Most athletes have experienced some of the effects of getting overly dehydrated when out on the trail, but few really understand the underlying physiological effects on performance as dehydration begins to take its grip. As I work endurance athletes, competing across a range of conditions, including the harsh heat and humidity of the Hawaii IRONMAN, I am well versed in building strategic fueling and hydration for longer duration events. Managing your hydration is not something that we want to leave to chance, if the aim is to have your best performance. Let's investigate.

Dehydration and performance: So what happens to your performance when you become dehydrated? It is worth understanding a layman version of the physiological challenge you will place on your body when out training and racing, so that you are able to adopt a plan to avoid some of the effects. I will aim to make it highly approachable, and won't blind you with scientific jargon. In basic terms, dehydration is mostly related to your blood. Really? Yes, it is best to think about hydration as it relates to the volume of blood in your body. As an athlete, your blood has three main roles to perform, related to performance:

1.     Deliver oxygen to the muscles to create energy for the work you are doing.
2.     Deliver heat generated from work to the skin, to avoid harmful heat build.
3.     Shift blood to the abdomen, to assist with absorption of any calories consume.

All sound important enough. You have, give or take, about 6 liters of blood in your body. This blood is make up for red blood cells (the red part!), and plasma (the clear fluid, or white part!). As you begin to get dehydrated, a part of the fluid you lose is the white plasma. In other words, there is less blood to go around. In short, as you become more and more dehydrated, there is an ever-increasing competition between the three roles stated above. In the battle, the winner will always be blood rushing to the skin, as accumulating additional heat within your body is highly corrosive, and even dangerous. This means not only that absorption of calories is compromised, but also that you will begin to experience a higher perception of effort at any work rate, then a greatly increased metabolism cost at that levels. Ultimately, it will become impossible for you to retain your desired pace, as the cost will be too high

Some dehydration is normal: Realize that some dehydration is normal, and one could argue, even a benefit. Most of the negative performance effects of dehydration only occur at about a 4% rate of dehydration. Before that loss, it can be assumed that there is no real performance loss with the minimal dehydration. This is important, as it influences our approach under different conditions. Imagine you run 60 minutes in a cool environment, it is unlikely that dehydration is going to be a massive performance limiter within that run, assuming you began the run fully hydrated. Now extend the run to multiple hours, and add the environmental cost of heat and humidity. Your first 45 to 75 minutes won't deliver any negative effects, but your mission will be about setting yourself up for success. While running, it is close to impossible to replenish all fluids you are losing, so the aim should be to prevent yourself from ever getting down to below that level of 'performance dehydration'. This takes planning and execution, all of which I lay out below

I should also note that, in addition to simply looking through the performance lens of hydration, you should also look at the fluids you take on as having the role of 'diluter' or 'transporter'. As mentioned above, your GI system becomes highly inefficient at absorbing calories during exercise, and even less so when becoming dehydrated. The fluids you consume will help offset the potential dehydration, but also should help to dilute the concentration of calories consumed to fuel your exercise. The more dilute the calories, the easier for an inefficient gut to absorb. No matter your fuel source, it is important that your hydration of choice be a very diluted solution that assists absorption, instead of inhibiting it. A general rule would be for your drink choice to be less than 3% concentration of carbohydrate, with some added electrolytes within the concentration. If you prefer drinking just water, I would add a dash of salt and a squeeze of citrus, just to get closer to natural body water chemistry, but a sports drink should be highly dilute and not a form of caloric replenishment in itself.

With this information, what is a starting point for approaching hydration for an endurance athlete? Here are my quick tips:

• Know the weather: Realize that you require more fluids in hot and humid conditions than cold conditions, as well as the fact that hydration becomes much more important for sessions lasting more than 60 minutes, over short sessions under that duration. Plan ahead, and begin learning your needs in your weekly training
• 10-12 ml/kg/hour: A general rule tends to be for about this amount of hydration per hour. You can read how to get a more personalized assessment of your needs and sweat rates from this great article from my colleague, Doug Casa.
• Frequent and often: If you are planning a longer session, begin early and drink frequently and often. I prefer athletes to consume fluids every 6 to 10 minutes, consistently, right from the start of the session.  
• Dilute those kcal: If you consume any calories, ensure you take on fluids. Similar to above, and realizing that for every 100 kcal you consume, it requires about 12 ounces of fluid to dilute, quick frequent hits are better than a calorie bomb consumed only when you feel the need.
• Rehydrate between sessions: Even if you nail your training and racing hydration, it is normal to finish in a dehydrated state. To facilitate recovery, limit stress, build a platform of health, and retain balanced energy in the day, ensure you rehydrate between sessions.
• Just because you can, doesn't make it great: It is very common for me to hear athletes tell me 'Yes Matt, but I can go on a 3-hour run and not consume a thing'. This may well be true, but is this approach maximizing your performance during the session or the adaptations from that session. Is it optimal? As I always say, 'I can ride my bike to Belgium on an apple and a flask of water, but why would I?'. Being able to 'survive' a session with hydration isn't the same thing as maximizing what you hope to achieve from the session.

I hope that helps frame the hydration puzzle in training and racing!

Cheers,

Matt Dixon