Hydration and running: what the science actually says about when and what to drink
Hydration in running is one of the most researched topics — and paradoxically, one of the most misunderstood by runners. Recommendations have evolved significantly in the past decade, and several widely-held beliefs ("drink before you're thirsty", "sports drinks are essential after 30 minutes") no longer match current scientific data. Here's what the research actually shows.
Key Takeaways
- Current recommendation: drink to thirst, not on a fixed schedule — it's the best guide for the vast majority of runners
- Overhydration (hyponatremia) is a real risk, especially in long events — and is often underestimated
- Sodium matters more than carbohydrates for efforts under 2 hours
- Sports drinks aren't necessary for runs under 60-75 minutes
- Pre-run hydration matters as much as during — and many runners start dehydrated
The big revision: drink to thirst vs drink on a plan
For decades, sports recommendations advised drinking on a schedule — say 5-8 oz every 15-20 minutes, whether thirsty or not. The goal was to prevent dehydration before it affected performance.
Research published since the 2000s (and consolidated in 2026 ACSM guidelines) shows a more nuanced picture:
- The thirst sensation is a very precise regulator for moderate efforts. Athletes who drink to thirst don't show inferior performance compared to those on a schedule — and they avoid overhydration risk.
- Overhydration (causing hyponatremia — dangerously low blood sodium) is a documented risk, particularly in marathons and ultramarathons. It can cause cramping, nausea, confusion, and in severe cases, serious complications.
- For intense efforts in high heat, needs increase significantly and thirst may slightly underestimate the deficit. In that context, proactive hydration makes sense — but not in excess.
Sodium: the underrated hydration nutrient
Most runners think of hydration in terms of water volume. But for efforts over an hour — especially in heat — sodium is the key variable to monitor.
Sodium serves several critical functions during exercise: it maintains cellular water balance, regulates plasma volume, and prevents hyponatremia. Sodium losses through sweat vary enormously between individuals ("heavy salters" often see white salt residue on their skin) — but for efforts over 90 minutes, replacing lost sodium is at least as important as replacing water. This is particularly relevant when training in summer heat, when sweat rates and sodium losses are substantially higher than in cool conditions.
In practice: sports drinks (with electrolytes), gels with sodium, or simply adding a bit of salt to your drink covers these needs. For efforts under 75 minutes, water alone is generally sufficient.
Before, during, and after: the summary guide
Before your run. Your hydration status at the start conditions everything else. A simple indicator: pale yellow urine (not colorless, not dark yellow) in the 2 hours before your start indicates good hydration. Drinking 14-20 oz in the 2 hours before starting is recommended by most guidelines.
During your run. For runs under 60 minutes: drink to thirst, water only. For 60-120 minute efforts: drink to thirst, with electrolytes if high heat. For efforts over 2 hours: proactive hydration recommended (roughly 14-27 oz per hour depending on heat and sweat rate), with sodium. Runners following a structured half marathon training plan will encounter several long runs in this range where sodium replacement becomes genuinely important.
After your run. Rehydrate with roughly 150% of weight lost during the effort (2 lbs of body weight lost = roughly 1 quart of sweat). Include sodium to facilitate fluid retention. Not necessary after short runs — your normal daily hydration is enough.