gels and water only.
Fourth gel, 200ml water
Two hours in. You are in full marathon fueling territory now. This gel keeps blood sugar stable through the dangerous miles 14 to 18 zone.
Electrolyte drink, 200ml
Sodium and potassium become critical at this point. Plain water at high volumes can dilute electrolytes and cause hyponatraemia.
Fifth gel, 200ml water
22 to 25g carbs. You have now covered 2.5 hours of racing. The end is in sight for most marathon runners. Finish what you carry.
Sixth gel or final top-up
For runners still out at 3 hours, one final gel at 165 minutes prevents a late bonk in the last 5 kilometres. Keep moving and drinking small amounts of water.
In-Race Fueling Products: UK Guide
The UK market has strong options across gels, chews, and drink mixes. The right choice depends on your gut tolerance, your pace, and the duration of your event. Whatever you choose, always use the same product in training as you plan to use on race day.
Science in Sport (SiS)
Isotonic gel22g CHOMaurten 100
Hydrogel25g CHOSiS Beta Fuel
Gel and drink40g (gel) CHOTailwind Nutrition
Drink mix50g per 600ml serving CHOHigh5 Energy Gel
Standard gel23g CHOPrecision Hydration PF 30 Gel
Gel30g CHOGels vs chews vs drink mix: what to choose
Gels are the most convenient format for racing. They deliver a concentrated carbohydrate hit in a small packet and are available at most UK road race aid stations. Chews (such as SiS GO Chews or High5 Energy Chews) work well for runners who find gel texture unpleasant and prefer to chew. Drink mixes like Tailwind replace both hydration and fueling in one bottle, which simplifies logistics but can be harder to manage at aid stations where plain water is provided. For most UK marathon runners, gels taken with water every 25 to 30 minutes is the simplest and most tested approach.
3 Case Studies: UK Runners Who Bonked
Every bonk story has a moment where a simple fueling decision went wrong. These are composite but realistic accounts based on common patterns seen among amateur UK runners.
Charlie, mile 19 of the London Marathon
Charlie had trained well for his third London Marathon. He carried four gels and planned to take them at miles 6, 10, 14, and 18. He took the first three on schedule but felt so good at mile 14 that he skipped the mile 14 gel thinking he did not need it. He took his fourth at mile 18 but by then his glycogen stores were already falling below the critical threshold. By mile 19 his legs felt like concrete and he slowed from 4:40 per kilometre to 6:20 per kilometre. The lesson: take gels to a schedule, not to feel. Feeling good is not a signal to skip fuel. It is a signal your fueling is working.
Priya, mile 10 wobble at the Great North Run
Priya had read that elite runners take 90 grams of carbs per hour, so she bought high-dose gels and planned three per hour for her first Great North Run. She had never practised taking gels during training. At mile 10 she started cramping severely and had to stop twice near a toilet. She crossed the finish line but at a pace four minutes per kilometre slower than her goal. Her glycogen was fine but her untrained gut could not absorb the volume she was forcing into it. The lesson: whatever you plan to take on race day must be rehearsed on every long run for at least four to six weeks beforehand.
Mark, 20-mile training run bonk before breakfast
Mark is an experienced runner who was three weeks out from a marathon. He went out at 7am for a 20-mile training run without eating breakfast, relying on a single gel at mile 10 and water. He bonked hard at mile 16 and had to call his wife for a lift home. His liver glycogen, which is depleted overnight during fasting, was already low before he even started. The one gel at mile 10 was not enough to compensate. The lesson: pre-run breakfast is not optional on long runs. 75 to 100 grams of carbohydrates two hours before is the baseline. You cannot out-gel a skipped breakfast on a 3-hour effort.
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MoteraWhat to Do If You Bonk Mid-Run
The bonk is not the end of the run. It is a metabolic signal that requires a specific response. The worst thing you can do is try to run through it at race pace. Here is exactly what to do in the first ten minutes.
Slow to a walk immediately
Walking dramatically reduces your glycogen burn rate and allows blood glucose to be redistributed to the brain and critical organs. Do not try to maintain pace. Walking is not failure, it is triage.
Take 30 to 40g of fast-acting carbs right now
A gel, a banana from an aid station marshal, sports drink, or even a cola from a spectator works. You need simple sugars that enter the bloodstream quickly. Slow complex carbs from a cereal bar will not act fast enough.
Wait 5 to 10 minutes before picking up pace
Blood glucose takes 5 to 10 minutes to rise noticeably after taking a gel. Resist the temptation to surge immediately after swallowing. Walk or very easy jog for 5 minutes, then test whether you can run comfortably.
Drop pace and keep fueling every 15 minutes
Once you can run again, do so at a pace 30 to 60 seconds per kilometre slower than race pace. Keep fueling more frequently than normal for the rest of the run. You will not return to your pre-bonk pace today.
Finish if you can, and review the fueling log
After the run, log exactly what you ate and when. The bonk point tells you where your fueling broke down. Did you skip a gel? Start too late? Go out too fast? Use that data to plan the next race.
Hydration vs Fueling: Not the Same Thing
One of the most common mistakes runners make is conflating hydration and fueling. Drinking enough water does not prevent the bonk. Water contains no carbohydrates. You can be perfectly hydrated and still bonk if you have not eaten enough. Conversely, taking gels without adequate water slows gastric emptying and can cause GI distress because concentrated carbohydrate solutions need to be diluted by your body before absorption.
Hydration target: 400 to 600ml of fluid per hour
On a cool UK day (10 to 15 degrees Celsius) you need approximately 400ml of fluid per hour. On a warm day (above 18 degrees Celsius) increase to 500 to 750ml per hour. Never drink to a fixed schedule without listening to thirst signals. Over-drinking plain water can cause hyponatraemia, a dangerous dilution of blood sodium.
Fueling target: 30 to 90g of carbs per hour (depending on duration)
Carbohydrate is the only nutrient that prevents the bonk. It must come from gels, sports drinks, or real food. Always combine gels with water. If you are using a sports drink that already contains carbohydrates, account for that in your total hourly carb target rather than taking full gel doses on top.
Electrolytes: sodium, potassium, and magnesium after 90 minutes
Electrolytes maintain fluid balance and prevent cramping. After 90 minutes of running in warm conditions, add a high-sodium gel or electrolyte tablet to your fueling plan. 400 to 1000mg of sodium per hour is a reasonable target for longer efforts in heat. Many UK runners significantly under-salt their race nutrition.
4-Week Gut Training Protocol
Your gut is trainable. Consistently practising in-run fueling over four to six weeks before your target race increases your body's ability to absorb carbohydrates during effort and reduces GI distress at higher intake rates. Do not leave this until race week.
Strategy: One gel at 30 minutes and one at 60 minutes. Use whatever brand you plan to race with. Always take with 150ml water.
Goal: Establish the habit of fueling on a schedule. Note any GI discomfort.
Strategy: One gel at 30 min, one at 60 min, plus 200ml sports drink at 75 min. Total rises to roughly 45g per hour.
Goal: Increase dose slightly. Monitor stomach comfort. If the first week caused no issues, this should be fine.
Strategy: Two gels per hour, one every 30 minutes. If using a standard 22 to 25g gel this lands at 44 to 50g per hour. Add 200ml of sports drink per hour to reach 60g total.
Goal: Hit the 60g threshold. This is the standard race-day target for most marathons. Note whether stomach feels trained and settled.
Strategy: Switch to a 2:1 glucose:fructose product such as SiS Beta Fuel or Maurten. One high-dose gel every 30 minutes delivers 75 to 80g per hour. Or combine two standard gels per hour with 300ml of sports drink.
Goal: Maximum absorption rate. Only relevant for runners who will race 3 hours or more and want to sustain a faster pace throughout.
How Conditions Change Your Fueling
The standard fueling targets assume moderate UK conditions. Heat, cold, and altitude each increase bonk risk in different ways and require adjustments to your plan.
Hot weather (above 18 degrees Celsius)
Cold weather (below 5 degrees Celsius)
High altitude (above 2000 metres)
Fueling Glossary
Sports nutrition guidance uses a set of specific terms. Understanding them helps you read product labels, interpret coaching advice, and make better fueling decisions.
The stored form of carbohydrate in your body. Muscle glycogen fuels your running muscles directly. Liver glycogen maintains blood glucose levels for the brain and other organs. Your total capacity is roughly 400 to 600 grams combined depending on body size and training status.
Standard scientific abbreviation for carbohydrate. When you see "60g CHO per hour" in sports nutrition guidance it means 60 grams of carbohydrate per hour. Glucose, fructose, maltodextrin, and sucrose are all forms of CHO relevant to running fueling.
The actual energy currency your muscles use. Carbohydrates are broken down into glucose, which is converted to ATP through glycolysis. Fat is converted to ATP through beta-oxidation. ATP from carbohydrate is produced roughly twice as fast as ATP from fat, which is why bonking at marathon pace feels so catastrophic.
Clinically low blood glucose. In running, hypoglycaemia means blood sugar has dropped below the threshold needed to maintain normal brain and muscle function. Symptoms include shakiness, cold sweat, confusion, and sudden severe fatigue. The cure is immediate simple sugar intake.
The speed at which food and liquid moves from your stomach into the small intestine for absorption. High concentration sugar solutions and stress both slow gastric emptying. Products designed for running fueling are formulated to leave the stomach quickly and reach the bloodstream fast.
A long-chain glucose polymer derived from starch. Used in gels and sports drinks because it provides a large amount of carbohydrate at a lower osmolality than simple sugar solutions, meaning it empties from the stomach quickly and is less likely to cause GI distress at moderate doses.
A simple sugar found in fruit and many sports nutrition products. Fructose uses a different gut transporter (GLUT5) than glucose (GLUT2), which is why combining them in a 2:1 ratio allows absorption rates above 60g per hour. Without fructose, glucose absorption caps at around 60g per hour.
The concentration of dissolved particles in a solution. Isotonic drinks match the osmolality of blood (around 290 mOsm/kg) and are absorbed fastest. Hypertonic solutions (high sugar concentration) slow gastric emptying and can draw fluid into the gut, worsening dehydration if taken without water.
Caffeine Timing and Dose for Endurance
Caffeine is the most extensively researched ergogenic aid in endurance sport. It delays perceived fatigue, reduces the rate of perceived exertion (RPE) at a given pace, and can improve marathon finish times by 2 to 4 percent when used correctly. The correct dose is 3 mg per kg of body weight, taken 30 to 60 minutes before peak effort. For a 70 kg runner that is approximately 200 mg, the equivalent of two espresso shots or one caffeinated running gel.
30 to 60 min before the gun
1 to 3 mg per kg body weight70 to 210 mg for 70 kg runnerPre-race coffee (one shot espresso = 60 to 80 mg), caffeine tablet, or caffeinated gel
Peak plasma concentration of caffeine occurs 45 to 60 minutes after ingestion, so taking it too close to the start means you are racing the first 10 kilometres without the full benefit. For a 9am start, take caffeine at 8am or 8:15am. For an early 7am start where you cannot face a coffee, a caffeinated gel 10 minutes before the gun still provides a partial benefit.
Mile 17 to 18 (or kilometre 27 to 29)
1 to 1.5 mg per kg body weight70 to 100 mg for 70 kg runnerCaffeinated gel (SiS Beta Fuel + Caffeine 75 mg, High5 Energy Gel + Caffeine 30 mg, Maurten GEL 100 CAF 100 mg)
A second caffeine dose at the point where bonk risk peaks is the most effective in-race intervention. Caffeine reduces perceived effort and delays the sense of fatigue, which helps runners hold pace when glycogen is low. It does not replace carbohydrates, but it reduces the felt severity of glycogen depletion during the critical final third of a marathon.
What to avoid
No caffeine in the 12 to 24 hours before race day if not a regular consumerRunners who do not regularly drink coffee or use caffeine should not take a large dose on race day for the first time. The side effects, including increased heart rate, jitteriness, and GI upset, can be significant without tolerance built up. Either use caffeine regularly in training runs four to six weeks before the race, or skip it entirely on race day.
UK caffeinated gel options
SiS Beta Fuel + Caffeine contains 75 mg per gel. High5 Energy Gel + Caffeine contains 30 mg. Maurten GEL 100 CAF contains 100 mg. For runners sensitive to caffeine, the High5 option allows a lower dose. For runners who want a single large hit at the critical point, Maurten GEL 100 CAF taken at mile 18 is a proven strategy used by club and sub-elite UK marathon runners.
Sodium and Sweat Rate: What a Salty Sweater Actually Needs
Sweat contains 500 to 1500 mg of sodium per litre on average, but the range between individual runners is enormous. A light sweater losing 400 ml per hour and 500 mg sodium per litre loses only 200 mg per hour of exercise. A heavy salty sweater losing 1.5 litres per hour with 1500 mg sodium per litre loses 2250 mg per hour. The same gel-and-water protocol that works for one runner causes cramping, nausea, and performance collapse in the other.
Identifying your sweat type allows you to personalise the sodium component of your fueling plan. The simple self-test below costs nothing.
DIY sweat type test
Pre-Race Meal Templates by Start Time
The timing of your pre-race breakfast is as important as what you eat. UK road races start at a wide range of times: 7am starts for large city marathons, 9am for local club races, 10:30am for the Great North Run, and occasional afternoon events. Each start time requires a different breakfast strategy.
7:00 am start
Wake: 4:30 to 5:00 am75 to 90g carbohydratesMeal options
- White toast (3 slices) with honey or jam and a small banana
- Porridge made with water, 50g oats, tablespoon of golden syrup, one small banana
- Two rice cakes with peanut butter and a 500ml sports drink
9:00 am start
Wake: 6:00 am75 to 100g carbohydratesMeal options
- Porridge with banana and honey (60 to 70g carbs), plus one slice of white toast with jam
- Two Weetabix with semi-skimmed milk and a glass of orange juice
- Three rice cakes with Nutella and a small banana
10:30 am start (Great North Run)
Wake: 7:00 am75 to 100g carbohydratesMeal options
- Porridge at 7:15 am, then a small banana or gel at 9:45 am
- White toast with honey at 7:30 am, then SiS gel at 10:00 am
- Two Weetabix, orange juice, and a 500ml sports drink sipped until 9:30 am
12:00 pm or afternoon start
Wake: 7:30 am75 to 100g carbohydrates (split across two small meals)Meal options
- Porridge at 8:30 am (50g carbs) plus white rice with a drizzle of soy sauce at 10:00 am (40g carbs)
- Toast and banana at 8:00 am, then rice cakes and sports drink at 10:30 am
Bonking in a Half Marathon vs Marathon vs Ultra
The bonk is not identical across distances. The underlying glycogen depletion mechanism is the same but the severity, timing, and recovery characteristics differ significantly between a half marathon, a full marathon, and an ultra. Understanding how bonking differs by distance helps you calibrate your concern and your fueling plan.
Half marathon
Typical wall point: Mile 10 to 13 (if it happens)Typical wall point: Mile 10 to 13 (if it happens)
Most runners completing a half marathon in 1:45 to 2:30 will not deplete full glycogen stores, but they will deplete liver glycogen and muscle glycogen in the specific muscles being used. The bonk risk is real for anyone racing hard in under-fueled state or running significantly over 2 hours.
One or two gels from the 45-minute mark. Sodium to prevent late cramps. The half marathon bonk is usually a pace collapse, not a total shutdown.
A half marathon bonk is usually recoverable within the same race. Blood glucose drops just enough to slow pace but rarely causes the severe cognitive symptoms seen in marathon bonking.
Marathon
Typical wall point: Mile 18 to 22 (almost always)Typical wall point: Mile 18 to 22 (almost always)
A 3 to 4.5 hour marathon at race effort will fully deplete most runners' glycogen stores without in-race fueling. The combination of muscle glycogen depletion and liver glycogen exhaustion causes simultaneous muscular and cognitive failure. This is the classic bonk at mile 18 to 22.
Consistent 45 to 90g carbs per hour from the 30-minute mark. This is the distance where gut training genuinely changes race outcomes.
The marathon bonk is severe and hard to recover from in-race. Even with a gel, you are unlikely to return to pre-bonk pace. Prevention is the only real strategy.
Ultra marathon (50K to 100 miles)
Typical wall point: Any point after 4 to 6 hours if fueling is inconsistentTypical wall point: Any point after 4 to 6 hours if fueling is inconsistent
Ultra running operates at a lower intensity than road marathon pace, so fat oxidation plays a bigger relative role. However, bonking still occurs at ultra distances when runners fail to maintain consistent caloric intake across many hours. Ultra bonking involves more fat-depletion fatigue and electrolyte failure than pure glycogen crash, and often hits alongside severe sodium depletion.
Real food becomes more important. Sandwiches, boiled potatoes with salt, bananas, and rice balls all work well alongside gels. Sodium becomes the critical electrolyte. 1000 to 1500 mg per hour in warm conditions.
Ultra bonking is often a gradual fade rather than a sudden wall. It is frequently triggered by nausea, which stops intake, which deepens the bonk. The response is small, frequent doses of bland real food rather than a single large gel hit.
Women-Specific Fueling: RED-S, Iron, and the Menstrual Cycle
General fueling guidelines are typically derived from research conducted primarily on male athletes. While the core principles apply to female runners, there are three areas where female-specific physiology meaningfully changes the picture: iron status, RED-S (Relative Energy Deficiency in Sport), and menstrual cycle phase. These are not fringe concerns. They affect a significant proportion of female distance runners who are training seriously.
Carbohydrate needs across the menstrual cycle
Research indicates that women have slightly higher carbohydrate oxidation rates during the follicular phase (days 1 to 14 of the cycle, pre-ovulation) compared to the luteal phase (days 15 to 28, post-ovulation). During the luteal phase, oestrogen and progesterone promote slightly higher fat oxidation, which theoretically reduces acute bonk risk at submaximal effort. However, the practical differences are small, and most female runners should follow the same gel-per-hour targets as male runners of equivalent size and pace.
Iron deficiency and fatigue that mimics the bonk
Iron deficiency without full anaemia, known as iron deficiency without anaemia or IDWA, is very common in female distance runners and causes fatigue that closely mimics the bonk. If you frequently hit the wall earlier than expected despite consistent fueling, and blood glucose checks are normal, iron deficiency is worth investigating with a full blood count and serum ferritin test. A ferritin level below 30 micrograms per litre in a female runner correlates with significantly impaired endurance performance and perceived energy. This is not a fueling problem and cannot be fixed with gels.
RED-S (Relative Energy Deficiency in Sport)
RED-S is a condition where total caloric intake is chronically below the energy demands of both training and basic bodily function. It was previously called the Female Athlete Triad. It affects both male and female runners but is more prevalent in women and in runners who have significantly restricted calories to improve body composition or race weight. RED-S causes chronically low glycogen stores, hormonal disruption, poor bone health, frequent illness, and a much higher bonk risk even on shorter runs. If you are frequently bonking on runs shorter than 90 minutes despite taking gels, consult a sports dietitian and consider whether your total daily caloric intake is adequate for your training load.
Practical fueling note for female runners
Female runners typically have smaller total glycogen stores than male runners of the same body weight, not because of physiology per se but because muscle mass (where most glycogen is stored) is proportionally lower. A 60kg female runner should use per-kilogram targets (8g carb per kg per day for carb loading, 1 to 1.5g per kg for race morning breakfast) rather than copying the gram totals used by larger male training partners. Two gels per hour may also exceed what a lighter runner needs if running at a comfortable aerobic pace, so adjust toward the lower end of targets and observe how your body responds.
When to seek specialist advice
If you are bonking regularly despite correct fueling, experiencing menstrual irregularity, repeated stress fractures, or persistent fatigue that does not resolve with rest, consult a GP or sports physician and ask for a full blood count including ferritin and a RED-S assessment. A registered sports dietitian can provide a personalised energy availability audit. These are not vanity concerns, they are performance and health issues that are well understood and treatable.
Frequently Asked Questions
What is the bonk in running?
The bonk is the sudden, severe fatigue that hits when your body runs out of stored glycogen, the fast-burning sugar your muscles and brain prefer during hard effort. Your liver holds roughly 80 to 100 grams of glycogen and your muscles hold another 350 to 500 grams depending on your size and training status. Once both stores are depleted your body switches to burning fat almost exclusively, which is far slower and cannot sustain race pace. The result is leaden legs, mental fog, and a pace drop of one to two minutes per kilometre that no amount of willpower will overcome without taking on simple carbohydrates.
How many carbs do I need per hour on a long run?
For runs lasting 60 to 90 minutes you need 30 to 45 grams of carbohydrate per hour. For runs lasting 90 minutes to 2.5 hours you need 45 to 60 grams per hour. For runs lasting more than 2.5 hours, including marathons, the target rises to 60 to 90 grams per hour, but hitting the upper end requires gut training because most untrained runners can only absorb 60 grams per hour from a single carbohydrate source. You can push absorption beyond 60 grams per hour by using products that contain a blend of glucose and fructose, which use different transport pathways in the gut.
Can I bonk on a 10K run?
Bonking on a standard 10K is very unlikely for most runners unless you have fasted for a long time beforehand or you are running at a very high intensity. At 10K effort your glycogen stores are rarely the limiting factor because most runners finish in under 70 minutes and your body carries enough stored glycogen to fuel approximately 90 to 120 minutes of moderate-to-hard running. However if you start the 10K in a glycogen-depleted state, for example after skipping breakfast or after a hard training session the day before, you can experience the warning signs of low blood sugar even in a shorter race.
What is the difference between hitting the wall and the bonk?
The two terms describe the same underlying event, glycogen depletion causing a severe performance decline, but they come from slightly different traditions. "Hitting the wall" is the phrase most commonly used in marathon culture and refers specifically to the notorious miles 18 to 22 of a marathon where glycogen stores run out after roughly two hours of racing. "The bonk" is older cycling slang that was adopted by distance runners and is particularly common in UK running culture. "The hunger knock" is a third term, also from cycling, used when the bonk is accompanied by intense hunger and shakiness. All three describe the same metabolic event.
How do I train my gut to take in more carbs per hour?
Gut training means practising high carbohydrate intake during training runs, not just on race day. Start with 30 to 45 grams per hour on runs of 90 minutes or more. After two to three weeks at that level, increase to 60 grams per hour. After another two to three weeks, try 75 to 90 grams per hour using a mixed glucose plus fructose product like a 2:1 maltodextrin and fructose gel. Your gut upregulates its transport proteins in response to regular carbohydrate exposure. Practise fueling every long run for four to six weeks before your target race and you will arrive at the start line with a trained gut that absorbs at race-day rates without cramping or GI distress.
Can you fix the bonk mid-run?
Yes, but recovery takes 10 to 15 minutes and is never complete during the same run. The fastest route back is to take 30 to 40 grams of fast-acting simple carbohydrates, a gel, sports drink, or a banana from a marshal station, then walk for five minutes to allow blood glucose to rise. Do not try to run through the bonk at race pace as this exhausts your remaining fat oxidation capacity faster and can cause a total shutdown. Once blood sugar stabilises you will regain enough energy to continue at a reduced pace. You will not return to pre-bonk pace in the same run but you will be able to finish. The lesson is to prevent it with earlier fueling rather than relying on mid-run rescue.
