We Run Ultras

(6) Cold Ultras Need Hydration Guardrails, Not a Fixed Drinking Rate

By Francesco//8 min read/Race execution & strategy

The invisible drain

Cold weather makes hydration harder to read. Sweat can remain inside clothing, urine output may rise during cold exposure, bottles freeze and gloves make drinking awkward. Respiratory water loss also continues, although it is usually smaller than sweat and urine losses. At the same time, thirst can be blunted. The result is a two-sided problem: an ultrarunner can drink too little because losses are inconspicuous, or too much by importing a fixed warm-weather rate into an environment with lower sweat loss. Cold-ultra strategy therefore needs individual guardrails, not one prescribed volume.

What cold does to thirst

Cold-induced peripheral vasoconstriction shifts blood centrally. Volume receptors can then register relative central fullness even when total body water is reduced, altering thirst and arginine vasopressin responses. In controlled USARIEM work, Kenefick and colleagues reported thirst sensations up to 40% lower in the cold during rest and exercise when participants were already 3–4% hypohydrated. A later trial at 0.4°C found lower voluntary intake and greater urine output than at 25°C, with a pattern consistent with blunted thirst. These studies involved short laboratory exercise in small groups, not a winter ultra, but the mechanism explains why waiting for strong thirst may delay drinking.

The performance paradox

Cheuvront and colleagues compared cycling performance after roughly 3% body-mass hypohydration in cold and temperate air. Work output fell significantly in temperate conditions; the smaller reduction in the cold condition was not statistically significant. Later reviews reach the same qualified conclusion: hypohydration is less likely to impair submaximal performance in cold or cool environments because heat strain and skin blood flow are lower. This does not make dehydration harmless. It means that preventing every gram of body-mass loss is neither necessary nor a sound basis for forcing fluid during a cold, low-intensity race.

Field data changes the calculation

In a 24-hour winter mountain race averaging −14.3°C, Chlíbková and colleagues measured 20 finishers. Plasma sodium, plasma osmolality, urine osmolality and urine specific gravity remained stable despite lower post-race body mass. Estimated intake averaged about 0.42 L/h: 0.30 in women and 0.46 in men. This small observational study cannot define an ideal rate, but it shows that 0.5–1.0 L/h may be excessive for some athletes in extreme cold and that body-mass change alone is an imperfect hydration proxy.

The opposite risk appeared in the 100-mile Iditasport. Seven of 16 studied runners and cyclists finished hyponatraemic. The hyponatraemic group reported about 0.5 L/h versus 0.4 L/h in the normonatraemic group and consumed less sodium, although the sample was too small to isolate one cause. The important boundary is individual: 500 mL/h was associated with overdrinking in that event, not proven dangerous for every cold-weather athlete.

Sodium does not turn excessive drinking into a safe strategy. Exercise-associated hyponatraemia is driven chiefly by fluid intake in excess of excretory capacity, often combined with non-osmotic vasopressin release; sodium intake can modify the balance but cannot guarantee protection. Conversely, avoiding fluid to prevent hyponatraemia creates a different problem. The usable target is neither maximal replacement nor deliberate dehydration, but intake appropriate to the athlete's observed losses and changing race conditions.

Running a dual-guardrail strategy

Start with training data from similar temperature, clothing, intensity and terrain. Record fluid carried and returned, food water, urination, thirst and pre-to-post body mass over several long runs. Use the result as a broad planning range, not a replacement mandate. During the race, set a timed prompt to reassess drinking because cold may mute thirst; do not turn that prompt into compulsory intake. Rising body mass, repeated clear urination, bloating, headache, nausea or confusion warrant stopping forced drinking and seeking medical assessment, because symptoms of exercise-associated hyponatraemia can overlap with fatigue and cold stress.

Conditions can also change during one event. A sheltered climb in heavy insulation may produce substantial sweat before an exposed ridge reduces it sharply. Fluid from soup, fruit and gels contributes to the total even when the bottle volume looks modest. Recalculate at aid stations when temperature, clothing, pace or food changes; an hourly rate derived from the opening section should not survive the whole race by default.

Logistics are part of the prescription. Insulate the reservoir and hose, keep a backup soft flask inside clothing, check that fluid remains accessible, and pair the drinking plan with the race's fuelling and sodium strategy. Avoid trying a new high-sodium or high-volume protocol on race day. The useful guardrails are concrete: do not wait passively for severe thirst, do not chase zero weight loss, and never keep drinking simply to hit a generic hourly target. That is the cold-specific decision inside a broader ultra race strategy.

Sources

Thirst physiology and cold attenuation

  1. Kenefick RW, Hazzard MP, Mahood NV, Castellani JW, "Thirst sensations and AVP responses at rest and during exercise-cold exposure" (2004), Medicine & Science in Sports & Exercise https://pubmed.ncbi.nlm.nih.gov/15354034/

  2. Freund BJ, Sawka MN, "Influence of Cold Stress on Human Fluid Balance" in Nutritional Needs in Cold and in High-Altitude Environments (1996), National Academies Press https://www.ncbi.nlm.nih.gov/books/NBK232870/

  3. U.S. Army Research Institute of Environmental Medicine, "Prevention and Management of Cold-Weather Injuries" TB MED 508 (2005) https://usariem.health.mil/assets/docs/partnering/tbmed508.pdf

  4. Kenefick RW, Sawka MN, "Hydration at the Work Site" (2007), Journal of the American College of Nutrition https://pubmed.ncbi.nlm.nih.gov/17921464/

Performance and hypohydration in cold

  1. Cheuvront SN, Carter R III, Castellani JW, Sawka MN, "Hypohydration impairs endurance exercise performance in temperate but not cold air" (2005), Journal of Applied Physiology https://pubmed.ncbi.nlm.nih.gov/16024524/

  2. Adam GE, Carter R III, Cheuvront SN, Merullo DJ, Castellani JW, Lieberman HR, Sawka MN, "Hydration effects on cognitive performance during military tasks in temperate and cold environments" (2008), Physiology & Behavior https://pubmed.ncbi.nlm.nih.gov/18166204/

  3. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS, "American College of Sports Medicine position stand: Exercise and fluid replacement" (2007), Medicine & Science in Sports & Exercise https://pubmed.ncbi.nlm.nih.gov/17277604/

  4. Sawka MN, Cheuvront SN, Kenefick RW, "Hypohydration and Human Performance: Impact of Environment and Physiological Mechanisms" (2015), Sports Medicine https://pubmed.ncbi.nlm.nih.gov/26553489/

  5. U.S. Army Research Institute of Environmental Medicine, "Hypohydration Effects on Physical and Cognitive Performance in Cold Environments" (technical report, 2008), DTIC https://apps.dtic.mil/sti/tr/pdf/ADA507114.pdf

Field data from cold ultra events

  1. Chlíbková D, Nikolaidis PT, Rosemann T, et al., "Maintained Hydration Status After a 24-h Winter Mountain Running Race Under Extremely Cold Conditions" (2019), Frontiers in Physiology https://pmc.ncbi.nlm.nih.gov/articles/PMC6336898/

  2. Stuempfle KJ, Lehmann DR, Case HS, Bailey S, Hughes SL, McKenzie J, Evans D, "Hyponatremia in a cold weather ultraendurance race" (2002), Alaska Medicine https://pubmed.ncbi.nlm.nih.gov/12497664/

  3. Stuempfle KJ, Lehmann DR, Case HS, Hughes SL, Evans D, "Change in serum sodium concentration during a cold weather ultradistance race" (2003), Clinical Journal of Sport Medicine https://pubmed.ncbi.nlm.nih.gov/12792212/

Respiratory water loss

  1. Mitchell JW, Nadel ER, Stolwijk JAJ, "Respiratory weight losses during exercise" (1972), Journal of Applied Physiology https://journals.physiology.org/doi/abs/10.1152/jappl.1972.32.4.474

  2. National Academies Press, "Nutritional Needs in Cold and in High-Altitude Environments" Chapter: fluid balance and respiratory losses (1996) https://www.ncbi.nlm.nih.gov/books/NBK232855/

Thermoregulation and sweating in cold

  1. Schobersberger W, et al., "Thermoregulatory responses in elite cross-country skiers during international competitions and training" (2025), Frontiers in Physiology https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2025.1709093/full

Cold-induced diuresis

  1. Dann EJ, Gillis S, Burstein R, "Effect of fluid intake on renal function during exercise in the cold" (1990), European Journal of Applied Physiology https://pubmed.ncbi.nlm.nih.gov/2289490/

  2. Young AJ, Muza SR, Sawka MN, et al., "Human vascular fluid responses to cold stress are not altered by cold acclimation" (1987), Journal of Applied Physiology https://pubmed.ncbi.nlm.nih.gov/3629738/

Hydration guidance and hyponatraemia

  1. Hew-Butler T, Verbalis JG, Noakes TD, "Updated fluid recommendation: position statement from the International Marathon Medical Directors Association (IMMDA)" (2006), Clinical Journal of Sport Medicine https://pubmed.ncbi.nlm.nih.gov/16858210/

  2. IMMDA, "Updated Fluid Recommendation: Position Statement" (2006), full text https://immda.org/wp-content/uploads/2015/08/Spring-2006-Updated-Fluid-Recommendations.pdf

  3. Noakes TD, Sharwood K, Speedy D, et al., "Three independent biological mechanisms cause exercise-associated hyponatremia: evidence from 2,135 weighed competitive athletic performances" (2005), Proceedings of the National Academy of Sciences https://pubmed.ncbi.nlm.nih.gov/16049340/

  4. Cheung SS, et al., "Does Hypohydration Really Impair Endurance Performance? A Critical Review with Methodological Considerations" (2019), Sports Medicine https://pmc.ncbi.nlm.nih.gov/articles/PMC6901416/

Cold-environment guidance

  1. National Academies, "Nutritional Needs in Cold and in High-Altitude Environments" (full volume, 1996) https://www.nationalacademies.org/publications/5197

  2. Castellani JW, et al., "ACSM Expert Consensus Statement: Injury Prevention and Exercise Performance during Cold-Weather Exercise" (2021), Current Sports Medicine Reports https://pubmed.ncbi.nlm.nih.gov/34752434/

Cold-exercise guidance

  1. HPRC, "Resources for operating in cold environments" (2019) https://www.hprc-online.org/physical-fitness/environmental-extremes/resources-operating-cold-environments

  2. Mears SA, Shirreffs SM, "Voluntary water intake during and following moderate exercise in the cold" (2014), International Journal of Sport Nutrition and Exercise Metabolism https://pubmed.ncbi.nlm.nih.gov/24762424/

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