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Hydration during endurance events: Is drinking based on thirst as simple as it sounds?

By Alister Gardner - Endurance athlete

Living in Quebec with its frigid winters, where -30°C is not a rare thing, I once made myself a promise to never complain about the heat in the summer, but only to embrace it fully, knowing another winter was always just around the corner. So, when an early season heat wave hit us in late May (32°C) and I was out for my long run in the middle of the afternoon with no more water left in my single bottle and a long way from home, the devotion to my promise was being thoroughly tested. I would have done anything for a cool breeze or a light shower, or even a hail storm.

That run certainly took me a few days to recover from. I had lost almost 7 lbs in under 2 hours, and I was no doubt suffering from heat exhaustion; the following night’s sleep was restless and my heart rate was still high when I woke up. But it was not really just the heat that was the problem, it was my hydration strategy. I realised that with such varying weather across the seasons, we have to remember to ignore the habits we have built up over the previous few weeks and adapt to the conditions in front of us. Hydration in the winter is almost an afterthought whereas in the heat of the summer it can be what makes or breaks a training, even under 60 minutes.

When it comes to hydration strategies there are typically two schools of thought:

  • Drinking to schedule
  • Drinking based on thirst

Drinking to schedule is a very simple guideline; usually along the lines of 1 cup every 15-20 minutes but the rates and quantities vary depending where you look (sports magazines and health sites). The problem is that it is not really suggesting to take in consideration of heat, humidity, duration, let alone body size or sweat rates. So, in recent years, the ‘drinking based on thirst’ strategy has gained popularity and it is easy to understand why; there is increased awareness of the risks of hyponatremia (excessive water consumption during effort) and the persuasive theories of researchers like Professor Timothy Noakes. Now throw in a handful of elite athlete testimonials and you have the recipe for a new trend.

The subject of how much body weight loss is acceptable while still maintaining performance is a whole article in itself and something I will write about in the near future, but what we can be sure of is that we want to stay sufficiently hydrated to maintain performance and not drink too much that we are uncomfortable, adding extra weight or going into hyponatremia (note, that is also related to what we drink as much as how much we drink). As a reference, the American College of Sport Medicine recommends staying within 2% of your original body weight.

If you are going to use the ‘drinking based on thirst’ strategy, here are some important things to consider which may affect how well you hydrate yourself:

  • Our day to day hydration habits and perception of thirst: there are two types of thirst that emerge under different circumstances: homeostatic thirst is evoked in response to an existing water deficit, whereas anticipatory thirst occurs before an impending deficit. So we need to think about if our feeling of thirst is based on the need or the anticipation of the need which could lead to overcompensation if dehydration is a major concern.
  • Age: as we get older our perception of thirst is reduced. Research has shown a decrease in thirst in age groups of 65+. However, it is unclear if the decrease in thirst is linear with age or increases significantly later in life.
  • Race stress: while we are racing, our minds become preoccupied with all sorts of things (usually the competitor in front, and, worse, the one right behind) as well transitions, gear, technicity of trails and so on. So our response to thirst may lose priority among all the things happening around us.
  • Fluid availability: like my monthly budget as a student, I often find myself making very little last as long as possible. So my thirst then becomes dependent on how much I have with me to drink. As with my overheated long run, I tried to make the 500 ml of water I had last as long as possible, but this was not helping my performance. On the flipside, I could have carried 2L of water in a hydration pack, giving me plenty, but also meaning an extra 5 lbs of weight. As a competitive racer I tend to lean towards staying as light as possible. The challenge is that I poorly predict my fluid needs and the time to the next aid station and risk coming up short. Leading us to the next point:
  • Guzzling: finally hitting that aid station on a hot day and chugging down a fresh full bottle of water, or 6 cups or sports drink is such a good feeling. But it does not necessarily mean I am directly consuming the right amount of fluids for my needs.
  • Food ingestion can stunt thirst. This is particularly important for ultra endurance events where stopping to eat is more common.
  • On the flipside, salty foods will likely increase our thirst.

Many of these points could be applied to scheduled drinking as well. The difference is that it is less random, so if we get thrown off our scheduled drinking strategy we can more easily reflect back on how much we have consumed and adjust accordingly.

There are also two more hydration strategies that can be used:

  • While drinking to thirst can be summed up as “focusing on the presence of thirst as the only stimulus to drink”, drinking ad libitum is described as “consuming fluid whenever and in whatever volume desired”. While some research has shown little difference between the results of the two during effort, the main difference was in recovery, where the ad libitum group would drink more after an event as part of their recovery. The key difference found in the research was that drinking ad libitum “frees athletes to focus on training and competition rather than being distracted by regular or continuous thoughts about sensations of thirst.”
  • The second alternative is to develop an individualised hydration plan; logging weight changes, times and feeling during training in different conditions, for different lengths of time and intensity. The reason being that we measure our own unique rate of sweating. This method will certainly be more time consuming, but would also help you better know your body and how to best prepare for different races in different conditions.

To be able to do this calculation yourself, you need to weigh yourself naked (or with minimum clothing) before and after your run. The difference between the two is a combination of your fluid loss and energy burnt (glycogen and fat), hopefully you did not gain weight!

It is hard to be precise about the weight loss in the form of energy, but for a moderate-hard workout of 90 minutes or more, is considered to deplete our glycogen reserves. This weight loss can be upto 2 kgs (4.4 lbs) as it is made up of a 1:3 glycogen to water ratio. As the energy is burned, the water associated with it is released for the body to use.

Example: a 4 hour trail run on a hot day:
Pre workout weight = 160 lbs
Post workout = 150 lbs
4.4 lbs lost in energy and associated fluid.
= total fluid loss of 5.4 lbs

Now add the fluids consumed during the run and divide by the number of hours running:

Drank 3 L during the run = 6.6 lbs.
6.6 + 5.4 = 12 lbs / 4 hrs = 3 lbs of fluid per hour or 1.35 L per hour in order to maintain our body weight within 2.75% of original body weight.

This calculation can be repeated in a variety of conditions and will vary each time, allowing you to build a better understanding of your hydration needs in different conditions.

Going back to the point about what an acceptable amount of weight loss is, is a complex subject and will once again vary depending on the person. This is why it is important to note sensations and performance along with the calculation to get a better understanding of your tolerance to dehydration.



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