Hydration

Hydration in ultramarathon racing is a fundamentally different challenge from marathon or half-marathon fueling. A 100-mile race lasting 20-30 hours subjects the body to cumulative fluid losses of 10-20 litres, electrolyte depletion measured in tens of grams of sodium, and physiological shifts that change how your kidneys, gut, and thermoregulatory system process fluid as the hours accumulate. The strategies that work for a 3-hour marathon will fail catastrophically at mile 60 of a mountain ultra.

The evidence base for ultra-distance hydration has expanded significantly in recent years. Studies from Western States, UTMB, and Comrades Marathon have revealed that both dehydration and over-hydration are common failure modes — with exercise-associated hyponatremia (EAH) accounting for 5-10% of medical tent visits at major ultras. Getting hydration right across 50-100+ miles requires a phase-based strategy that accounts for aid station availability, self-supported segments, overnight temperature drops, and the progressive decline in gut absorption that occurs beyond 12 hours of continuous exercise.

Flask vs Vest Capacity Planning

Your hydration carry system determines the ceiling of what you can consume between aid stations. Most ultra runners choose between handheld flasks (500ml each) and hydration vests (1.0-2.0L bladder plus 2x 500ml soft flasks). The right choice depends on aid station spacing:

• Aid stations every 5-8km: Two 500ml soft flasks are sufficient. Refill at every station regardless of how full your flasks are — never pass an aid station without topping up
• Aid stations every 10-15km: A 1.5L bladder plus one 500ml flask provides adequate capacity. At a moderate pace of 7-8 min/km with a sweat rate of 500-800ml/hr, you need 900-1,600ml between stations
• Self-supported segments over 15km: Carry the maximum your vest allows (2.0-2.5L total). In mountain ultras with 20km+ between water sources, calculate your minimum requirement as: segment distance (km) x pace (hours) x sweat rate (ml/hr) x 1.2 safety margin

Pre-load 400-500ml of sodium-containing fluid 30-45 minutes before entering any self-supported segment longer than 2 hours. This expands plasma volume and buys a buffer against unexpected delays from navigation errors, technical terrain, or deteriorating conditions.

Aid Station Drinking Strategy

Aid stations are the critical control points for ultra hydration. Most runners spend 2-5 minutes at each station — use this time systematically rather than grabbing a cup and moving on:

• Drink 200-300ml immediately on arrival. Your gut absorbs fluid fastest when you are standing still. This is the most efficient hydration moment between stations
• Refill all carry bottles completely regardless of how much remains. Partial refills create a false sense of security — always leave with maximum capacity
• Consume a sodium-rich broth or soup if available (typically at aid stations from mile 30+ in 100-mile races). A cup of broth provides 800-1,200mg of sodium — equivalent to 2-3 electrolyte capsules — plus psychological warmth during night sections
• Assess urine colour if you use the toilet. Pale yellow is optimal; dark amber means you are behind on fluid; clear or very light urine with bloating may indicate developing hyponatremia

Sodium Targets and Electrolyte Management

Sodium replacement is the single most critical electrolyte priority during ultras. Target 500-1000mg of sodium per hour, adjusted based on conditions and individual sweat composition:

• Cool conditions (under 15°C): 500-600mg/hr is usually adequate. Sweat rates drop significantly, and overnight sections in mountain races may require as little as 400mg/hr
• Moderate conditions (15-25°C): 600-800mg/hr. This is the standard range for most ultra runners during daytime sections
• Hot conditions (above 25°C): 800-1000mg/hr. Heavy sweaters may need up to 1,200mg/hr. Monitor for muscle cramping, which often indicates sodium depletion before other symptoms appear

Deliver sodium through a combination of electrolyte drink in your flasks (300-500mg per 500ml), salt capsules every 30-45 minutes (200-250mg each), and salty foods at aid stations (crisps, pretzels, broth, pickles). Diversifying sodium sources reduces GI fatigue from any single input.

Overnight Hydration Shifts

Races lasting through the night introduce hydration challenges that daytime-only events never encounter. Between 10pm and 5am, several physiological changes alter your fluid needs:

• Core temperature drops: As pace slows and ambient temperature falls, sweat rate can decrease by 40-60%. Maintaining daytime fluid intake rates overnight leads to over-hydration — reduce intake to 300-500ml/hr during cool night sections
• Antidiuretic hormone (ADH) increases: Nocturnal ADH secretion reduces urine output, meaning fluid is retained more efficiently. Drinking the same volume overnight as during the day dilutes blood sodium — a key driver of overnight hyponatremia
• Gut absorption slows: After 12-18 hours of exercise, gastric emptying rate decreases by 20-30%. Switch to smaller, more frequent sips (100-150ml every 15 minutes) rather than larger boluses that sit in the stomach and cause nausea

Weight Monitoring at Checkpoints

Many well-organized ultras provide scales at aid stations. Use them. Body weight changes during an ultra provide the most objective hydration assessment available in the field:

• Weight loss of 0-2%: Normal and expected. No intervention needed beyond maintaining your current drinking rate
• Weight loss of 2-4%: Meaningful dehydration. Increase fluid intake by 200-300ml/hr for the next segment and add an extra sodium capsule per hour
• Weight loss over 4%: Significant dehydration requiring aggressive correction. Sit at the aid station, drink 500ml over 15 minutes, and reassess before continuing
• Weight gain of 1% or more: A red flag for over-hydration and potential hyponatremia. Reduce drinking to thirst-only, increase sodium intake, and avoid plain water until weight stabilises

Record your weight at each checkpoint if possible. A progressive downward trend suggests you are systematically under-drinking; a stable or upward trend during the second half may indicate fluid retention problems.

Hyponatremia Risk Management

Exercise-associated hyponatremia (blood sodium below 135 mmol/L) is the most dangerous hydration-related complication in ultra-distance racing. It occurs when fluid intake exceeds the body's ability to excrete water, diluting blood sodium to dangerous levels. Risk factors include:

• Drinking more than 800ml/hr consistently for multiple hours — especially plain water without sodium
• Weight gain during the race (the most reliable predictor)
• Slower runners who spend more time on course with more access to aid station fluids
• NSAID use (ibuprofen reduces kidney function and impairs water excretion)
• Female athletes, who have lower body water volume and are statistically more susceptible

Symptoms progress from mild (nausea, bloating, headache) to severe (confusion, seizures, loss of consciousness). The treatment in the field is to stop drinking, consume salty foods, and seek medical attention. Prevention is straightforward: drink to thirst during low-intensity sections rather than forcing a fixed schedule, maintain sodium intake at 500-1000mg/hr, and monitor body weight at every opportunity. Build your personalised ultra hydration plan with the NorthLine Sweat Rate Calculator and carry NorthLine electrolyte drink in your flasks for consistent 300mg/500ml sodium delivery across the full race distance.