Hydration & Electrolyte Timing for Triathletes: Numbers That Hold Across the Bike and Run

Here is what you can actually measure and feel, in order. Within one well-fuelled long ride you will notice your legs hold power deeper into the back half when fluid and sodium are on track. Across a hot brick, you will feel the run come together instead of falling apart in the first kilometre off the bike. And over a season of weighing in around sessions, you will watch a clear pattern emerge: your sweat rate is a number, it differs between disciplines, and once you know it the guesswork disappears.

Triathlon punishes a vague hydration plan more than any single sport. You train nine to thirteen sessions a week, you race for one to many hours, and your fluid logistics differ on every leg, no aid in the water, bottles on the bike, cups on the run. A plan that ignores those differences either leaves you dehydrated by the marathon or, worse, dangerously overhydrated by drinking 'to be safe' at every station.

This is the data-led version: the timeline of what to expect, the sodium-and-carb numbers per hour, the science underneath, and the specific scenarios, bricks, doubles, and race day, where it all gets tested.

1. What You Can Measure: Sweat Rate by Discipline

Start with the single most useful number in your sport: your sweat rate, measured separately for the bike and the run. Weigh in nude before a roughly one-hour session, weigh out straight after, and account for everything you drank. Each kilogram lost is about a litre of sweat. Add back your fluid intake and you have a per-hour rate for that discipline and temperature.

Expect the two numbers to differ. Cycling moves air over your skin and offloads heat efficiently, so losses often sit in the 0.5-1.5 L/hr range. Running generates more heat for the same effort and traps more of it, so run sweat rates commonly run higher, and in heat or humidity either can climb past 2 L/hr. Heavy and salty sweaters, the ones leaving white crust on the kit, sit higher still.

The point of the number is not a fixed prescription, it is calibration. Once you know you lose, say, 1.2 L/hr on the bike and 1.6 L/hr running in summer, your bottle math and aid-station plan write themselves, and you stop drinking on superstition.

2. The Per-Hour Protocol Across Long Course

Translate the sweat number into a plan that bends to discipline and duration. On the bike you control intake, so drink to a tested schedule. On the run you drink to thirst, because forcing fluid you cannot absorb at run-jostle is how stomachs and blood sodium both go wrong. The overarching rule across both: keep total body-mass loss within roughly 2-3% of your starting weight, and never aim to finish heavier than you started.

Test every line of this in training, never on race morning. The bike is where you load most fluid and carbohydrate because you can; the run is where you live off what you banked and sip to thirst. Nothing in this table is allowed to make its debut on a start line.

3. The Science: Why Sodium Carries the Long Effort

Sodium does three jobs that matter once an effort passes the 60-90 minute mark. It stimulates the thirst that keeps you drinking adequately, it helps your small intestine absorb both water and glucose, and it helps your body retain the fluid you swallow instead of urinating it away. That is why sodium, not potassium or magnesium, is the electrolyte that earns its place in a long-course bottle.

Carbohydrate rides alongside it for a separate reason: fuel. The 30-60 g/hr you target, climbing toward 90 g/hr with mixed glucose-and-fructose sources in very long events, is about sustaining pace, not about the electrolytes boosting performance directly. Sodium and carbohydrate are partners with different jobs in the same bottle.

Potassium and magnesium are lost in sweat too, but in small amounts your normal diet covers easily, and the popular magnesium-for-cramps story is weakly supported, cramps are multifactorial, not a simple deficiency. For short or easy sessions, none of this applies; plain water and meals are enough, and the sugar and sodium of a sports drink are calories you do not need.

4. The Ironman Hyponatremia Scenario

The longer the race, the slower the back of the field, and the more time there is to overdrink, which is exactly how exercise-associated hyponatremia happens. It is not a dehydration problem; it is the opposite, too much plain fluid diluting blood sodium, and in long-course racing it is the more dangerous mistake of the two because the instinct to 'drink at every aid station to be safe' drives it.

The symptoms are a trap. Nausea, headache, and confusion appear in dehydration and overhydration alike, so the field cannot rely on how you feel. The one reliable clue is your weight trend. Finishing or mid-race heavier than your start, with puffy hands, a swollen face, and a sloshy gut, points to overhydration, and the answer is to stop drinking, not to add water.

Prevention is built into the plan above. Drink to thirst on the run, never aim to gain weight on course, and keep sodium flowing through the long hours so the fluid you take is held in the right place. If you ever feel that swollen, sloshy, foggy combination late in a race, slow down and get medical eyes on it rather than reaching for another bottle.

5. Bricks, Doubles, and Honest Product Choices

Your training week is where the per-hour numbers either become automatic or never get rehearsed. Treat brick sessions as race-fuel dress rehearsals: drink and fuel the bike portion to plan, then practise running off it on the sodium and carbs you actually carried. Doubles need their own bookkeeping, the morning session's losses follow you into the evening, so rehydrate with about 125-150% of what you dropped, salted, between them rather than starting the second session already down.

On products, be honest with your wallet. Across nine to thirteen weekly sessions, plenty of them are short or easy enough that water and food cover them completely; a branded electrolyte powder there is buying flavour and convenience, not physiology. The genuine value is concentrated in the long, hot, and heavy-sweat work and in rapid post-session rehydration, where the sodium does real work holding fluid.

Two cautions specific to your volume. Heat illness and hyponatremia are the long-course risks to respect, and chronic low energy availability across huge training hours is a real and separate problem, hydration plans must not double as calorie restriction. Fuel and fluid are infrastructure, not extras.

Race-Plan Hydration Questions

How do I take hydration across doubles and brick days?

Treat them as connected, not separate. After a morning session, replace about 125-150% of the weight you lost, with salty food or an electrolyte mix, so you start the second session topped up rather than already in deficit. On brick days, fuel and drink the bike leg to your tested plan, then run off what you carried so race-day fuelling is rehearsed under real fatigue, never improvised.

What's the race-week and Ironman-day fluid protocol?

Arrive euhydrated; in the 2-4 hours before, take 5-10 ml/kg, roughly 350-700 ml for a 70 kg athlete, aiming for pale urine. On the bike, drink to your tested schedule with sodium and 30-60 g/hr of carbs, up to 90 g/hr in very long events. On the run, switch to drinking by thirst. Cap total body-mass loss near 2-3% and never aim to gain weight.

Which electrolyte actually matters across the three disciplines?

Sodium, in every leg. It drives thirst, helps your gut absorb water and glucose, and helps you retain fluid rather than pee it out, which is why it dominates sweat replacement. Potassium and magnesium are lost too but in small amounts a normal diet covers. Heavy and salty sweaters, white crust on the kit, need more sodium per hour; test your own saltiness in training, not on race day.

Will overhydrating during an Ironman actually hurt me?

Yes, and it is the more dangerous error in long course. Drinking plain fluid beyond your sweat losses dilutes blood sodium into exercise-associated hyponatremia, which can become life-threatening. The giveaway is gaining weight on course with puffy hands and a sloshy stomach. More water makes it worse, not better. Drink to thirst on the run, keep sodium flowing, and never aim to finish heavier than you started.

Disclaimer: This article is for educational purposes only and is not medical advice. Consult a qualified healthcare professional before starting any supplement, nutrition, or training protocol — especially if you are pregnant or breastfeeding, under 18, taking medication, or managing a health condition.