Dehydrated or Overhydrated? How to Read the Signs Mid-Race

The Problem Nobody Talks About

Everyone knows dehydration is bad. Drink more water. Take your electrolytes. Don't skip the aid stations.

But here's the thing most hydration advice leaves out: drinking too much is just as dangerous as drinking too little. It's called hyponatremia — a condition where your blood sodium drops to dangerous levels because you've diluted it with excess fluid. It sends runners to the hospital every single race season.

The tricky part? Early symptoms of dehydration and overhydration look almost identical. Nausea. Headache. Confusion. Fatigue. If you're 15 miles into a marathon and feel awful, you need to know which problem you're dealing with — because the fix for one makes the other worse.

What Dehydration Actually Looks Like

Dehydration happens when you lose more fluid through sweat than you take in. Your blood volume drops, your heart works harder to push less blood to your muscles, and performance tanks.

Here's how it tends to show up during a race, roughly in order of severity:

Early signs (1–3% body weight loss):

• Thirst (obvious, but it's a lagging indicator — you're already behind)
• Dry mouth and sticky saliva
• Darker urine at aid stations
• Heart rate creeping higher than expected for your pace
• A subtle feeling that your effort level doesn't match your speed

Moderate signs (3–5% body weight loss):

• Headache that builds over miles
• Muscle cramping, especially in calves and quads
• Dizziness when you look up or turn your head quickly
• Noticeable drop in pace despite the same perceived effort
• Reduced sweat output — your body is conserving fluid

Severe signs (5%+ body weight loss):

• Confusion or difficulty focusing
• Nausea and possible vomiting
• Chills or goosebumps despite hot conditions
• Rapid, shallow breathing
• At this point, stop running and seek medical help

The key pattern: dehydration builds gradually. It rarely hits you all at once. If you started your race well-hydrated and you've been drinking nothing, you'll feel it escalate over 60–90 minutes.

What Overhydration Looks Like

Overhydration — clinically called exercise-associated hyponatremia (EAH) — happens when you drink more than your kidneys can process during exercise, diluting the sodium in your blood. It's most common in slower runners, in cooler conditions, and in athletes who follow the outdated advice to "drink as much as possible."

Early signs:

• Bloating or a sloshing feeling in your stomach
• Puffiness in your hands, fingers, or face
• Clear, almost water-like urine
• Weight gain during the race (you've added fluid, not lost it)
• Mild nausea that feels like stomach distress from gels

Moderate signs:

• Worsening nausea without an obvious cause
• Headache
• Confusion or disorientation
• Fatigue that feels disproportionate to your effort

Severe signs:

• Vomiting
• Seizures
• Loss of consciousness
• This is a medical emergency — call for help immediately

The key pattern: overhydration symptoms can appear suddenly. An athlete may feel fine for hours and then deteriorate quickly, because the sodium drop crosses a physiological threshold.

How to Tell the Difference Mid-Race

This is the question that matters. You feel terrible at mile 18. Your head is pounding. You're nauseous. Do you drink more or drink less?

Here are the practical signals you can use during a race:

Check your hands and face. Puffy, swollen fingers or a bloated face suggest overhydration. Sunken eyes, dry lips, and tight-feeling skin suggest dehydration.

Think about how much you've been drinking. Have you grabbed a cup at every single aid station and drained it? Have you been sipping from a handheld bottle non-stop? You may be overhydrated. Have you skipped multiple stations or only taken small sips? Dehydration is more likely.

Check your urine colour (if you can). Dark yellow to amber means dehydration. Completely clear and copious means you may be over-diluted.

Consider the conditions. Hot, humid race with heavy sweating and minimal drinking? Dehydration. Cool, overcast race where you drank on a schedule regardless of sweat output? Overhydration risk is higher.

Consider your pace. Slower finishers (4:30+ marathons) are at higher risk for overhydration because they're on the course longer and may drink more total fluid relative to their sweat output. Faster, heavier sweaters in hot conditions skew toward dehydration.

The stomach test. A stomach that's sloshing, heavy, and bloated points to overhydration. A stomach that feels hollow, empty, or crampy points to dehydration.

What to Do When You Suspect Dehydration

If the signs point to dehydration, the fix is straightforward — but don't panic and chug a litre of water at the next aid station. That can trigger nausea or push you toward overhydration.

Immediate actions:

• Take small, frequent sips at every aid station (100–200ml per stop)
• Switch to an electrolyte drink if available — water alone won't replace the sodium you've lost
• If you have salt tabs or electrolyte capsules, take one with a small amount of fluid
• Slow your pace by 15–30 seconds per mile to reduce sweat output and cardiac load
• Pour water over your head and neck to cool down externally (this doesn't count as hydration, but it reduces heat stress)

What not to do:

• Don't try to "catch up" by drinking large volumes at once — your gut can only absorb about 200–250ml every 15 minutes during exercise
• Don't ignore escalating symptoms — if you're confused or can't maintain a straight line, walk to the nearest medical tent

What to Do When You Suspect Overhydration

This one is counterintuitive for most runners, but the answer is simple: stop drinking.

Immediate actions:

• Stop all fluid intake until symptoms ease
• If you have salt tabs or salty snacks (pretzels at aid stations), take them — sodium is what your blood needs
• Slow your pace or walk
• If symptoms are moderate or severe, seek medical attention immediately — do not try to "run it off"

What not to do:

• Do not drink more water — this will make hyponatremia worse
• Do not assume your nausea is from gels if you've been drinking heavily — it may be your sodium levels

The Real Fix: Stop Guessing Before the Race Starts

Reading symptoms mid-race is damage control. The better question is: why were you guessing in the first place?

The reason most runners end up dehydrated or overhydrated is the same: they followed a generic plan that wasn't built for their body.

"Drink 400–800ml per hour" is the standard advice. But that range is enormous. A 55kg runner in cool conditions might need 400ml. A 90kg runner in 30°C heat might need 1,200ml. Telling both of them to drink "400–800ml" is the kind of advice that sends one to the medical tent for dehydration and the other for hyponatremia.

What actually determines how much you should drink:

• Your personal sweat rate — how many millilitres of fluid you lose per hour of running at race pace. This varies enormously between individuals and changes with temperature, humidity, and fitness.
• Your sweat sodium concentration — some runners lose three times more sodium per litre of sweat than others. This is partly genetic and doesn't change much with training.
• The conditions on race day — temperature and humidity directly affect sweat output. A plan built for a 15°C spring marathon will under-hydrate you on a 28°C summer race.
• Your pace and body weight — slower runners need less fluid per hour but are on the course longer. Heavier runners tend to sweat more. The math changes for every individual.

You can calculate your sweat rate manually. Weigh yourself before and after a one-hour run (no bathroom breaks, note exactly how much you drank). The weight difference plus fluid consumed gives you your hourly sweat loss. Repeat in different conditions to build a profile.

Or you can let your wearable data do the work. Your Apple Watch and Garmin already track heart rate, workout intensity, pace, and environmental conditions. Sweatr pulls this data together and calculates your personal fluid and electrolyte needs for every run — adjusted for temperature, duration, and intensity. No spreadsheets. No guessing. Just a plan that matches your body.

A Pre-Race Hydration Checklist

Whether you build your plan manually or use an app, here's what a good race-day hydration plan looks like:

1. Know your sweat rate for the expected conditions (temperature, humidity, intensity). Not a range — your actual number.
2. Set a drinking schedule based on your sweat rate, not based on aid station availability. If you need 700ml/hr and aid stations are every 3km, you know how much to take at each one.
3. Include electrolytes — especially sodium. If you're a salty sweater (white residue on your kit after runs), you need more.
4. Practise your exact plan on at least three long training runs before race day. Your gut needs to rehearse, too.
5. Adjust for conditions. If race morning is hotter than expected, increase fluid intake by 10–20%. If it's cooler, scale back.
6. Set upper and lower limits. Never drink more than ~800ml/hr (above this, your kidneys can't keep up). Never drink less than ~400ml/hr in hot conditions.

The Bottom Line

Dehydration and overhydration are both preventable. But you can only prevent what you can predict — and prediction requires data, not guesswork.

If you're training for a race this summer or fall, don't wait until mile 18 to find out your plan was wrong. Build a hydration strategy based on your actual sweat rate, your actual electrolyte losses, and the actual conditions you'll race in.

Sweatr calculates your personalised hydration plan using your Apple Watch and Garmin data — adjusted for temperature, pace, and duration. No generic ranges. No guesswork. Just your numbers.

[Download Sweatr free and build your race-day hydration plan →]