Hydration Periodisation: Why Your Fluid Plan Should Change Every Phase of Marathon Training

The Problem With "Drink 500ml Per Hour"

You've probably read a hydration guide that told you to drink a set amount per hour during your runs. Maybe 400ml. Maybe 600ml. Maybe "drink to thirst."

Here's the issue: that number was calculated for one version of you. The version that ran a particular pace, in a particular temperature, at a particular fitness level, on a particular day.

But you're not the same runner in week 4 of base building as you are in week 14 of peak training. Your cardiovascular system has adapted. Your sweat response has changed. Your thermoregulation is more efficient — or it's been completely thrown off by a seasonal shift from spring to summer.

A single hydration number can't account for all of that. What you need is hydration periodisation: adjusting your fluid and electrolyte plan at each phase of training, just like you adjust your mileage and intensity.

What Changes Across a Training Cycle

Your body's fluid dynamics shift in ways that directly affect how much you need to drink.

Sweat rate increases with fitness. As your aerobic capacity improves, your body gets better at cooling itself — which means you sweat more, earlier. A runner who measured their sweat rate at 0.8 litres per hour in March might be losing 1.1 litres per hour by July, even at the same pace. The fitter you get, the more fluid you lose.

Thermoregulation adapts to heat. If you train through summer, your body acclimatises: blood plasma volume expands, you start sweating sooner, and your core temperature stabilises faster. These adaptations are real and measurable. They also mean your hydration needs in August look nothing like your needs in April.

Intensity changes fluid demand. An easy 10K recovery run and a 20-mile long run at marathon pace don't just differ in distance — they differ in how hard your cardiovascular system works, how much blood is diverted to working muscles (and away from your gut), and how much heat you generate. Your hydration plan needs to match the session, not just the distance.

Sodium losses vary. Sweat sodium concentration is partly genetic but also influenced by heat acclimatisation. As you acclimatise, your sweat becomes more dilute — you lose less sodium per litre of sweat. If you were loading up on salt tablets in June, you might need fewer by September.

Phase-by-Phase Hydration Adjustments

Here's how to think about hydration across the four major phases of a marathon training cycle.

Base Building (Weeks 1–6)

This is your calibration phase. The runs are shorter and easier, and the stakes are low — which makes it the perfect time to establish your baseline.

What to do: Measure your sweat rate at least twice during this phase. Weigh yourself before and after a 60-minute easy run: the difference in kilograms, plus any fluid consumed, equals your hourly sweat loss. Do this in the conditions you'll typically be training in.

Hydration target: Start with replacing about 70–80% of your measured sweat loss. For most runners in mild weather, that's somewhere between 400–700ml per hour. Don't try to replace 100% — mild dehydration (up to 2% body weight loss) is normal and not performance-limiting.

Electrolytes: Unless you're running over 90 minutes, plain water is typically sufficient during base building runs. Save the electrolyte tabs for when the intensity and duration increase.

Build Phase (Weeks 7–12)

Training load increases. Long runs get longer. Tempo sessions get harder. Your body starts adapting in ways that change your hydration needs.

What to do: Re-test your sweat rate now. If it's been two months since base, especially if the season has changed, your numbers will be different. Test once during an easy run and once during a tempo or long run — you'll see the difference intensity makes.

Hydration target: You'll likely need 10–20% more fluid per hour than your base phase numbers, because your sweat rate has increased and your sessions are longer and harder. For long runs over 90 minutes, start planning your fluid intake in advance rather than relying on thirst alone.

Research shows that thirst is a lagging indicator during prolonged exercise. By the time you feel thirsty, you may already be 1–2% dehydrated. That's not catastrophic, but in a three-hour run at marathon pace, it means your heart rate drifts upward, your perceived effort increases, and your pace slows — all of which affect the quality of the session.

Electrolytes: This is where electrolyte supplementation starts to matter. On runs over 90 minutes, add 300–500mg of sodium per litre of fluid. Your exact needs depend on your sweat sodium concentration, which varies between roughly 200mg/L and 2,000mg/L depending on genetics and acclimatisation.

Peak Phase (Weeks 13–16)

The highest-volume and highest-intensity weeks. Your longest long runs happen here. Your body is at its most adapted — but also most stressed.

What to do: Test your sweat rate one more time, ideally during your longest long run. This is your most race-relevant data point. If you can test in conditions similar to your race (same time of day, similar temperature), even better.

Hydration target: Use your peak-phase sweat rate as the foundation for your race-day plan. Replace 70–80% of measured loss. Calculate how much fluid you need per hour, then work backwards from the race's aid station spacing to figure out how much you'll drink at each stop.

A common mistake in peak phase is under-hydrating on long runs because carrying fluid is inconvenient. If you train with less fluid than you'll have access to on race day, you're training your gut to handle less intake — which creates problems when you try to drink more during the race.

Electrolytes: Lock in your electrolyte plan now. If you're using a specific drink mix, gel, or salt capsule on race day, practise with it during every long run in peak phase. Your gut needs to be trained to absorb both the fluid and the electrolytes under race-pace effort.

Taper (Final 2–3 Weeks)

Training volume drops dramatically, but your hydration strategy shouldn't drop with it.

What to do: This is the most commonly mismanaged phase for hydration. Runners reduce their mileage and assume they need less fluid overall. That's true for training runs — but your daily baseline hydration matters more than ever because you're building the fluid reserves you'll start the race with.

Hydration target: During training runs, your per-hour intake drops naturally because the runs are shorter and easier. That's fine. But your daily fluid intake — the water, tea, and food-sourced fluids you consume throughout the day — should stay consistent or even increase slightly. You want to arrive at the start line euhydrated (normally hydrated), not dehydrated from two weeks of reduced attention.

The 48-hour window: In the final 48 hours before your race, be deliberate about hydration. Sip consistently rather than guzzling. Include sodium in your fluids (a light electrolyte drink, or adding a pinch of salt) to help your body retain fluid rather than just flushing it through.

Don't overdo it. Drinking excessive fluid in the days before a race doesn't "pre-hydrate" you — it just makes you pee more and can dilute your sodium levels. The goal is steady, moderate intake, not a water-loading blitz.

How to Know Your Plan Is Working

Your wearable data gives you real signals to track across the training cycle.

Resting heart rate trends. A rising resting heart rate — especially if it creeps up over several days — can signal chronic dehydration. If you see your resting HR climb 5–8 bpm above your baseline during a training phase, check your fluid intake before assuming it's overtraining.

Heart rate drift during long runs. If your heart rate climbs significantly (more than 10–15 bpm) at the same pace over the course of a long run, dehydration is a likely contributor. Compare drift across long runs through the training cycle — if it's getting worse as the weeks go on, your hydration plan isn't keeping up with your sweat rate.

Post-run body weight. Weigh yourself before and after key sessions. If you're consistently losing more than 2–3% of body weight despite following your hydration plan, your intake targets need to increase.

Morning body weight stability. A steady morning body weight (measured before eating, after using the bathroom) across a training week suggests good daily hydration. A downward trend over several days, especially during a high-volume week, can mean you're not replenishing between sessions.

Why One Calculation at the Start of Training Isn't Enough

A sweat rate test in April gives you April's number. But you'll race in October — after months of adaptation, seasonal change, and fitness gains.

The athletes who get hydration right for race day are the ones who treated it like any other variable in their training plan. They measured. They adjusted. They tested again.

That's a lot of manual work: weigh-ins, fluid logging, cross-referencing weather data with heart rate and pace. It's the kind of thing most runners know they should do and almost nobody actually does consistently.

Sweatr does this automatically. It pulls your training data from your Apple Watch, Garmin, or Strava, tracks how your metrics shift across your training cycle, and recalibrates your hydration targets as your body adapts. When your sweat rate changes because you got fitter, or the season shifted, or your training intensity increased, your plan updates with it.

You don't need to remember to re-test. You don't need a spreadsheet. You just need to keep training with your watch on, and Sweatr does the math.

The Takeaway

Hydration isn't a number you set once. It's a moving target that shifts with every phase of your training. Your body in week 4 is not your body in week 14, and a plan that treats them the same will leave you either under-hydrated when it matters most or over-drinking and risking hyponatremia.

Periodise your hydration the way you periodise your mileage. Measure your sweat rate at least three times across a training cycle. Adjust your electrolytes as you acclimatise. Pay attention to your taper-phase daily intake. And use your wearable data — resting heart rate, heart rate drift, body weight trends — to catch problems before race day.

Your watch has been collecting this data all along. The question is whether you're using it.