How much do I need to drink? How often do I need to drink? How do I know if I’ve had enough? What can I do to keep cool? These are common questions amongst runners, especially in hot weather, or when considering a race. There are huge variables, so read on to get some ideas of how runners should approach hydration and heat management.

Hydration for runners

When you exercise, the body is “burning” fuel to allow the muscles to move. There are no flames, but the chemical reactions required to break down Glycogen in the muscles and liver release heat. Since your body has to stay around ~37°C for everything to function as expected, it does a few amazing things to get rid of that heat.

You expel heat through your mouth when breathing out. You might have noticed dogs doing this - it’s their primary heat management function. Fortunately, when you exercise, you have to breath harder and more often, so you also expel more heat. Tidy!

Your body will also shed heat via the skin, favouring capillaries near the surface - you’ll notice your veins are more visible above the surface too when hot.

That movement towards the surface works in conjunction with your sweat glands, which are a human’s primary method of dissipating heat. When you exercise, moisture is excreted and then has the chance to evaporate. Evaporation from liquid to gas requires heat transfer and the gas molecules have more energy than liquid, so heat is removed from the body, plus the remaining moisture on the skin feels cool.

Dew Point

There are various factors that affect how easily the sweat can evaporate; these are Temperature, Relative Humidity and Pressure.

Higher pressure = harder for sweat to evaporate.

Higher humidity = harder for sweat to evaporate

The relation between these factors is called Dew Point - this is the temperature below which air saturated with water vapour would start to condense into water droplets. This is important because the closer the dew point is to the real temperature, the harder it is for your sweat to evaporate. It means that you could be more comfortable running on a “hot” day of 20°C with a low pressure, humidity and dew point, than on a “moderate” day of 15°C with a high pressure, humidity and dew point. And the key issue isn’t really comfort - when it comes to high levels of exercise like running, it’s safety.

Some weather apps will show you the dew point, so if you do a quick check of temperature vs dew point, you can tell if it will be an easier or harder run in terms of heat management than a previous run.

What happens if you don’t stay hydrated?

Once sweat evaporates, it’s gone. That water is lost from the body and needs to be replenished. As exercise levels, dew points and temperatures rise, the rate that you will sweat increases accordingly. If you are only exercising for a short amount of time, this isn’t an issue. Most people can go out and do a 30 minute run in most temperatures without losing much of their body’s water content.

However, in hot, humid weather, it’s impossible to safely replace fluids at the same rate they are being used, unless you reduce the amount of effort, which might mean running very slowly. So, if you don’t want to slow down, eventually, your body will become dehydrated to a level that affects performance, which will slow things down for you anyway. Around 1% body weight lost in sweat is when you start to feel thirsty and your performance will start to decrease. By 4% of body mass lost as sweat, your performance will swiftly decrease by 20-30% and if you continue past this point, you will risk entering a slightly confused state of mind where your body is suggesting you should stop, but your mind is thinking it should be capable of more.

Hydration for runners

Here are the splits from a run I did in 25-29°C at 9am to 11.30am, so the sun was continually getting stronger, with less shade from the trees at the side of the road. I did have a mouthful of water every 15 minutes, but not nearly enough (deliberately). Note that it was an aerobic base HR run (for me, around 140bpm), so I ran at a consistent heart rate the whole way, and thus pace is expected to drop off gradually with each passing mile.

Hydration for runners

You can see that performance gradually slows (on average) by a couple of seconds per mile as you would expect, but at mile 14, dehydration starts becoming an issue as pace starts dropping off a cliff. Mile 15 is a whole 30 seconds slower again and mile 16 is another 30 seconds slower than that. Time to call it a day! I was able to do this run safely by doing a small loop, so that I wasn’t out in the middle of nowhere when I jumped over the performance cliff, plus I picked a route that was at least a little shaded.

Now that you can see the importance of staying hydrated, the next thing is to figure out how much to drink and when. It’s not just as simple as guzzling as much as possible. If anything, drinking too much water is far more of a risk than not drinking enough.

Risks of drinking too much

If you become properly dehydrated while running, you’d normally be unable to continue enough to put yourself at risk. You’d experience cramps and nausea, both of which would stop you being such a danger to yourself, with your only options being to slow down, or stop and take on board more fluid. The only way to get so dehydrated you’d risk serious harm or death is if you were deprived of water completely over a long stretch of time, losing 10+% of body weight.

If you drink too much, you can quickly put yourself at risk of irreversible damage. It’s unusual for people to attempt to drink this much, but you might have heard of healthy people entering marathons or taking Ecstasy, drinking a lot of water and dying from hyponatremia. This is where the amount of sodium in the blood becomes too diluted for your body to regulate cell size, plus overloading the kidneys with water and potentially leading to coma or death.

How much is going to work then?

Studies have shown that 400-800ml/hour is what most people should be aiming at during exercise. http://cjasn.asnjournals.org/content/2/1/151.full As your level of exertion increases, the amount you’d drink would move towards the top end of this range, so heavier people, longer races and hotter weather. Drinking more than 1L/hour may decrease performance puts you at risk of hyponatremia. Besides, any excess water your body can’t use effectively will end up uncomfortably sloshing around in your stomach which can’t absorb it quickly enough.

You could measure your weight before and after a longish run, done without food or drink, to determine how much sweat you lose personally per hour and then aim to get close to that without exceeding the typical amounts above. You’d have to adjust for weather conditions. However, most casual athletes are not going to bother doing this sort of test, or have access to accurate scales.

Another method of managing your fluid intake is to pay close attention to your body’s own indicator: thirst. When you need fluid, you get thirsty, due to the brain noticing that sodium levels are no longer correct. If you are carrying water, you can respond to this quickly. However, this is an immediate need and is not a suitable method if you are relying on aid station water which might be another 40 minutes away. Regardless, you should also keep track of how much you are drinking to ensure you aren’t drinking too much.

The Run Company stocks a variety of hydration products and can advise you on what will work best for you, from Race Vests to Electrolyte Drink Mix.

Hydration for runners

What about outside of exercise?

Having the right amount of fluid during an exercise session or race is one thing, but you need to approach hydration as a long term part of your exercise schedule. There is no point having a perfect race hydration plan if you didn’t bother drinking anything the day before.

Day to day hydration comes from the food you eat and the drinks you have. In hotter weather, you’ll need to drink more than usual, but again, you aren’t aiming to drown yourself. If your wee is regularly clear, you are drinking too much. If it’s regularly dark, you aren’t drinking enough.

After a hard exercise session or in the heat, you’d expect your wee to be darker because, at higher levels of exertion, you cannot safely take on fluids fast enough. So, after you finish an exercise session, think about whether you need to continue to take on board fluids.

Hydration for runners

Ways to assist the body in keeping cool beyond hydration.

Your first action when feeling the effects of heat during exercise should be to reduce effort. As you can see in my training run splits above, when trying to maintain a consistent effort (as determined by my heart rate), once the sun got very strong and dehydration started to kick in, my only option to maintain that heart rate was to massively reduce my pace. If I had not slowed down, my heart rate would have shot up sharply accordingly.

Reducing your effort in hot weather might mean slowing to a walk, or even stopping, which also gives you a moment to take on board fluids. Welcome these breaks and think positively about them as necessary rewards. Insisting on running through it will lead to sub-standard training sessions or perhaps not finishing a race.

Water applied externally will briefly help reduce heat stress. In Bali, I ran along the beach path which had showers on it at each hotel, so just stopped for a second for a quick spray off whenever the heat got too harsh. You could also wear a bandana on your head and pour water on to that.

Wearing the right clothing in the heat is a major factor for heat management. Cotton clothing will soak up the sweat, which is the opposite of you need for efficient evaporation. Instead, great technical running garments wick moisture away from the skin, assisting with the process, with the added bonus of reducing the chance of chafing. The Run Company has all the garments you’ll need for this.

Hydration for runners

Wait, should I run in the heat at all?

Wow, it’s hot outside. Wouldn’t I be better off just staying at home and hiding in the fridge? Actually, running in hot weather, if approached sensibly, is highly beneficial to overall running fitness. A few sessions of heat training have been shown to increase overall blood plasma volume and might even be more useful than altitude training. Once the temperature drops again, if you have continued training, you’ll be a more efficient running machine than if you had trained at lower temperatures throughout the summer. You just need to adjust your pace in the heat so that the effort remains the same as if it was a lower temperature. Your absolute top end speed training might suffer, but endurance gains will surpass the gap.

So get out there and embrace the heat. As long as you plan your hydration safely, seek out shady routes and reduce your pace to stop your effort from being higher than normal, you’ll have a great run and get fitter. If you need any advice on any hydration products, pop into store and we’ll get you sorted.