A sports scientist's flexible framework for fluid and sodium intake during endurance exercise, by duration and sweat rate.
Sponsored content · In partnership with Precision Fuel & Hydration
You might think hydration should be simple: just put back what you lose through sweating. But full like-for-like replacement is not a good target, because several factors confound it: different athletes tolerate different levels of dehydration, sweat and sodium loss can outpace your gut's absorption rate, your pre-exercise hydration status varies, drinks are not always available, and your goal (getting through today's session versus setting up tomorrow's) changes the math. A hydration plan should be built on a few pieces of information and refined in the real world.
A quote from economist Tim Harford applies well here: "Show me a successful complex system, and I will show you a system that has evolved through trial and error." Experimenting is the best way to get hydration right. Precision's Personalised Hydration Plans are designed as starting points you optimize in training, not fixed prescriptions.
It is tempting to oversimplify a complex problem, either by replacing an arbitrary percentage of your estimated losses or by drinking a flat number of milliliters per hour. Others swing the other way and rely entirely on drinking to thirst. Both have merits, and they are not binary. You do not have to choose between drinking rigidly to a plan and drinking entirely to thirst.
Most successful athletes have a basic framework, honed through trial and error, that is flexible enough to adjust on the fly. This best-of-both-worlds approach is common among elite performers because they are more in tune with their bodies and can lean on instinct and experience.
2016 IRONMAN 70.3 World Champion Tim Reed put it this way when asked how he approaches hydration:
Drinking to thirst is likely to be a good approach in day to day life, or endurance training completed at a very low intensity. But, in my opinion, it's a terrible approach for those looking to maximize their performance in endurance events. My thirst response doesn't really kick in until I've already lost 1 liter of fluid or more. While that shouldn't drastically affect my performance, I still continue to lose far more than I replace after my thirst response has kicked in, and so inevitably have to slow my speed as my blood volume continues to decline. When drinking to thirst, I can lose 4 to 4.5kgs in a 2 hour run, leaving my heart rate very high and my performance very lethargic. During a race, my thirst response is even more subdued due to my fight or flight system running on overdrive. Perhaps there are athletes whose thirst response provides a more reliable guide, but in my experience with both coaching and racing, drinking to a schedule, particularly in the first half of events, leads to vastly better performance outcomes.Tim Reed, 2016 IRONMAN 70.3 World Champion
On the flip side, plenty of elite athletes rely more on instinct and drink largely to thirst during events. Less experienced athletes may benefit from leaning toward a pre-planned approach, but building in flexibility is always key.
Ideally, a hydration plan draws on four things:
Before you can dial in the numbers, it helps to gather data on your sweat rate and how salty your sweat is, either through an Advanced Sweat Test or by estimating your sodium losses. Duration then has a large bearing on your needs, so here is a framework by event length. The rates are guidelines to experiment within; if prior experience points somewhere different for you, factor that in.
For short endurance activities under about 90 minutes, the smartest move is to begin well hydrated; trying to play catch-up mid-event is not. (Precision covers this in a separate piece on how to start hydrated and why it matters.) Assuming you start properly hydrated, there is often little to gain from significant water or sodium during the activity itself.
That does not mean you must not drink. Listen to your instincts and drink if you feel thirsty, and keep fluids and electrolytes available. A few cases where drinking during short bouts can help:
Generally, for activities under around 90 minutes, hydration is mostly a concern for before and after the session.
This is where fluid intake starts to have a pronounced impact on performance, especially in the heat or for anyone with a high sweat rate or high sweat sodium loss. Start well hydrated, then use your sweat-rate and sodium estimates to guide intake.
| Sweat Rate | Fluid Per Hour |
|---|---|
| Low (under 1 L / 34 oz per hour) | 300 to 500 ml (10 to 16 oz) |
| Moderate (1 to 1.5 L per hour) | Around 750 ml (24 oz) |
| High (over 1.5 L per hour) | Up to 1 L (34 oz) |
Some athletes with very high sweat rates can absorb as much as 1.5 L (51 oz) per hour in very hot conditions, but that is not the norm; if you are drinking that much, consider whether you might be overdoing it.
As sodium losses become significant beyond a couple of hours, some supplementation is strongly recommended within your experimentation:
| Sweat Sodium | Sodium Per Liter |
|---|---|
| Low-salt sweater | 500 mg |
| Moderate | 1,000 mg |
| Salty sweater (or prone to cramp) | 1,500 mg |
The hourly fluid rate may not differ dramatically from medium and long events, but a few things shift. Because fending off fatigue is a key goal, drinking slightly more per hour early on can be advisable, most applicable to fitter, faster athletes who sustain a high work rate and sweat rate throughout. Less fit athletes often reach a point where their sweat rate drops, so their drinking rate can decrease too. That nuance is often missed in generic advice, and may partly explain why hyponatremia from over-drinking is more common in slower-moving athletes than in elites.
Sodium is arguably even more crucial in ultras, but count the sodium in your food and other supplements so you do not over-salt. A good place to start experimenting in training is at least 500 mg of sodium per liter for low-salt sweaters, 1,000 mg per liter for moderate, and up to 1,500 mg per liter for saltier sweaters. As with everything in ultra racing, there is no substitute for hard-won experience, so record what you actually consume and the performance outcomes.
It helps to know the signs that you are drinking too much or too little. Signs you are drinking too much:
Signs you may not be drinking enough:
The bottom line: an actionable hydration strategy needs guardrails so you do not vastly under or over do your fluid and electrolyte intake, plus enough intuition to refine it on the fly. The more experience you build, the more you can trust your instincts, and honing those instincts is part of the goal. Organized trial and error is the best way to reach a flexible plan that works across scenarios.
We have covered how much to drink here; for what to drink, see our guide to the different types of sports drink and when to use them, and for fueling energy rather than fluid, see the IRONMAN guide to [[race-day fueling|VERIFY-LIVE-URL: race-day-fueling]].
This is general performance guidance, not medical advice. Both under-drinking and over-drinking carry real risks: over-drinking can cause hyponatremia (dangerously low blood sodium), which can be serious. If you have a medical condition, take medication affecting fluid or sodium balance, or experience symptoms like confusion, severe headache, or nausea during or after exercise, seek medical advice.
This article is produced in partnership with Precision Fuel & Hydration, the Official Hydration of the IRONMAN Global Series. You can build a personalised starting point with their sweat-rate and sweat-test resources.
How Much Should You Drink Per Hour During Endurance Exercise?
It depends on your sweat rate. For sessions of 90 minutes to 4 hours, a low sweat rate (under 1 liter per hour) suggests starting around 300 to 500 ml (10 to 16 oz) per hour, a moderate rate (1 to 1.5 liters) around 750 ml (24 oz) per hour, and a high rate (over 1.5 liters) up to 1 liter (34 oz) per hour. These are starting points to experiment from, not fixed rules.
Do You Need To Drink During Exercise Lasting Less Than 90 Minutes?
Usually not much, if you start well hydrated. For most short sessions, hydration is mainly a before-and-after concern; drink to thirst during the session and keep fluids available. Exceptions include racing again soon after, very high-intensity elite events, or extreme heat and very high sweat losses.
How Much Sodium Should You Replace When Sweating?
For events over a couple of hours, start experimenting with roughly 500 mg of sodium per liter of fluid for low-salt sweaters, around 1,000 mg per liter for moderate, and about 1,500 mg per liter for salty sweaters or those prone to cramp. Factor in sodium from food and other supplements, especially in ultras, so you do not over-salt.
Should You Drink To Thirst Or To A Plan?
Neither extreme. Most successful athletes use a flexible framework built through trial and error, then adjust on the fly. Drinking to thirst suits day-to-day life and easy training, but thirst can lag well behind real fluid loss in hard events, so a planned starting point with room for intuition tends to work best.
What Are The Signs You're Drinking Too Much Or Too Little?
Too much: feeling bloated or full, fluid sloshing in your stomach, needing to pee frequently, or struggling to get drinks down. Too little: thirst or a dry mouth, light-headedness, lethargy, or a higher-than-normal heart rate for a given pace or power. Persistent over-drinking can cause hyponatremia, which is dangerous, so use these signs to stay in range.