RunFuel quick check
- Most running nutrition mistakes are small habits, not dramatic failures.
- Heavy legs, poor recovery, stomach problems and late long-run fade-outs are patterns worth checking.
- The first fix is usually food, carbs, recovery or hydration around the sessions that need them most.
- Race day should repeat what worked in training, not test something new.
- Persistent fatigue, dizziness, recurring injuries or RED-S warning signs need professional support.
Most running nutrition mistakes are not dramatic. They are small habits that quietly make training feel harder than it needs to.
Your legs feel heavy during workouts. Long runs fall apart late on. Your stomach complains at the worst possible time. You are constantly hungry during marathon training, or tired even when your plan looks sensible.
Food is not always the only cause. Sleep, training load, stress, illness, iron status, heat and life outside running all matter. But fuelling is one of the first places to check, because small changes can quickly make running feel steadier.
This guide is a troubleshooting tool. It explains what runners often get wrong, why it happens, how it can affect training and what to try instead.
Why fuelling mistakes make running feel harder
Running creates a simple demand: your body needs enough energy to train, enough carbohydrate to support harder work, enough protein to repair, enough fluid to replace sweat losses and enough consistency to do it again.
When one part is missing, the first sign is not always obvious. It may feel like low motivation, heavy legs, poor sleep, a sensitive stomach, a disappointing workout or a long run that suddenly turns into survival.
That is why "healthy eating" and "fuelling well" are not always the same thing. A high-fibre salad, a very low-carbohydrate meal or a protein shake can all have a place in a runner's diet. They may not be the right tool before intervals, during a long run or after a hard session.
Use this guide to spot the pattern, then make the smallest useful fix.
| If you notice | Check first | Try this |
|---|---|---|
| Heavy legs on workouts | Total energy, carbohydrates and recovery | Add a familiar carbohydrate option before key sessions and eat properly afterwards |
| Long runs fall apart late | Starting fuel, during-run fuel and hydration | Practise taking carbohydrates during runs over 90 minutes |
| Stomach problems while running | Meal timing, high-fat foods, high-fibre foods and untested gels | Keep pre-run food simpler and test changes in training |
| Poor recovery between sessions | Carbohydrates, protein, fluids and total food across the day | Eat a proper recovery meal or snack after demanding runs |
| Constant hunger in marathon training | Whether daily food has increased enough for the training load | Add food around long runs, workouts and heavy training days |
| Dizziness, repeated fatigue or recurring injuries | Possible under-fuelling, low energy availability, training load or medical issues | Get personalised support rather than trying to solve persistent symptoms alone |
For a full guide on what to eat before, during and after running, read the RunFuel guide on what to eat before and after running. This article focuses on mistakes, warning signs and simple fixes.
Mistake 1: not eating enough to match your training
The mistake: You are asking your body to train more, but your total food intake has not increased enough to support it.
Why runners do it: Mileage often creeps up gradually. Busy days squeeze meals. Some runners lose their appetite after hard sessions. Others are trying to lose weight, avoid snacks or assume feeling tired is just part of training.
How it affects running: Not eating enough can show up as heavy legs, poor recovery, constant hunger, irritability, low mood, poor sleep, recurring illness, stalled progress or workouts that feel harder than they should. In more serious cases, chronic under-fuelling can contribute to low energy availability and RED-S.
What to do instead: Start with consistency. Build your day around regular meals, then add extra food around harder training blocks, long runs and double-session days. Do not only think about the snack before the run. Your overall day matters too.
Simple example: If you add a midweek workout and a longer Sunday run, you may need a bigger breakfast, a recovery snack or an extra evening snack on those days. That is not "overeating". It is matching food to the work.
Mistake 2: cutting carbohydrates too far
The mistake: You keep carbohydrates low because you think they are unnecessary, unhealthy or likely to slow progress.
Why runners do it: Low-carbohydrate messages are everywhere. Some runners over-correct towards protein because it feels more disciplined. Others reduce carbohydrates when trying to lose weight, then wonder why harder running feels flat.
How it affects running: Carbohydrate is stored as glycogen in your muscles and liver. It is especially useful for intervals, hills, tempo runs, long runs and races. If you avoid carbohydrates around key sessions, you may struggle to hit pace, concentrate, finish strongly or recover well.
What to do instead: You do not need to eat the same amount of carbohydrate every day. Put more of it where it helps most: before harder or longer sessions, during long runs when needed, and after demanding runs.
Simple example: Toast with jam before a workout. Rice, potatoes or pasta with a protein food after a long run. That is basic running fuel, not a complicated diet plan.
Mistake 3: under-fuelling hard or long sessions
The mistake: You run demanding sessions on an empty stomach or with too little fuel, then hope willpower will cover the gap.
Why runners do it: Early runs are convenient. Fasted running can feel simple. Some runners believe training low will automatically make them fitter. Others just forget to plan food before a hard session.
How it affects running: Fasted easy runs can be fine for some runners. The problem is repeatedly doing long runs, intervals, race-pace sessions or hill workouts under-fuelled. That can make sessions feel harder, increase the chance of fading and leave you hungrier or slower to recover later.
What to do instead: Match the fuel to the session. If the run is short and easy, you may not need much. If the run is long, hard or important, give your body something to work with.
Simple example: Before an early interval session, have a small carbohydrate top-up such as a banana, toast, a few dates or a sports drink. Then eat a proper breakfast afterwards.
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The mistake: You eat a normal-sized meal too close to running, then blame the food when your stomach complains.
Why runners do it: Life gets tight. Lunch runs, after-work sessions and evening club nights often happen in the awkward gap between meals. Many runners also fear starting hungry, so they eat too much too late.
How it affects running: Digestion and running both compete for comfort. Eating too close to a run can contribute to cramps, nausea, reflux, stitches, bloating, toilet stops or a heavy feeling.
What to do instead: Let the size of the meal match the time you have. A normal meal usually needs more space. A lighter snack can sit closer to the run. A small carbohydrate top-up is best when you only have a short window.
Simple example: If you are running at 6 pm, a normal lunch plus a small snack at 4-5 pm may work better than a large meal at 5.30 pm.
Mistake 5: choosing hard-to-digest foods before running
The mistake: You choose foods that are nutritious in general, but difficult to run on.
Why runners do it: This is one of the most common eating-before-running mistakes. High-fibre and high-fat foods often look like the "healthy" choice: beans, lentils, big salads, bran cereal, nuts, avocado, creamy sauces or heavy meals.
How it affects running: Fat and fibre can slow digestion. That is not bad in normal meals, but before running it can increase the chance of stomach problems, urgency, bloating or feeling sluggish.
What to do instead: Keep high-fibre and higher-fat meals further away from hard or long runs. Close to running, choose simpler foods you already know sit well.
Simple example: Before a hard evening session, a plain bagel or toast with jam may work better than a big lentil salad. Save the lentils for later.
Mistake 6: trying to survive long runs without fuel
The mistake: You treat long runs as something to survive without fuel, then keep hitting the wall late on.
Why runners do it: Some runners think using fuel in training is cheating. Some save gels for race day. Others underestimate how long 90 minutes feels once fatigue, hills, heat and pace are involved.
How it affects running: Long-run fuelling mistakes can lead to hitting the wall, heavy legs, poor concentration, low mood, late-run pace drop-off and rough recovery. For marathon training, it also means you miss the chance to practise your race-day stomach plan.
What to do instead: For most runs under 60 minutes, you usually do not need fuel during the run. Between 60 and 90 minutes, it depends on intensity and how well you ate beforehand. Over 90 minutes, practise taking carbohydrate during the run.
Simple example: For marathon training, start with a realistic target such as 30-60 g carbohydrate per hour, then build gradually if needed. Do not jump from no fuel to an aggressive race plan overnight.
Mistake 7: forgetting carbohydrates in recovery
The mistake: You finish a hard or long run without replacing the carbohydrate you used, especially if you only drink water, have a protein shake on its own, or wait hours before eating.
Why runners do it: Protein gets a lot of attention. Carbohydrates are often treated like something to earn or restrict. Some runners also lose appetite after hard sessions, especially in heat.
How it affects running: Carbohydrates help replace glycogen, the stored fuel you use while running. If you skip recovery carbohydrates after long runs, races, workouts or double-session days, the next run can feel heavier than it needs to.
What to do instead: After easy runs, your next normal meal may be enough. After hard or long runs, aim for carbohydrates, protein and fluids within the next couple of hours, especially if you are training again soon.
Simple example: Greek yoghurt with granola and fruit, or eggs on toast with a smoothie. The point is carbohydrate replacement: recovery is not only about muscle repair.
Mistake 8: treating protein products as the whole recovery plan
The mistake: You make protein products the whole recovery plan.
Why runners do it: Protein feels simple. It is linked with muscle repair, strength training and recovery. It is also heavily marketed in bars, powders and shakes.
How it affects running: Protein matters for runners, but it does not replace carbohydrate, fluids, sodium or enough total energy. If you only focus on protein products, you can still be under-fuelled, low on glycogen, thirsty or missing the food needed to adapt to training.
What to do instead: Use protein as one part of the recovery picture. After harder or longer sessions, many runners do well with roughly 20-40 g protein alongside carbohydrates and fluids. Whole foods are fine. Shakes can be useful when appetite, timing or convenience make food difficult, but they are not required.
Simple example: A protein shake on its own may not be enough after a long run. Add a bagel, cereal, fruit, oats or a proper meal if the session took a lot out of you. Or skip the shake and use normal food: yoghurt with granola, a tuna wrap, tofu rice bowl or chicken rice bowl can all do the job.
Mistake 9: guessing hydration and electrolytes
The mistake: You either ignore fluid and sodium, or force so much plain water that you create a different problem.
Why runners do it: Hydration advice can sound extreme. Some runners barely drink after hot sessions. Others carry the idea that more water is always better. Many are unsure when electrolytes matter.
How it affects running: Dehydration can make running feel harder and slow recovery. Heavy sweat losses can also mean sodium matters, especially in long, hot or humid runs. But drinking excessive plain water can be dangerous because it may contribute to exercise-associated hyponatraemia, where blood sodium becomes too low.
What to do instead: For easy, short runs, water and normal meals are usually enough. For long, hot or very sweaty runs, use a more intentional plan: fluids, sodium through food or electrolytes, and drinking to thirst rather than forcing large amounts.
Electrolytes can help replace sodium, but they do not make overdrinking safe. The bigger mistake is forcing large amounts of fluid beyond thirst or need during long events.
For more detail, read the RunFuel running hydration guide.
Simple example: A relaxed 40-minute run may only need normal drinking afterwards. A two-hour summer long run may need fluid during or after, plus sodium from salty food or an electrolyte drink.
Mistake 10: testing new fuel on race day
The mistake: You use race day to test a new breakfast, gel, drink, caffeine dose or "healthy" high-fibre meal.
Why runners do it: Race expos, course sports drinks, last-minute advice and nerves all push runners into experimenting. It can feel like you should do something special because the race matters.
How it affects running: New fuel can cause stomach problems, unexpected toilet stops, nausea, reflux, too much caffeine, too little carbohydrate, or a plan you cannot repeat once the race gets hard.
What to do instead: Race day should feel almost boring. Practise breakfast, gels, chews, drinks, caffeine and timing in training. If the race provides an on-course drink, test it before the event or carry what you know.
Simple example: Use a long run to practise your race breakfast and during-run fuel. If it works, write it down. That becomes your race plan.
Warning signs runners should not ignore
Low energy availability and RED-S are not just "mistakes". They are safety concerns that can affect male and female runners and deserve careful support. This guide can help you spot warning signs, but it cannot diagnose RED-S or rule out other causes.
Persistent symptoms may also involve iron status, thyroid issues, illness, training overload, sleep, stress or another medical issue. Speak to a registered dietitian, SENR-registered sports nutritionist or healthcare professional if you notice:
- Persistent fatigue that does not match your training.
- Dizziness, fainting, shakiness or feeling unusually cold.
- Rapid or unintentional weight loss.
- Missed, irregular or changed menstrual periods.
- Loss of libido or hormone-related changes.
- Recurring illness, stress fractures or repeated injuries.
- Poor recovery despite easier training.
- Declining performance despite consistent training.
- Anxiety, guilt or rigidity around food.
- Bingeing, restriction, purging or compensating with exercise.
- Regular stomach problems that do not improve with simple changes.
- Diabetes, pregnancy, gastrointestinal disease or another medical condition that affects fuelling.
Missed periods, stress fractures, fainting, purging, rapid weight loss or persistent dizziness need professional support. If symptoms are severe, sudden or worrying, seek medical help promptly.
How to fix your running nutrition without overcomplicating it
You do not need to rebuild your whole diet in one week. Start with the mistake most likely to affect your running.
- Fuel the sessions that need fuel. Easy short runs are different from long runs, races, intervals and hill sessions.
- Put carbohydrates around harder work. Carbohydrates before and after key sessions often solve more problems than another supplement.
- Recover after demanding runs. Think carbohydrates, protein and fluids within the next couple of hours.
- Practise long-run fuel. Build your stomach plan in training, not on race day.
- Hydrate with context. Heat, humidity, sweat rate, duration and sodium losses all change the plan.
- Keep a simple fuel note. Track what you ate, when you ate it, how your energy felt, how your stomach felt and how recovery went.
- Fix the basics before supplements. Powders, tablets and pills cannot replace enough food, enough carbohydrates, enough fluids and enough consistency.
The best running nutrition plan is not the most complicated one. It is the one that helps you train, recover and repeat without turning food into a second sport.
Running nutrition mistakes FAQs
These answers cover the grey areas runners ask about most: under-fuelling, carbohydrates, fasted running, recovery, protein, long-run fuel, hydration, injury risk and simple fixes.
What is the biggest nutrition mistake runners make?
The biggest mistake is usually not one dramatic thing. It is not eating enough for the training you are asking your body to do. That might mean too little total food, too few carbohydrates around harder sessions, poor recovery after long runs, or skipping fuel during longer efforts.
The fix is not to eat perfectly. It is to match food to the work: more support around long runs, workouts, races and heavy training weeks.
How do I know if I am under-fuelling as a runner?
Possible signs include constant hunger, heavy legs, poor recovery, low mood, poor sleep, dizziness, frequent illness, recurring injuries, missed or irregular periods, and performance dropping despite consistent training.
These signs can have other causes too, including medical issues or training overload. If they persist, get personalised advice rather than trying to push through.
Should runners avoid carbohydrates?
No. Carbohydrates are a useful fuel for runners, especially for harder sessions, hills, races and long runs. Avoiding them can make those runs feel harder and can slow recovery afterwards.
You do not need to eat the same amount every day. Many runners do well by placing more carbohydrates around the sessions that need them most.
Is it bad to run on an empty stomach?
Not always. Some runners feel fine doing short, easy runs before breakfast. The problem is making fasted running the default for hard workouts, long runs or race-pace sessions, especially if it leaves you dizzy, flat, ravenous or slow to recover.
If fasted running works for an easy run, fine. If the session matters, fuel it.
When do runners need fuel during a run?
Most runs under 60 minutes do not need fuel during the run. Between 60 and 90 minutes, it depends on intensity, heat, how well you ate beforehand and how you usually feel late in the run.
Over 90 minutes, carbohydrate becomes much more useful. A common starting point is 30-60 g carbohydrate per hour, practised gradually in training.
What should runners eat after a hard run?
After a hard or long run, think carbohydrates, protein and fluids. Protein helps repair muscle, while carbohydrates help replace the energy you used.
That could be a proper meal or a recovery snack followed by a meal later. The exact food matters less than getting the mix in consistently.
Do runners need protein after every run?
Not in a dramatic, stopwatch way. After an easy short run, your next normal meal may cover it. After hard runs, long runs, races or strength sessions, protein becomes more useful because it supports repair and adaptation.
The bigger win is regular protein across the day, not panicking about one perfect shake after every jog.
Can drinking too much water be bad for runners?
Yes. Drinking excessive amounts of plain water during long events can contribute to exercise-associated hyponatraemia, a dangerous drop in blood sodium.
This does not mean you should ignore hydration. It means more water is not always better. Drink to thirst, consider sodium for long or sweaty runs, and build a sensible plan in training.
Can poor nutrition increase injury risk?
Poor nutrition can contribute to the conditions that make injury more likely: low energy availability, poor recovery, low mood, poor sleep, fatigue, menstrual disruption, low bone health and repeated illness.
Nutrition is not the only factor in injury risk, but it is one runners can often improve. If you have recurring injuries, stress fractures or RED-S warning signs, get personalised support.
How do I fix my running nutrition?
Start with the one mistake most likely to affect your training. If long runs fall apart, practise fuel during long runs. If workouts feel flat, add carbohydrates before harder sessions. If recovery is poor, fix the post-run meal.
Do not try to optimise everything at once. One repeatable improvement beats a perfect plan you cannot keep.
Simple RunFuel rules
- Most running nutrition mistakes are fixable.
- Do not treat every run the same.
- Fuel hard and long sessions more deliberately.
- Carbohydrates help running; protein helps repair; fluids help recovery.
- Long-run fuel needs practice.
- Race day is for repeating, not experimenting.
- Warning signs are not weakness.
- Keep the plan simple enough to repeat.
Running nutrition does not need to be perfect. It needs to be useful. If one small change makes your next month of training feel steadier, that is a win.
Sources
- Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine, "Nutrition and Athletic Performance", 2016.
- Jeukendrup, "A step towards personalized sports nutrition: carbohydrate intake during exercise", 2014.
- International Society of Sports Nutrition position stand, "Protein and exercise", 2017.
- Sports Dietitians Australia, "Recovery Nutrition".
- Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, 2015.
- International Olympic Committee consensus statement on Relative Energy Deficiency in Sport, 2023.
