Hydration without overdoing it

Endurance sports hydration advice has swung from one extreme to the other. The 1990s mantra of “drink as much as possible, drink before thirst” produced a wave of exercise-induced hyponatremia cases and at least several deaths. The backlash produced the opposite extreme: only drink to thirst, water is enough, hydration products are unnecessary. Neither extreme is correct. Current evidence supports a nuanced middle ground, which is considerably more practical than either.

The Physiology of Hydration in Running

Water is lost during running primarily through sweating and respiration. Sweat rates vary wildly between athletes and conditions – from less than five hundred millilitres per hour in cool, easy conditions to over two and a half thousand millilitres per hour in hot, hard-intensity conditions.

The performance effects of dehydration are well-attested. At one to two per cent body weight loss, perceived exertion rises and heart rate rises slightly at the same load. At two to three per cent, endurance performance drops measurably and core temperature rises. At three to five per cent, performance degrades significantly, cognition genuinely drops, and heat risk increases. At over five per cent, there is severe impairment and medical risk.

The practical conclusion is that over two per cent dehydration by body weight incurs significant performance costs and should be avoided in competition.

The Problem With Just "Drinking to Thirst"

Thirst is a delayed signal. By the time it is consciously perceived, dehydration is already underway. In cool conditions, the delay is manageable. In hot conditions, under race stress, with cognitive function already impaired by fatigue, thirst alone is unreliable.

Thirst is also blunted by exertion. Many athletes in hard efforts don’t feel thirsty despite significant fluid loss.

"Drink to thirst" is a useful general principle for low-stakes training. It is not sufficient as the sole hydration strategy in a competitive ultra in warm conditions.

The Problem With Aggressive Over-Hydration

Drinking large volumes of plain water without sodium replacement dilutes blood sodium concentration, causing hyponatremia. At mild levels, it causes nausea, confusion, and headaches. At severe levels, it causes brain swelling and is potentially fatal.

The key risk factor is consuming large volumes of plain water without equivalent sodium replacement during a long event. It is more common in slower athletes, who spend more time on course, and in cool conditions, where thirst does not naturally limit intake. The solution is not to drink less. It is to include sodium in the hydration strategy.

A Practical Hydration Framework

A useful starting range is four hundred to eight hundred millilitres per hour. Adjust upwards to eight hundred and beyond in temperatures above twenty degrees Celsius, for heavy sweaters, at higher intensities, or for larger athletes.

Always include a source of sodium. Options include electrolyte tablets dissolved in water, electrolyte powder added to a soft flask, or salty aid station food alongside plain water.

Keep hydration separate from carbohydrate fuelling. Manage water intake according to conditions and thirst. Manage carbohydrate intake according to target pace and schedule. Mixing them removes the ability to adjust either independently.

When possible, monitor urine colour. Pale yellow indicates adequate hydration. Dark yellow or amber indicates dehydration. Clear can indicate over-hydration. Aid station bathroom visits are useful hydration checkpoints during a long event.

Cold Weather Hydration

In cold conditions, thirst is heavily blunted, and dehydration is less symptomatic. In cold conditions, drink on a schedule – at least three hundred millilitres per hour – irrespective of thirst. Include a source of sodium just as in warm conditions.

Pre-Race Hydration

Arrive at the start line already hydrated. This means normal hydration throughout race week, five to seven millilitres per kilogram body weight in the four hours before the start, and pale yellow urine on race morning.

Do not try to hyper-hydrate by drinking large volumes just before the gun. The kidneys will excrete the excess, and the attempt to carry extra fluid to the start line is physiologically inefficient.

Summary

The correct hydration strategy for ultrarunning is neither aggressive over-drinking nor solely thirst-based drinking. It is moderate, scheduled fluid intake with consistent sodium replacement, adapted to conditions and individual sweat rate. Keep it separate from fuelling. Monitor urine colour when possible. Always include sodium.

References

Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine and Science in Sports and Exercise. 39(2), 377–390.

Hew-Butler T, Rosner MH, Fowkes-Godek S, Dugas JP, Hoffman MD, Lewis DP, et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine. 25(4), 303–320.

Noakes TD. (2007). Drinking guidelines for exercise: what evidence is there that athletes should drink as much as tolerable, to replace the weight lost during exercise or ad libitum? Journal of Sports Sciences. 25(7), 781–796.