How much should you be drinking when you're sweating?

By Andy Blow | 9 Minute Read

Once you’ve gathered some data on your sweat rate and how salty your sweat is (either through taking an Advanced Sweat Test at one of our Sweat Test Centers or by estimating your sodium losses), you’re ready to start formulating/refining an actionable hydration strategy to use out in the real world.

When it comes to creating a successful hydration plan, a quote from economist Tim Harford comes to mind…

 

"Show me a successful complex system, and I will show you a system that has evolved through trial and error."

 

Experimenting using a process of trial and error is definitely the best way to get hydration right and our Personalised Hydration Plans are very much designed to be starting points from which you optimise in training.

You might be thinking that hydration doesn’t sound all that complex. It ought to be simple, right? You should simply aim to put back in what you’ve lost through sweating.

But, 100% ‘like for like’ replacement is not something we’d encourage aiming for because there are several confounding factors, including…

  • The fact that different athletes can tolerate different levels of dehydration
  • That the rate of sweat and sodium loss can outpace your gut’s absorption rate
  • Your pre-exercise hydration status
  • The availability of drinks before, during and after exercise.
  • And whether you’re only hydrating to get through the current session or you’re aiming to set yourself up for a good performance in another bout of exercise later that day or the next day.

A natural human reaction to complex problems is to oversimplify them. When it comes to the fairly complex problem of hydration, it’s tempting to simplify things by aiming to replace an arbitrary percentage of your estimated losses, or to simply drink ‘X’ millilitres (or fluid ounces) per hour.

Some athletes decide to go entirely the other way and choose to ignore the idea of planning altogether, instead relying entirely on ‘drinking to thirst’.

 

Road sign 

Both of these approaches have merits in certain situations and they shouldn’t be treated as binary options. You don’t have to choose between ‘drinking rigidly to a plan’ or ‘drinking entirely to thirst’.

Most successful athletes have a basic framework for their hydration strategy which they’ve honed through trial and error in training and competition but - and this is crucial - this framework is flexible enough to allow them to make adjustments on the fly.

This ‘best of both worlds’ approach is very common amongst elite performers because they tend to be more in tune with their bodies and can rely more on their instincts and experience.

2016 Ironman 70.3 World Champ Tim Reed said this when I asked him how he approached 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 4-4.5kgs 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.”

On the flip side, we work with plenty of elite athletes who rely far more on their instincts and drink largely to thirst during events. Less experienced athletes may benefit from leaning a bit more towards a pre-planned approach, but again, building in some flexibility is key.

 

Flexibility

 

So, what should a framework hydration strategy be based on?

Well, ideally, a hydration plan should be based on a few pieces of information…

  1. Past experience of what has worked and not worked
  2. The athlete’s individual sweat rate and sweat sodium losses
  3. The likely access to drinks and supplements during the session or event
  4. The duration of the exercise

Duration is a logical element to expand on because it has a large bearing on your likely hydration needs. Let’s go through a few scenarios for events of different lengths...

The following framework is based on a combination of scientific evidence (albeit slightly conflicting evidence, but that'd often the case for complex topics!) and our experience working with a lot of athletes in real world scenarios.

The rates of fluid consumption recommended are very much meant as guidelines within which to experiment but, if prior experience indicates that something wildly different may be more appropriate for you or your athlete, then it’s important to factor that in to your experimentation.

 

Track meet

 

'Short' activities lasting < 90 mins 

So, for short endurance activities lasting less than about 90 minutes, ensuring that athletes begin events well hydrated is a smart place to start. Trying to play catch up once an event is underway is not so smart.

This is so important that we’ve covered it in a separate blog titled How to START hydrated and why that's so important (when the word important is in the title, you know it must be, well, important).

Assuming that you start a short activity properly hydrated, there's often little to be gained from taking in significant amounts of water or sodium during the activity itself.

This doesn’t mean that you categorically should not drink during short sessions and events, you should absolutely still listen to your instincts and drink if you feel thirsty. Try to ensure fluids/electrolytes are readily available so that you can respond to the dictates of thirst.

There are a few particular instances where it might be beneficial to drink during shorter bouts of exercise...

  1. If you'll be training or competing again soon after the initial activity ends (e.g. a soccer or tennis tournament, or during an intense training camp)
  2. If you're an elite athlete competing in very high intensity aerobic events, as there’s evidence that ingesting small amounts of a carb based drink - or even just rinsing it the mouth - can improve performance
  3. If you're exercising in extreme heat and/or at an intensity that elicits very large sweat losses

But, generally speaking, when it comes to activities lasting less than around 90 minutes, the bottom line is that hydration should largely be a concern for before and after the session.

 

Olympic triathlon

 

Medium/Long activities lasting 90 mins to 4 hours

This is where fluid intake can start to have a much more pronounced impact on performance - especially in very hot conditions or for anyone with a high sweat rate and/or a high level of sweat sodium loss.

As with shorter duration activities, it’s smart to make sure athletes start whatever they’re doing well hydrated.

Estimates of sweat rates (see here for formulas and a spreadsheet for capturing some sweat rate data) and levels of sweat sodium loss start to become useful when you’re figuring out what to drink during medium to long duration events.

For those with low sweat rates of less than around 1 litre (34oz) per hour, an intake in the region of around 300-500ml (10-16oz) of fluid per hour would be a good ball-park figure to start experimenting with.

For athletes whose sweat rates are more moderate at around 1 to 1.5 litres per hour, an intake of around 750ml (24oz) of fluid per hour would be the kind of region in which to start your trial and error.

For athletes whose sweat rates are high or very high at over 1.5 litres per hour, as much as 1 litre (34oz) of fluid might be needed per hour to maintain performance, especially when you’re engaging in activities lasting 3 to 4 hours.

Whilst some athletes who do have very high sweat rates seem to be able to absorb as much as 1.5 litres (51oz) of fluid per hour in very hot conditions, this isn't the norm, so if you're drinking that much it’s worth considering whether you might be overdoing it.

 

Replacing the sodium you're losing in your sweat

As sodium losses start to become a significant factor in events lasting more than a couple of hours, some level of supplementation is also strongly recommended within the experimentation process.

For those losing low amounts of sodium in their sweat, a ratio of around 500 milligrams of sodium per litre (34oz) of water consumed is a good starting point for testing purposes.

For ‘salty sweaters’, and often for anyone experiencing regular muscle cramps in longer or hotter sessions, 1,500 milligrams of sodium per litre (34oz) of water is likely to be in the ballpark of what’s required to maintain performance.

 

Ultra marathon

 

Ultra duration activities lasting more than 4 hours

The rate of fluid intake may not be dramatically different to those you would consume during medium to long events to be honest.

But, there are a few things that can influence your hydration needs somewhat during ultra distance exercise...

Firstly, because fending off fatigue is a key goal, drinking slightly more per hour early on in the activity may be advisable. This is most applicable to fitter and faster athletes, who are able to keep up a very high work rate for the entire duration of the race and so have a high sweat rate throughout.

Less fit athletes will often reach a point where their sweat rate drops and therefore the rate of drinking can also decrease. This is often not considered in generic recommendations for fluid intake during ultra distance events, and may partly explain why hyponatremia from over-drinking tends to be more prevalent in slower-moving athletes, rather than elites.

 

Replacing the sodium you're losing in your sweat

Sodium intake is also arguably more crucial in ultras than during events of a shorter duration, but factoring in sodium consumed in foods and other non-hydration supplements is important here so as not to 'over-salt'.

Playing around with a sodium intake of at least 500 milligrams of sodium per litre (34oz) of water for low salt sweaters, 1,000 milligrams per litre (34oz) for moderate sweaters and rising to 1,500 milligrams per litre (34oz) for saltier sweaters is where we'd generally recommend starting your experimentation in training.

Like many other aspects of ultra distance racing, hydration is an area where there’s no real substitute for relevant, hard won experience gained through lots of trial and error (both on the coach’s side and the athlete’s side), so being organised in recording what you actually do consume in events and very long training sessions, along with the performance outcomes, is very important if progress is to be made over time.

 

So, how do you adjust a hydration plan on the fly?

Whilst some athletes seem to be more naturally self-aware and attuned to the feedback that their bodies give them during exercise, it can be useful to think about the key signs and symptoms that might indicate that you're either drinking too much or too little.

The acute signs that you're is drinking too much include...

  • Feeling feel bloated and full
  • Sensing fluid sloshing around in your stomach
  • Needing to pee frequently
  • Struggling to get drinks down

On the other end of the scale, signs that you may not be drinking enough include…

  • Feeling thirsty or having a dry mouth
  • Feeling light headed
  • Feeling lethargic
  • Experiencing a higher than normal heart rate for a given power output or pace

The bottom line with planning an actionable hydration strategy for any sort of endurance activity is that it needs to include some guardrails to prevent you from vastly under or over doing your fluid intake and, where relevant, your electrolyte intake.

At the same time, the strategy needs to encourage the use of a healthy element of intuition and instinct to help refine your plan on the fly and get intake into the most appropriate zone in any scenario.

Generally, the more experience you build up, the more you can rely on instinct and intuition to guide your fluid intake. Part of the goal of the development of a robust hydration plan should be to hone your instincts so you can become more reliant on them over time.

Organised trial and error is the best way to iterate your way to a flexible hydration plan that works for you in different scenarios.

In this blog we’ve covered how much to drink, but you might have noticed we’ve largely ignored the subject of what to drink. But, don’t worry, we’ve covered this in our blog on the different types of sports drinks and when to use them.

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