The Race-Week Hydration Protocol Most Runners Skip

You have spent months training. Your long runs are done. Taper is underway. And right now, your browser history probably looks like some variation of "carb loading plan marathon 3 days."

Fair enough. Glycogen matters. But here is the thing almost nobody talks about: your hydration status when you cross the start line matters just as much as your glycogen stores, and it takes days to optimise, not hours.

This is the race-week hydration protocol most runners skip entirely.

Why race-week hydration is not the same as "drink more water"

When people think about pre-race hydration, they picture chugging water the night before. That is not a protocol. That is a recipe for a 3am bathroom trip and diluted electrolytes.

Genuine race-week hydration is about optimising two things over several days:

Plasma volume — the liquid portion of your blood. Higher plasma volume means your heart pumps more blood per beat, delivers more oxygen to working muscles, and dissipates heat more efficiently. Studies on endurance athletes show that plasma volume expansion from deliberate hydration strategies can improve cardiovascular efficiency during prolonged exercise.

Electrolyte balance — specifically sodium. Your body does not just need fluid; it needs fluid with the right concentration of sodium to hold onto it. Drink plain water without adequate sodium and your kidneys excrete much of it within hours. Drink water with sodium and your body retains more of it where it counts.

This is why the "8 glasses a day" advice is useless for race week. You need a structured approach that accounts for your body weight, your training-week sweat losses, and your individual sodium needs.

The 5-day race-week hydration protocol

Here is a practical, day-by-day framework. Adjust quantities based on your body weight and sweat rate. If you weigh more or sweat heavily, trend toward the higher end. If you are a lighter runner or a light sweater, trend lower.

Days 5 and 4 before the race (Monday-Tuesday for a Saturday race)

Goal: Establish a hydration baseline. Top off fluid reserves without overdoing it.

• Drink consistently throughout the day — roughly 35-40ml per kg of body weight. For a 70kg runner, that is about 2.5-2.8 litres spread across the day.
• Include electrolytes with at least two of your drinks. A tab or sachet with 300-500mg sodium per serve is a good starting point.
• Cut or reduce alcohol. Even one or two drinks can impair fluid retention for 12-24 hours.
• Monitor urine colour as a rough guide. Aim for pale straw, not clear. Clear urine often signals you are drinking more than you are retaining.

Days 3 and 2 before the race (Wednesday-Thursday)

Goal: Begin sodium pre-loading alongside your carb load.

This is where most runners diverge. They start eating more carbohydrates (good), but they do not match the increased fluid and sodium needs that come with glycogen storage.

Here is the science: every gram of glycogen stored in your muscles binds roughly 3 grams of water. If you are storing an extra 300-500 grams of glycogen during your carb load, that is an extra 900-1,500ml of water pulled into your muscles. If you do not increase your fluid and sodium intake to compensate, the water for glycogen storage comes from your plasma volume — the exact thing you were trying to protect.

• Increase fluid intake slightly — aim for 40-45ml per kg of body weight per day.
• Add an extra electrolyte serving, bringing your total sodium intake from supplements to around 1,000-1,500mg per day (on top of what you get from food).
• Salty foods count. Pretzels, salted nuts, miso soup, and broth all contribute. This is the one week where adding salt to your meals is a strategy, not a habit to feel guilty about.
• Keep sipping throughout the day. Front-loading a litre in the morning and then forgetting until dinner does not work.

The day before the race (Friday)

Goal: Fine-tune without overdoing it. Protect your sleep.

• Maintain the same fluid intake as Days 3-2, roughly 40-45ml per kg.
• Have your last large electrolyte drink with dinner or by early evening, roughly 500ml with 400-500mg sodium.
• Stop drinking large volumes 2-3 hours before bed. Small sips are fine. The goal is to avoid the 2am wake-up that fragments your sleep before the most important day of your training cycle.
• Avoid new foods or supplements. Race week is not the time to try a different electrolyte brand. Use what you have trained with.

Race morning

Goal: Top off without sloshing.

• Drink 400-600ml of fluid with electrolytes 2-3 hours before the gun. This gives your kidneys time to process the excess and lets you use the bathroom before you line up.
• Sip another 200-300ml in the 30 minutes before the start, ideally with 200-300mg sodium. This is your final pre-load.
• Do not chug a litre on the start line. Your stomach has limited capacity, and starting a marathon with fluid sloshing around your gut is a fast track to nausea in the first few miles.

The sodium pre-loading detail most runners miss

Sodium loading is not a new concept in sports science, but it has not filtered down to recreational marathon runners the way carb loading has. Research on endurance athletes shows that consuming a sodium-rich solution before exercise can increase plasma volume and delay the onset of dehydration compared to plain water alone.

The practical version: in the 24 hours before your race, aim for a total sodium intake of around 50-60mg per kg of body weight from all sources (food, electrolyte drinks, and supplements combined). For a 70kg runner, that is roughly 3,500-4,200mg of total sodium across the day.

That sounds like a lot, but consider that a typical Western diet already contains 2,000-3,000mg of sodium per day from food alone. You are adding an extra 1,000-1,500mg through deliberate electrolyte drinks and salty snacks. It is achievable without force-feeding salt tabs.

One important caveat: if you have high blood pressure or a medical condition affected by sodium intake, consult your doctor before increasing salt consumption. For most healthy runners, short-term sodium pre-loading is safe and effective.

Why this matters more than you think

Here is the uncomfortable truth about marathon fueling: you can nail your carb loading, have the perfect gel timing plan, and still have a terrible race if you start dehydrated.

A 2% loss in body mass from dehydration — which can happen within the first 60-90 minutes of a marathon, especially in warm conditions — increases heart rate, reduces blood flow to working muscles, and impairs your body's ability to regulate temperature. Your perceived effort goes up. Your pace goes down. And no amount of grabbing cups at aid stations will fully undo a deficit that started before the gun went off.

The runners who run their best marathons do not leave hydration to chance. They treat race-week hydration with the same intentionality as their carb load, their taper, and their race-day pacing.

How your training data changes the equation

Everything in this article gives you a solid framework. But the truth is, the exact numbers — how much fluid, how much sodium, how aggressively to pre-load — depend on factors unique to you.

Your sweat rate. Your sweat sodium concentration. Your body weight. The weather forecast for race day. Whether you are a heavy sweater who loses 1.5 litres per hour or a light sweater who barely hits 0.7.

This is where generic advice breaks down and personalised planning takes over. And this is exactly what Sweatr is designed to do.

Sweatr pulls your training data from your Apple Watch, Garmin, or Strava, estimates your sweat rate from your workout history and environmental conditions, and builds a complete race-week hydration protocol tailored to your body. Not generic ranges. Your numbers. Down to the millilitres and milligrams.

If you have spent months training for this race, your hydration plan deserves more than a Google search the night before. Try Sweatr free and see what a personalised race-week protocol looks like for your body, your race, and your data.

The bottom line

Carb loading is important. But it is only half of the pre-race equation. A structured, 5-day hydration protocol with deliberate sodium pre-loading ensures you arrive at the start line with optimised plasma volume, balanced electrolytes, and the physiological foundation to execute the fueling plan you have spent months rehearsing.

Your race is not won at mile 20. It is won — or lost — in the five days before it starts.