Episode 5 | Ask the Expert | Guest: Robbie Green – Registered Dietitian and Endurance Sports Nutrition Specialist
Marathon runners, triathletes, long-distance cyclists – the world of endurance sport is one of the fastest-growing areas of participation sport. It is also one of the most nutritionally demanding, and one where poor fuelling [Podcast] decisions can derail months of training in a single event.
In this episode of the London Bridge Sports Medicine podcast, Dr Ajai Seth is joined by Robbie Green, a registered dietitian with a specialist interest in endurance sport nutrition [Article] and a serious endurance athlete himself. What follows is a comprehensive guide to fuelling for endurance – from the underlying energy systems to race-day strategy.
What counts as endurance sport?
Robbie defines endurance sport as sustaining submaximal effort for prolonged periods – typically beyond an hour. Running, cycling, swimming, and triathlon are the classic examples, but the metabolic demands of the event matter more than the distance alone. Events that severely tax the body’s carbohydrate stores and require a strategic approach to fuelling qualify, whether that is an Olympic-distance triathlon or a 24-hour ultra.
One thing Robbie is clear on: “It takes planning and determination and a lot of strategy. These are things that the majority of people won’t be able to just put their running shoes on and undertake.” But the gradual building of training – and fuelling – over time is exactly what makes these events achievable.
The energy systems: a brief guide
Understanding why nutrition matters [Podcast] so much in endurance sport starts with understanding how the body produces energy. Robbie outlines three key systems:
- Anaerobic system: produces energy without oxygen – fast and powerful, but lasts only seconds to minutes. Think sprinting or heavy lifting. Produces lactic acid as a byproduct, which is why muscles burn and fatigue quickly.
- Glycolytic aerobic system: produces energy using carbohydrate and oxygen. This is the primary system for sustained moderate-to-high intensity effort – running pace, cycling pace. The fuel is glycogen, stored in the muscles and liver, and there is roughly 2,000 calories of it available. The critical point: it runs out in approximately two to three hours without being topped up.
- Fat oxidation system: produces energy from fat stores. The average person has 50,000–60,000 calories of fat available – essentially limitless. The catch is that it produces energy more slowly, meaning it cannot sustain the same exercise intensity as the glycolytic system.
The goal of endurance training – and endurance nutrition – is to make the body better at using fat for fuel while preserving glycogen for when it really needs it. Running out of glycogen is what causes “hitting the wall” or “bonking”: a sudden, dramatic loss of energy that is very difficult to come back from mid-event.
Training nutrition: train low, compete high
One of the best-researched strategies in endurance nutrition is “train low, compete high” – deliberately exposing the body to low-carbohydrate states during certain training sessions to drive fat-burning adaptations, while arriving at race day with glycogen stores fully topped up.
This can be implemented in several ways:
- Eating a lower-carbohydrate diet during certain training blocks
- Training twice in a day and eating inadequately between sessions (going into the second session with depleted glycogen)
- Fasted training – running or cycling before breakfast
Robbie is honest that these approaches are often unpleasant, and typically result in compromised performance during the session itself. The physiological adaptations do occur – the research supports this – but evidence for improved performance outcomes is more limited. His key message: “Don’t push yourself into making yourself feel unwell when you’re training. Find that happy medium.”
The most important variable is variety. Training at the same pace, the same distance, and with the same nutritional approach every day produces limited adaptation. Mixing fasted sessions with fully fuelled sessions, shorter efforts with longer ones, and varying intensity produces far greater gains. “The biggest differences to my own performance came when I started mixing things up,” says Robbie.
Carbohydrate loading: what the evidence actually says
The classic image of eating three bowls of pasta the night before a race reflects the principle of carbohydrate loading, but the practice has evolved considerably from its origins.
Early protocols recommended first depleting glycogen stores completely, then loading heavily. That approach has since been replaced. Current recommendations focus on scaling back training in the taper period and increasing carbohydrate intake during those final days. The target range is wide – five to 12 grams of carbohydrate per kilogram of body weight per day, depending on the event and the individual. At the upper end (12g/kg) you are talking about Tour de France volumes – approaching a kilogram of pasta – which requires liquid nutrition supplements to be achievable.
Practically, during the pre-race loading period, Robbie suggests temporarily shifting away from the usual health-focused approach to carbohydrates. High-fibre whole grains are normally recommended – but in the days before a race, refined, quickly digested carbohydrates are actually preferable. “The priority shifts from achieving optimal health to simply getting those fuel stores where you want them for the event,” he says.
The golden rule: never try anything new on race weekend. Whatever the loading strategy, it needs to have been tested in training.
Race morning: fuelling before the start
Most endurance events start early. Most athletes will not be eating a full meal at 4am. But getting carbohydrate on board before the start – and doing so without upsetting the gut – is critical.
The research (from sports nutritionist Asker Jeukendrup, a leading figure in this area) provides a useful framework based on timing:
- Four hours before: up to 4g carbohydrate per kg body weight
- Three hours before: 3g per kg
- Two hours before: 2g per kg
- One hour before: 1g per kg
These are guidelines, not rules. What matters is finding what works for your gut. Robbie’s own solution, arrived at through trial and error: waking four hours before the event and eating oats made with water – carbohydrate only, nothing that might slow gastric emptying or sit heavily. Not the most exciting breakfast, but one that works.
The principle: on race morning, this is not a health breakfast. It is a fuelling exercise. Prioritise carbohydrate, minimise anything that could cause gut issues, and stick to what you have practised.
During the event: keeping the tank from running dry
The most important conceptual shift in race-day nutrition: do not wait until you feel empty before eating. By the time glycogen is depleted, the damage is done. The wall is not recoverable mid-race – at least not without significant time lost.
The strategy is to maintain a trickle of carbohydrate into the bloodstream throughout the event, reducing the rate at which glycogen stores are drawn down. This means fuelling from early on, even when you feel fine.
Research guidance:
- Under 90 minutes: additional carbohydrate during the event is generally not necessary; hydration is the priority
- Over 90 minutes: aim for around 60 grams of carbohydrate per hour (roughly equivalent to two energy gels), with variation depending on the event, intensity, and individual tolerance
What you use during the event – gels, bananas, raisins, sports drinks, real food on the bike – matters less than getting the carbohydrate in consistently. Gut tolerance during exercise varies enormously between individuals. The digestive system is diverted during hard effort as blood flow goes to the working muscles, which suppresses appetite and can cause nausea. Training the gut to accept food during exercise is a real process, and one that needs to happen in training, not on race day.
Hydration: enough but not too much
Dr Seth raises an important medical point: over-hydration during endurance events – drinking too much plain water – is a genuine risk. It dilutes sodium and other electrolytes in the blood (a condition called hyponatraemia), which can be dangerous. Yet many athletes drink far less during training than they do on race day, simply because of nerves and easy access to water.
Robbie’s guidance: train your hydration strategy just as you train your nutrition strategy. Drink consistently and regularly rather than in large volumes. Urine colour is a reliable practical indicator – pale straw (or “champagne”) is the target; dark yellow signals dehydration; clear signals over-hydration. Isotonic drinks that combine carbohydrate and electrolytes are preferable to plain water, particularly in longer events.
The three principles to take away
- Have a plan – and practise it. Know what you will eat the night before, the morning of, and during the event. Trial it during training. Nothing new on race day.
- Vary your training nutrition. Expose your body to different states – fasted, carb-loaded, mid-session fuelled – to drive adaptation, but always keep your race-day plan in mind.
- Ask for help if you’re stuck. Online resources abound, but individual variation is enormous. If you cannot find what works for you, a sports dietitian can help identify the root cause of performance barriers or gut issues.
The bottom line
Endurance sport rewards preparation, and nutrition is inseparable from training. The energy systems are well understood, the strategies are well researched, and the framework is clear: keep glycogen stores from running out, train the body to use fat efficiently, hydrate consistently, and practise everything in training before race day.
The application of all of that is deeply individual. What works for one athlete – oats and water at 4am, a specific gel every 20 minutes, isotonic drinks throughout – may not work for another. Finding what works takes time, experimentation, and often a willingness to feel uncomfortable in training so that race day is not the experiment.
Robbie Green is a Registered Dietitian specialising in endurance sport nutrition. He works with recreational and competitive endurance athletes across running, cycling, and triathlon.
To discuss your nutrition needs with the LBSM team, visit lbsm.co.uk/booking
Listen to the full episode below ↓
