Hydration hydration hydration: everything you need to know about hydration in endurance running

I was given a last minute place at Hackney Half a couple of weeks ago which meant (full disclosure here) that I ran under someone else’s name, thereby committing one of running’s seven deadly sins. I haven’t yet come up with a list of the other six, but perhaps that’s a blog for another day…

I did it as a progressive tempo training run, aiming to finish in something like half my target marathon time for London in October. Naturally, I got slightly overexcited towards the end (big crowds, banging tunes etc.) and sped up a bit, finishing about a minute ahead of schedule in 1h17.

It was the first time I’d run Hackney, and while this blog isn’t intended as a race review, the atmosphere was great, and I’ll run it again one day (under my own name) – just not as a PB race. For one, the course is quite winding and not particularly flat. But most importantly, being at the end of the spring race season in late May, it always seems to be roasting hot in East London.

I watched my girlfriend (now wife) run in it in heatwave conditions back in May 2016, and it really wasn’t much cooler this year. I have been lucky that most of my target half and full marathons over the years have been on the cool side, and in fact I can say with certainty that these were the hottest conditions I’ve done anything more than a 10k race in.

On the day of the race, I posted an Instagram story about how I thought the Hackney emergency services would be in for a busy day, and sadly this was very much the case. I received response after response to that story confirming they’d seen people literally dropping like flies around the course.

In the (enormous) race village, all the pre-race chat coming over the loudspeaker had been about ‘hydration, hydration, hydration’, but besides ‘drinking a lot’, what does this really mean, and why is it so important in hot conditions? What does the evidence say about hydration in endurance sport, and how can we apply that to ourselves? It’s undoubtedly a tricky area and one that so many runners get wrong, especially when race conditions aren’t kind.

First then, why hydrate? i.e. what happens if we don’t?

Good hydration is essential, both for health and exercise performance. A quick GCSE biology recap: when we exercise, muscles generate heat as a by-product. Sweating is one of our go-to physiological responses to counteract this; the evaporation of water from the skin surface allows us to lose heat from blood vessels near the skin surface. The lost water needs to come from somewhere though, so if this process continues unchecked (i.e. without rehydration), blood plasma volume will reduce significantly (hypovolaemia) or core body temperature will steadily rise (called hyperthermia when exceeding 40 celsius), or both.

As a lot of those Hackney runners found out, this can result in heat-stroke, which at its worse can be fatal. However, well before this grim endpoint is reached, evidence suggests that, although there is huge individual variability, dehydration even at the level of >2% of body weight (i.e. 1.4kg of fluid loss for a 70kg runner) can affect aerobic performance and also cognitive function in hot conditions. This latter effect might be particularly important in the context of, say, decision making about fluid intake or pacing, and exacerbate the problem. These effects all increase as dehydration worsens, and when we reach 6-10% bodyweight loss, cardiac output, sweat production and muscular blood flow can all be compromised. Nasty.

How much to drink?

So, what can we do to prevent this? In general, it’s best to start a race (or hard training session) in a state of ‘euhydration’, that being one of neither over- nor under-hydration, with pale, straw-coloured urine. Clearly, starting in a dehydrated state would be a bad idea, whereas overhydration can cause GI discomfort and unwanted mid-run toilet trips!

During the run / race itself, hydration advice has shifted in the past decade or so. The traditional viewpoint used to be that ‘drinking to thirst’ was unsafe, because if you’re already thirsty, you’re already dehydrated, with runners therefore advised to pre-empt and avoid thirst by drinking early and regularly.

However, this method has its own risks, chiefly that of overhydration and exercise-associated hyponatraemia (EAH). This is known to be very common among recreational endurance athletes, and is essentially when the blood becomes diluted (specifically with regard to sodium content), due to replacement of fluid but not electrolytes (which I will get to later). Without wanting to sound too dramatic, again, at its most severe, EAH can be fatal.

Endurance runners are now advised, therefore, that following their instinctive thirst mechanism is a useful starting point to avoid EAH. There is phenomenal variability in sweat rates (anywhere from around 300ml to 2.5L / hour), both between and within individuals, depending on conditions and exercise intensity. However, for the majority of endurance runners, a range of 400-800ml fluid intake per hour is appropriate, with the higher end applied to faster or heavier athletes or hotter conditions (and vice versa).

Again though, this range remains just a starting point. A dietitian can help individualise the plan, but an athlete should also learn through their own experience about what they need. Sweat rate and lab composition testing can be helpful, but runners can also get into the habit, from time to time, of weighing themselves pre and post run (naked, wiping off any skin surface sweat) and working out sweat losses per hour in different conditions (accounting for any drinks consumed during the session, of course).

What to drink?

So now we know roughly how much to drink – or at least how to go about learning how much. What about what to drink? Let’s kick that off with another quick GCSE biology recap by way of some key definitions. These terms are thrown around freely in relation to sports drinks, but are, in my experience, quite poorly understood:

Hypotonic (e.g. water, dilute squash)

  • less concentrated than blood
  • best for rapid hydration

Hypertonic (e.g. hydrogels, most ‘recovery’ drinks)

  • more concentrated than blood
  • best for recovery and rapid carbohydrate delivery

Isotonic (e.g. most traditional sports drinks)

  • similar concentration to blood
  • all-rounder: compromise between hydration and carbohydrate delivery

When we talk about concentration, what we are really referring to here is the amount of sugars and salts (electrolytes) per litre. And when we consider our drink of choice, it’s important we can understand and weigh up the options, because ultimately it becomes a trade-off between optimal hydration and optimal carbohydrate delivery.

During a run, if rapid (re)hydration is needed above all else (e.g. from a safety perspective), a hypotonic solution such as water could be prioritised. If energy (carbs) are an urgent priority (but no other options e.g. gels are immediately available), then a hypertonic, sugar-heavy drink could be chosen.

However, in most other circumstances, isotonic is the way to go during a run. The rehydration rate is not far behind that of a hypotonic drink, and you have the added bonus of a gentle infusion of carbs (to be factored in alongside your race day fuelling plan) and electrolytes. In fact, current endurance sport guidelines consider the sodium levels found in most isotonic sports drinks to be optimal during prolonged exercise, for most athletes. Again though, there is great variability in sweat composition. Some people regularly see white salt marks on clothes or have salty-tasting sweat. In such ‘subjectively salty sweaters’ (or those that have lab-tested their sweat composition), alternative electrolyte plans are likely to be needed, especially when these runners also have either naturally high sweat rates and / or hot or humid conditions.

Remind me what electrolytes are?

Before moving on, a very quick recap on electrolytes and why they are important. Sodium is the one we hear most about, but potassium, calcium, magnesium, chloride and phosphate are all crucial too. Electrolytes are so called because they carry electric charge across cell membranes, and when this balance is disturbed, heart, nerve and muscle function can all be compromised.

And what about post-exercise rehydration?

One of the other important roles that electrolytes (especially sodium) play is regulating fluid balance, which takes us on nicely to post-exercise rehydration. Despite the pre-run and mid-run hydration guidance above, it is expected that any prolonged endurance exercise is likely to end up with some bodyweight fluid deficit (especially in conditions like Hackney) and the recommendations are to rehydrate with 150% of that lost weight. So, if you’re 1 kg lighter after the run, drink 1.5L.

The most efficient and optimal way to do this is with a surprisingly salty drink. Guidelines state that >60 mmol sodium per litre is optimal for fluid absorption, and it should be pointed out that this is where commercially available sports drinks really don’t cut it. They fall well below this level, because it wouldn’t be palatable to the general consumer and wouldn’t exactly fly off the shelves! So this is where my homemade special recipe comes in handy (and cheap):

  • 500ml orange juice
  • 500ml water
  • 2/3 of a tsp of salt

…mix together and enjoy the salty smugness of optimal rehydration!

Very concise summary of recommendations:

  • Pre-run: start euhydrated (pale straw urine)
  • Mid-run: drink to thist is an acceptable starting point
  • Most fall within 400-800ml / hour range (higher end if heavier, faster or hotter)
  • Isotonic usually wins for mid-run
  • Individualised plans (for volume and electrolytes) are best
  • Rehydrate post-run with 150% of lost weight, and go homemade for optimal results

Right, so there you have it. Hopefully that is a one-stop-shop with everything you needed to know about hydration in endurance sport, but as ever, your friendly sports dietitian (me) can help make sure you’re getting everything right with an individualised hydration plan. Anyone else thirsty?

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