Ultra-endurance nutrition in the heat requires much more precise planning than in temperate conditions. High temperatures increase heat stress, fluid and electrolyte losses, and the risk of gastrointestinal problems, making hydration, carbohydrate intake and sodium replacement decisive for maintaining performance. This article reviews the main nutritional strategies for tackling ultra-endurance events in hot environments with an individualised, safe approach.
What characterises ultra-endurance events in the heat?
- Extreme physiological demands.
This is no ordinary challenge: preparing the body to run for hours in the sun pushes systems that operate comfortably in temperate conditions to their limits. Energy efficiency falls and cardiovascular demands soar simply to keep the athlete moving.
- Thermal and metabolic stress.
The body becomes a furnace fighting on two fronts: sending blood to the muscles to sustain performance while redirecting it to the skin for cooling. This internal conflict forces metabolism to work harder, accelerating fatigue.
- Associated risks.
The line between extreme exertion and real danger is easily crossed when temperatures remain high. Risks range from severe cramps and a sharp performance decline caused by dehydration to critical conditions such as heatstroke, which can incapacitate an athlete within minutes.
Hyponatraemia must also be considered: focusing solely on water and neglecting electrolyte replacement for too long can create a very serious situation for the athlete.
How heat affects performance and digestion
- Redistribution of blood flow
To cool the body, substantial blood flow is diverted towards the skin to facilitate sweating. That blood cannot be in two places at once: more flow to the skin means less is available to working muscles. The heart must beat much faster to maintain pace, raising the athlete’s heart rate and reducing running efficiency.
- Increased gastrointestinal risk
Because cooling the body and moving the legs take priority, the stomach and intestines receive very little blood during exercise in the heat, a condition known as splanchnic ischaemia. Without sufficient blood supply, digestion stalls. The stomach loses its capacity to absorb the gels and water the athlete consumes, leading to nausea, stomach cramps or difficulties assimilating nutrients.
- Changes in perceived exertion
The brain acts as a safety thermostat. When core temperature rises too far, the central nervous system immediately alters fatigue signals to protect us. Even at the athlete’s usual running pace, perceived exertion and mental fatigue become much greater, limiting performance long before the muscles run out of energy.
Hydration during ultra-endurance events in high temperatures
- Fluid losses
In extreme heat, sweat rates can easily exceed 1–1.5 litres per hour. Athletes lose not only water but also large quantities of electrolytes, particularly sodium. Without continuous fluid replacement, blood volume falls and blood becomes more viscous, impairing oxygen transport and accelerating dehydration.
- Hyponatraemia vs dehydration
Balance requires particular care. While dehydration means insufficient body water, hyponatraemia can result from the opposite problem: drinking too much plain water without enough sodium. This dilutes blood sodium and can be extremely dangerous, causing disorientation, muscle weakness or collapse. That is why we emphasise consuming electrolytes as well as fluid.
- Personalised strategies
There is no universal formula: every athlete is different and sweats differently. An effective strategy requires measuring the athlete’s sweat rate in hot conditions through preliminary weighing tests. Only then can we accurately prescribe the millilitres of fluid and, above all, milligrams of sodium needed per hour of racing to maintain performance and avoid problems.
Carbohydrate intake strategies in extreme heat
- Types of carbohydrate
During the day in scorching heat, the priority is to minimise digestive effort. We choose rapidly assimilated options with easy-to-tolerate textures, including carbohydrate powders, hydrogels, chews and high-water-content fruit such as watermelon or melon. To avoid overloading digestion, we set aside fats, proteins and even denser fruit such as bananas unless accompanied by water, reserving more solid or dry foods for cooler night-time conditions.
- Osmolarity and tolerance
The concentration of what enters the stomach determines how quickly it is absorbed. In extreme heat, overly concentrated products force the body to divert water towards the stomach to dilute them, slowing gastric emptying and increasing gastrointestinal risk. We therefore seek easily assimilated formats such as hydrogels or liquids, combined where necessary with electrolyte capsules so that water and energy reach the bloodstream without causing heaviness.
- Timing and dividing intake
With digestion slowed by heat, large servings are unsuitable. The effective strategy is to divide energy intake into small, regular doses throughout the race. Complex carbohydrates and more substantial foods such as rice, pasta or broth are reserved for low-intensity periods, sheltered aid stations or cooler night-time sections. The golden rule remains: never try these solid-food strategies for the first time during competition. Race day is not a testing ground.
Sodium and electrolytes: when and how to adjust them
- Realistic replacement
Do not assume that drinking plain water is enough. When athletes sweat heavily in the heat, sodium losses are very high. Replacing water alone dilutes blood salt concentration, opening the door to exercise-associated hyponatraemia. The key is to compensate for those losses with scheduled electrolyte tablets, isotonic drinks or other sodium-rich supplements throughout the event.
- Avoiding excess
Drinking for its own sake or taking excessive electrolyte tablets without control is no solution either. Excess sodium can disturb fluid balance and cause digestive problems or fluid retention. We need the right balance. Swollen hands or feet, nausea and disorientation are clear warning signs. Sweat sodium losses vary considerably with acclimatisation, genetics, diet and other factors. For greater precision, a patch that measures both fluid loss and sodium loss through sweat can be useful.
- Individualisation
Generic recommendations may be sufficient in many cases, but every body has a unique sweat rate and sweat sodium concentration. For precise planning, a sweat test in conditions similar to race day is practically essential. This identifies the grams of salt and quantity of fluid the athlete needs to perform safely.
Gastrointestinal adaptation in ultra-endurance
- Gut training
The stomach can be trained too, especially when heat reduces its blood supply. Accustoming the digestive system to processing nutrients and fluids while running under heat stress improves its efficiency. By race day, it can tolerate chews, gels or fruit more effectively without breaking down along the way.
- Prior testing
The nutrition professional’s golden rule is that nothing is improvised at the start line. Everything the athlete will consume—gels, electrolytes, fruit such as watermelon, or night-time solid foods such as rice—must have been thoroughly tested during training. Indoor sessions in controlled environments are ideal for simulating these situations and assessing digestive responses before the big day.
- Reducing race-day risk
Minimising gastrointestinal breakdown in the sun requires an impeccable strategy. Prioritise textures that are easy to assimilate and place little demand on the stomach. Dividing intake and avoiding heavy combinations or dense foods during the hottest hours helps sustain energy absorption without nausea or cramps, preventing the familiar post-race complaint: “My stomach shut down.”
Strategies before, during and after the event
- Pre-race loading
The days before the event are crucial for filling glycogen stores and ensuring the athlete arrives fully hydrated. Carrying a water bottle throughout the day and sipping regularly is the simplest way to support hydration at the start. Beginning even slightly dehydrated puts the athlete at a disadvantage.
A less frequently discussed but useful strategy is hyperhydration with glycerol and sodium. It can help retain around 1 extra litre of water, delaying dehydration and its effects.
The protocol involves 1.2–1.4 g/kg of glycerol with 25 ml/kg of water and 3 g/L of sodium. Intake should be gradual; dividing it into 4 equal doses, one every 30 minutes and finishing 60 minutes before the start, often works well. This must always be tested in training beforehand.
- Nutrition during the event
Organisation and discipline are essential. Athletes often start well but neglect food and drink as fatigue develops. Watch alarms reminding them when to take electrolytes, eat or drink are an excellent strategy. Carrying up to 2 kg of water may feel burdensome, but this is minor compared with the fatigue caused by dehydration.
- Recovery in the heat
After finishing, or during cooler night-time sections following more than 8–10 hours of exertion, the focus changes. This is an ideal time to introduce recovery products that support muscle regeneration and replenish stores. Monitoring urine colour afterwards and during the race also indicates hydration status: very dark urine or less frequent urination calls for increased hydration until normality returns.
Problems that may arise during events and how to prevent them
Nothing can be left to chance in ultra-endurance racing. Even beyond our particular specialty, our experience and advice can spare athletes major difficulties.
- Blisters and chafing. Increased sweating in hot conditions can increase friction around the groin, arms and feet. Lightweight, breathable technical socks, changes at aid stations if needed, and petroleum jelly on high-friction areas can be very helpful.
- Exercise-associated hyponatraemia. This occurs when athletes hydrate with water without consuming sodium. Nausea and confusion may develop, with swollen hands and feet being a distinctive symptom.
Electrolyte tablets, isotonic drinks and other sodium-containing supplements can help prevent it.
- Dehydration. Dehydrated athletes experience reduced performance, dizziness, nausea and general fatigue. It is more common than we might think, and urine is a useful indicator. Very dark urine during the event or many hours without urinating should prompt immediate attention to hydration.
This is why sweat-rate testing and hydration training are practically essential for an ultra-endurance event.
- Overhydration. This may occur when the hydration plan is not adjusted to individual sweat rates as temperature and humidity change. Sweat rates early in a race can easily be five times lower than during the hottest part of the day. If athletes urinate too frequently, for example every one or two hours, water intake should be reduced.
- Fluid overload. Although rarely discussed, this is not difficult to recognise in many amateur athletes. Drinking a large amount of water at once can cause water-loading diuresis: increased blood water volume -> inhibition of antidiuretic hormone (ADH) production -> reduced water reabsorption in the kidneys -> water excretion before it has hydrated cells.
The problem with drinking large amounts at once is inefficiency. Rather than hydrating cells, the excess volume triggers water diuresis, and the athlete excretes not only the newly consumed water but valuable electrolytes too. We therefore emphasise small, regular sips.
Common mistakes in ultra-endurance nutrition in the heat
- Copying standard strategies
In practice, athletes frequently try to copy an elite athlete’s strategy or advice from Instagram without adaptation. Under substantial heat stress, there is almost no metabolic margin for error. Prescribing a standard “90 g/h at a 1:0.8 ratio and a standard electrolyte capsule” without assessing sweat rate or digestive history greatly increases the likelihood of dehydration or withdrawal through gastrointestinal problems.
- Failing to train nutrition
Many runners accumulate hundreds of kilometres but arrive with a digestive system unprepared for exertion. Without a specific gut-training protocol in the preceding weeks, their intestinal transporters cannot absorb the necessary carbohydrate load in 30°C heat, when splanchnic blood flow is minimal.
- Ignoring digestive signals
Athletes can be overly disciplined, continuing scheduled intake despite early reflux, bloating or mild nausea. We must teach them that these signals indicate gastric emptying is becoming overloaded. Forcing more carbohydrate can completely stall digestion, turning mild discomfort into severe gastrointestinal failure. Skipping a serving or delaying it briefly can prevent a much bigger problem later.
How INDYA supports nutrition planning for extreme events
- Detailed planning by phase
An ultra such as Val d’Aran by UTMB requires a dynamic strategy for each section. INDYA allows nutrition professionals to divide the plan according to the temperature profile and intensity of each race phase. We can prescribe light hydrogels or easily assimilated liquids for daytime periods of maximum thermoregulatory demand, then switch to savoury solid foods such as rice, potatoes or sodium-rich broth at night or during longer aid-station stops. This provides complete freedom to plan nutrition minute by minute.
- Adjustments according to tolerance
Theory may look perfect on paper, but the athlete’s actual tolerance takes priority. If training tests reveal a dislike of sweet chews in the heat or a preference for refreshing watermelon, the platform allows immediate adjustment of menus and equivalents. Hourly carbohydrate and sodium targets stay intact while food choices fully reflect individual preferences and responses.
- Recording digestive symptoms
INDYA’s feedback tool is invaluable in practice. Systematic recording of heaviness or digestive problems after long runs or indoor simulations provides the clinical information needed to refine the strategy. It helps identify which foods and textures are reliably tolerated and which should be excluded from the final race plan.
- Learning between events
Ultra-endurance nutrition is a process of continuous improvement. Post-race analysis in INDYA compares what was planned with how the athlete felt and performed. This history provides a valuable foundation for the next goal, allowing carbohydrate ratios and sodium amounts to be adjusted using actual evidence rather than assumptions.
The nutrition professional’s role in ultra-endurance sports
- Individualisation
When digestion is a limiting factor, standard textbook protocols are insufficient. Our clinical work involves analysing each athlete’s sweat rate, sweat sodium concentration, gastrointestinal history and taste preferences under fatigue. A tailored strategy helps ensure the nutrition plan supports performance rather than becoming a reason to withdraw.
- Working with the coach
Nutrition and training planning must work together. Coordination with the coach lets us align carbohydrate loads and hydration protocols with specific training blocks. We need to know when indoor sessions will take place to schedule controlled dehydration simulations, and when long runs are planned so that gut training matches real race intensities.
- Safety and evidence
As health professionals, our responsibility is to protect performance without compromising health. In an ultra in the heat, where exercise-associated hyponatraemia and severe dehydration are real risks, every recommendation must be supported by current scientific evidence. From sodium per litre to glycerol hyperhydration, everything needs a metabolic rationale and thorough testing in safe conditions before race day.
FAQs – Ultra-endurance and heat
How much carbohydrate per hour is recommended in the heat?
As we like to say at INDYA, it depends. Although current evidence supports 90–120 g/h in ultra-endurance events, digestive tolerance generally comes first in extreme heat. A range of 60–90 g/h during the hottest hours, using easily digested textures with rapid gastric emptying, is ideal. At night or during low-intensity sections, a more ambitious intake towards the upper limit can include solid complex carbohydrates.
How can gastrointestinal problems be avoided in ultra-endurance?
Protecting splanchnic blood flow and avoiding stomach overload are key. Three pillars support this: prior digestive training (gut training), precisely managed hydration with sufficient sodium for optimal water and glucose transport, and small, regular servings. Emphasise fluid formats during the day and reserve solids for night-time or quieter periods.
Is sodium supplementation essential?
Absolutely. Sodium losses through sweat during ultra-endurance events in high temperatures are too high to rely solely on ordinary food or plain water. Inadequate replacement inevitably leads to a sharp drop in plasma volume, muscle cramps and, in the worst case, dilutional hyponatraemia.
Can digestive tolerance be trained?
Yes. The digestive system is highly adaptable. Gut-training protocols increase glucose and fructose transporter density, improve gastric emptying and reduce bloating during exertion. Training should progress over several weeks, simulating race intensity and, if possible, its thermal conditions.
What are the most common mistakes in these events?
Last-minute improvisation is the main mistake: trying a new gel during the race or untested food at aid stations. Next is drinking large volumes of plain water at once, triggering water diuresis that further dehydrates the athlete and removes electrolytes. Finally, central fatigue can undermine timing: switching off nutrition alarms and stopping intake for two hours makes late-race metabolic collapse inevitable.
