Registered Dietitian Charlie Watson on how to fuel before a run or workout: what the evidence says about carbs, timing, hydration, fasted training and training your gut.
Key Takeaways
Carbohydrates are your main pre-workout fuel, and quick-digesting, low-fibre options beat "healthier" wholegrains right before a session.
Timing matters as much as food choice: a balanced meal 2–4 hours out, or a fast-digesting carb snack 30–60 minutes before.
Fasted training burns more fat in the session but doesn't improve performance or fat loss over time; fuelled sessions are higher quality.
Hydration is the most overlooked lever: even 2% dehydration can cut endurance capacity by 10%.
Gut training works. Practising your race-day fuelling for a few weeks can dramatically reduce digestive issues on the day.
Charlie is a registered dietitian with six years of NHS experience, currently specialising as a neurology dietitian with a particular focus on Motor Neuron Disease. A former Deputy Cookery Editor at Good Housekeeping Magazine, she retrained as a dietitian out of a conviction that nutrition guidance should be evidence-based and led by qualified practitioners. Charlie is also a keen runner and passionate about helping everyday runners improve their performance and enjoyment through smarter fuelling.
There are as many different approaches to pre-workout fuelling as there are types of workouts themselves because, honestly like the training itself, fuelling is very individual. What works for one person won't quite hit right for someone else. However there are general principles with solid evidence behind them that create the basis for the majority. Some people swear by fasted training (although there are lots of people that this is likely more detrimental than beneficial for!), others rely on simple carbs, and then we all know that person who could eat a huge meal right before a long run and seem to suffer zero consequences.
What you eat before your run or workout can make or break the session though. And while it's true that everybody is different, there is some solid science behind what type of pre-workout nutrition can be most effective. This guide talks through what the research actually supports, so you can make the best choices for you.
Our ability to sustain high-intensity efforts is hugely affected by how much fuel our muscles have available – and they burn through that fuel quickly. Studies show that just one hour of high intensity exercise can deplete your muscle glycogen (carbohydrate) stores by up to 70%.⊕ Carbohydrate is our body's favourite, and easiest accessed, fuel. If we don't top up glycogen stores with a pre-workout or run carbohydrate based meal or snack, then we need to start using our reserves immediately to fuel our workout – and these only last a finite amount of time. When your body runs low on these stores, it takes longer to convert other fuel sources in the body into energy, making exercise feel harder – or the feeling of "hitting the wall" aka running out of energy.
Past diet-culture made carbs out to be the enemy, but they're absolutely essential for everybody, and especially for athletes. Carbohydrates are the body's preferred source of fuel for any moderate to high intensity exercise. Think tempo runs, intervals, strength training – anything where your perceived effort is about 60–70% essentially.⊕
Carbohydrates don't just fuel your muscles. They are essential for brain function. Your brain relies on a regular supply of glucose, which mostly comes from carbohydrates. Eating enough carbs before exercise can help you stay focused, make quick decisions and feel mentally sharp throughout your workout.
Carbohydrates come in many forms but low-fibre and fast-digesting options are generally preferable before exercise, as they're less likely to cause you GI issues. When we workout our blood goes to our heart and muscles and away from our stomach, slowing down digestion. Quick digesting carbs are absorbed into the bloodstream making them more readily available for your muscles to use them rather than sitting in your stomach, potentially upsetting your gastrointestinal system while you workout.
Think white rice, white or sourdough bread, bananas, oats and sports drinks. This is the opposite to the sorts of slow-releasing, higher fibre carbs you might have otherwise been told are "healthier" (whole grains etc) and we want to include as part of a well balanced diet.
From carb-loading the night before a long race to gels on the start line, choosing when to fuel can be as much of a minefield as what to eat, if not more.
Research shows that eating a carbohydrate snack in the 30–60 minutes before exercise can help ensure glucose is available in your bloodstream as you begin exercising, particularly for longer or higher-intensity sessions.⊕ You'll want to choose something that will be digested quickly like a banana, cereal bar or piece of white toast with jam. If you're eating 2–4 hours before exercise, you'll typically benefit more from a larger, balanced meal that has time to digest.
Carbohydrates are the main source of fuel for moderate to high intensity running, so as running distance/time increases, carbohydrate/glucose availability becomes more important.
You want to go into each session with adequate glycogen (carbohydrate) stores, so eat a balanced diet with between 50–70% of your total daily calories coming from carbs. For runs under 60 minutes a pre-run snack or meal should suffice for fuelling.
For runs of between 1–2 hours, research supports taking on 30–90g carbohydrates per hour (in addition to a pre-run meal 2+ hours beforehand). For runs over 2 hours, 30–90g carbohydrates per hour can be beneficial (with gut training). The easiest way to take on carbs on the run is from foods like sweets, dried fruits or sports-specific gels or drinks to maintain blood glucose and delay fatigue. You want to start fuelling early rather than waiting to feel tired or for your energy to dip.
Ideally you want to choose a combination of glucose and fructose for improved gut absorption (most sports products are designed with the ideal ratio of 1:0.8). Real food versions that work in similar ratios are honey and maple syrup.⊕
It's easy to spend a lot of time thinking about your optimum carbs per hour, the best supplements to boost health and performance but to overlook one of the most simple factors for improving your performance: hydration.
Research shows that dehydration levels of even 2% can reduce your endurance capacity by 10%.⊕
Daily hydration is key for overall health and will lead to better performances too. Pre-workout a good target is to take on 400–600ml of water in the 1–2 hours before training, rather than waiting until the session starts and you feel thirsty. Monitoring your urine colour is a straightforward way to keep check of hydration levels – we are looking for transparent to pale yellow/light gold. You can also have your sweat and hydration levels tested professionally for more personalised hydration (and sodium) recommendations.⊕
For longer sessions (60+ mins, or shorter in very warm conditions), you'll want to start thinking about electrolytes too. Electrolytes are minerals like sodium, magnesium and potassium that help regulate fluid balance and allow muscles and nerves to function properly. We lose these electrolytes in higher concentrations in our sweat than others.
Fasted training regularly has moments of popularity, with the main advantages cited being that it helps with fat loss or "fat adaptation" – encouraging our bodies to fuel from existing fat stores, rather than requiring additional intake of glucose/carbs. And some research does back this up: studies show that exercise performed while fasted does in fact increase fat oxidation (burning fat to produce energy) compared to training after eating.
But – and there is a but – this single-session fat oxidation doesn't actually translate to better performance or increased fat loss over time. It doesn't train the body to rely on fat reserves any better. There are plenty of associated risks though: reduced training quality, increased perceived effort, potential long-term health consequences from consistently training in an energy deficient state.⊕
Studies generally show that for any higher-intensity or longer sessions, adequate fuelling beforehand is pretty much always preferable. You'll likely get better results from a higher quality session while fuelled, than a lower quality session while fasted.
Overall there isn't a huge amount of evidence against fasted training – some have shown decreased performance while others showed no impact. Generally speaking we want to make sure that fasted training is not in combination with under-fuelling overall, and is mostly for low intensity workouts vs high intensity sessions, to avoid negative impact on the body.
Gut training may sound a bit gory but it essentially just means getting your stomach and digestive system used to consuming whatever your race-day nutrition will be. That can mean both better tolerating training in the hours after a larger meal/higher carb snack (especially if you're used to training in a fasted state), and enabling you to take on fuel and hydration during the run or workout itself.
This is a big deal because gut issues are incredibly common amongst endurance athletes – many a well-trained for marathon or ultra has been ruined by digestive problems. Research suggests that 27% of recreational runners have suffered severe digestive issues during races.⊕ That can look like nausea, bloating, cramping and diarrhoea.
The good news is that these can mostly be solved by practice. Consistent exposure to carbohydrates improves our ability to absorb them and leads to less undigested carbs reaching the colon and causing issues. You're likely to run into trouble if you typically eat a low-carb diet, and then suddenly try and consume them on race day.
Just two weeks of training with 30–90g of carbohydrate per hour can make a huge difference though, reducing discomfort by up to 47% and reducing carbohydrate malabsorption by 45–54%. So even if your event is coming up soon, it's not too late to start gut training.
What does gut training really look like though? Although you can see a meaningful improvement in just a couple of weeks, the ideal is to start your gut training 8–12 weeks out from race day, and at least 6 weeks.
If your event is going to take 1–2 hours, then training with 30–60g carbs per hour during your training will help you prepare for race day. For events taking over 2 hours then you may want to play around with upping that carbohydrate intake to 60–90g per hour, increasing gradually during training.
Use the same products as you will on race day. Lots of runners prefer to save their favourite gels because, let's be honest, they can be pricey. However, gut training only works if you replicate the exact stress your gut will experience on race day though, so it's worth splashing out for at least some of your key training sessions.
Different people respond differently to different products. It might take a while to find the best sports nutrition for you – hence why starting gut training early is important. If GI issues persist, it's worth looking at your hydration and sodium levels, as dehydration can make problems worse especially during hot conditions and long events. When we are dehydrated, our body sends more blood to muscles and less to our digestive system, delaying gastric emptying. This makes the stomach more sensitive, increasing risk of bloating, nausea and diarrhoea and reduces carbohydrate absorption.
Carbs are key before a workout, regardless of length or type. Ideally eat a balanced meal 2+ hours prior to exercising or easy to digest, carb based snack 30–60 minutes beforehand. If you're strength training then adding in a portion of protein can maximise muscle gains.
The closer you eat to your workout, the smaller the portion and the more simple carbs.
30–60 minutes – 20–30g quick release carb snack
1–2 hours – light carbohydrate based meal with a little protein
3–4 hours – full balanced meal that is rich in carbohydrates with a portion of protein. If you have a sensitive stomach then going easy on the fat and fibre might help reduce risk of GI issues.
If exercising fasted it should be for easy efforts only and making sure you are not chronically underfuelling to reduce risk of LEA (low energy availability).
You should prioritise fuelling before your run or workout for more intense, longer efforts to improve performance, reduce injury risk and lower stress on the body.
The International Society of Sports Nutrition position statement notes that "maximal endogenous glycogen stores are best promoted by a high-carbohydrate diet, with pre-exercise carbohydrate intake being one of the key strategies for sustaining performance."⊕
Part of it is mindset too. Repeatedly, research shows that well-fuelled athletes will score the same exercise as feeling less difficult than those who are under-fuelled.⊕ This element of perceived effort has a huge impact on how much you can actually get out of a session.
This isn't to say that you need a large meal before every single workout though (this probably wouldn't end well for most of us!). It's all about timing, the type of workout you're doing, and the length of your planned session.
Some ideas for good pre-workout carb options might include banana and peanut butter on white toast, oats with honey, crumpets with jam, rice krispie bars, a sports drink or glass of fruit juice.
While carbs are still the main source of energy, protein before a session (not just after!) has been shown to support muscles during resistance training. Amino acids are the building blocks of protein; the nine essential amino acids are the ones that our body cannot make and have to be consumed as part of our diet or as a supplement. Consuming essential amino acids usually as part of a protein based meal or snack of 20–40g has been shown to optimally promote muscle protein synthesis.
To maximise both performance and post exercise "gains", you want to combine carbohydrates and protein pre-workout. Meal ideas: combo of carbs and protein 1–2 hours before to tick both boxes. E.g. greek yoghurt with oats, protein shake with fruit, chicken and rice.⊕⊕
It can be tempting to opt to undertake your early morning sessions fasted to maximise sleep and in the thought of reducing risk of GI issues. However studies show that even small amounts of carbohydrates can improve performance compared to training fasted, as well as improving your recovery and injury resistance.
For those that regularly exercise early in the morning, a carb based evening meal to top up glycogen stores the night before can be beneficial. Prior to an easy session, a quick glass of orange juice, cereal bar or other quick digesting snack will suffice (followed by a balanced meal post workout).
For more intense or longer morning workouts, it's important to consume something with sufficient carbohydrates (and protein depending on your workout of choice) before working out. This may take some trial and error into timing and type to see what suits your body and lifestyle best.⊕
Hydration needs vary a lot from person to person. It's not just the volume of liquid you lose through sweat – it's also about your sodium levels. If you see white streaks on your clothes, have crusty skin or regularly suffer from cramps, then you might be a heavy sweater or have high sodium levels in your sweat and may be worth getting tested to more accurately understand your hydration needs.
You could also consider a four-week course of multi-strain probiotic. The evidence is increasingly compelling that probiotic supplementation helps runners go faster, which could in large part be down to a reduction in gut issues. Worst case, it's minimal risk.⊕⊕
30–60 minutes – glass of orange juice, banana, piece of toast with jam, cereal bar, hot cross bun
1–2 hours – crumpet with honey, bowl of porridge with banana, yoghurt with granola and berries, bagel with peanut butter and jam.
3–4 hours – spaghetti bolognese or pasta with tomato sauce with cheese, chicken sandwich, jacket potato with tuna and sweetcorn, salmon potatoes and vegetables, prawn stirfry with noodles or rice, breakfast bagel with egg.
Training your gut is as important as training your legs for performance. Practising what you're going to eat before and during your workouts/runs is crucial for optimal carbohydrate absorption while exercising. Regular exposure to high carbohydrate improves our ability to absorb and utilise the carbs ingested on the go, and is likely to reduce GI issues.
Practise taking on 30–60g carbs per hour on your longer runs or rides in training so that on race day your stomach is used to the dump of glucose/fructose and can utilise them efficiently.
