Hydration wins – especially on hot, humid runs

Hydration isn’t just about quenching thirst.  It’s the quiet engine behind temperature control, joint lubrication, digestion, nutrient transport, and overall performance. Your body water lives in intracellular and extracellular “compartments,” and it supports everything from cushioning your joints to carrying heat away from working muscles.  When fluid drops, those systems strain and performance follows.

Now picture a 45-year-old, well-trained woman logging a long run on a 90°F, humid Chicago day. Heat itself raises skin blood flow and sweat rate, but humidity is the joker: the wetter the air, the less your sweat can evaporate, meaning you drip, but don’t cool effectively. That’s “useless water loss” that dehydrates you without much temperature relief.

Why that matters:

Even small fluid deficits amplify physiologic strain.  As dehydration progresses and plasma volume falls, sweating and peripheral blood flow drop, heart rate climbs, core temperature rises, and fatigue hits earlier. Losses as little as ~1 – 2% of body mass impair thermoregulation and aerobic capacity, with larger losses eroding VO₂max and endurance further.

A simple, athlete-friendly way to personalize your plan is to weigh before and after key runs. Each 1 lb (~0.45 kg) down is roughly ~450 mL of sweat loss. Use that to estimate your hourly sweat rate and schedule sips every 10 – 15 min.   Afterward, target 125 – 150% of the deficit over the next few hours to account for urine losses (e.g., if you’re 1 kg down, drink 1.25 – 1.5 L). Thirst alone under-shoots needs for many people, especially in heat, so don’t rely on “I’m thirsty” to start taking sips.

What to drink and when?

The American College of Sports Medicine (ACSM) recommends starting exercise euhydrated “several hours” before, then individualizing during-exercise intake to limit body-mass loss to ≤ 2%.   Planned drinking (rather than strict “drink to thirst”) is particularly useful in long, hot sessions with high sweat rates. Practically, many runners do well beginning around  0.4 – 0.8 L/h and then adjusting to their measured sweat rate; include electrolytes and 30 – 60 g carbohydrate per hour on runs > 60 – 90 min. (Sawka et al., 2007; Kenefick, 2018; Jeukendrup, 2014).

Race-week specifics for our Chicago runner:

• Pre-run (3 – 4 hours out): eat normally and sip fluids to produce pale urine; if it’s still dark near start time, take a modest “top-off” (~150 – 250 mL).
• Start line: arrive euhydrated; avoid large boluses right before the gun.
• During: use your sweat-rate baseline to pace sips every 10–15 min; in hot, humid conditions, include sodium/carbohydrate sports drink to support absorption and fueling (6 – 8% carb solutions work well).
• Post: replace 125 – 150% of body-mass loss with fluids plus sodium-containing foods/drinks. (Sawka et al., 2007).

One caution:

More isn’t always better. Over-drinking can dilute blood sodium (exercise-associated hyponatremia), leading from headache and confusion to rare but life-threatening complications. Avoid weight gain during races; use electrolytes and individualized intake instead of “drink as much as possible.” (Hew-Butler et al., 2015).

Bottom line:

On a steamy Chicago long run, heat + humidity raise the stakes. Personalize your plan (weigh in/out), pace your sips, include electrolytes and carbs when the run is long, and replace what you lost afterward. That’s how you keep the engine cool, the legs turning, and the brain sharp.

This newsletter is educational and not a substitute for personal medical advice. If you have a history of heat illness, kidney/cardiac issues, or are on diuretics, consult your clinician before changing hydration practices.

References

Hew-Butler, T., Rosner, M. H., Fowkes-Godek, S., Dugas, J. P., Hoffman, M. D., Lewis, D. P., … Verbalis, J. G. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine, 25(4), 303–320. https://journals.lww.com/cjsportsmed/fulltext/2015/07000/statement_of_the_third_international.2.aspx

Jeukendrup, A. E. (2014). Carbohydrate intake during exercise and performance. Sports Medicine, 44(S1), S25–S33. https://pmc.ncbi.nlm.nih.gov/articles/PMC4008807/

Kenefick, R. W. (2018). Drinking strategies: Planned drinking versus drinking to thirst. Sports Medicine, 48(S1), 31–37. https://pmc.ncbi.nlm.nih.gov/articles/PMC5790864/

Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine Position Stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377–390.

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