IRONMAN Hawaii and a few other hot and humid races such as Malaysia are looming on the horizon so it's timely to address the topic of hydration in detail. I often tell my athletes, amateurs and pros, that they need to focus on hydrating on the bike in order to 'set up a successful run' so let me explain why.

Before we dive into things, a couple of key facts that are important to understand:

1. With a proper hydration approach you will still become dehydrated. The question is by how much. Don't make the mistake of proper hydration in endurance events being maintenance of 100% hydration status. It is impossible, at least without running into health issues and performance decline.
2. Performance decline only occurs after about 4% dehydration, or about 4% of your body weight lost in fluids. Before that it can, ironically, be seen as a performance enhancer, as you are carrying less 'weight' without performance loss.
3. In hot and humid conditions, while running, it is close to an impossible task to consume enough fluids to get anywhere close to hydration needs. We will be 'shedding' fluids at a high rate in order to cool the body but it has a high impact on hydration status.

These three facts (as we understand with today's science) should inform every athletes approach to hydration. By way of example, if you are competing in a two-hour event, there is little need to follow a hydration protocol except drinking to thirst, which is an excellent evolutionary mechanism to help support hydration. It is highly unlikely you need to do more than this. However, in an IRONMAN, especially a hotter one, it is a completely different game.

Impact of hydration status on performance

Hydration status is heavily linked to blood volume, as blood volume is made up of red blood cells and plasma. When we become dehydrated, blood volume drops, and this is where we start to understand the performance ramifications. Keeping it simple, blood has a few basic roles in endurance performance:

• Deliver oxygen to the muscles to create energy (okay, it's more than this, but let's keep it simple)
• Carry heat, generated by the work we are doing, to the skin, so that we can dissipate it into the air. This avoids all sorts of corrosive issue that will harm bodily functions.
• Assist with the absorption of calories and fluids consumed.

Some pretty important and basic roles. Of these, the most important role is for the body to get rid of the heat generated, as a build up becomes deadly to brain and other organs. The second priority is oxygen delivery, and byproduct removal, from the muscle. Lastly is the functioning of the gut, and you likely don't need me to tell you that the rate of absorption decreases during exercise and/or when fatigued. With this in mind, as we become dehydrated, there's a competition between skin and muscle - the gut is already defeated - and the only winner can be the skin. So, for longer events completed in the heat, hydration plays a big role in overall performance.

So, with that backdrop, how should you hydrate in hot and humid endurance events?

The mission:  We know you will lose plenty of fluids over the course of your marathon, especially if you are at the pointy end of the race. This means our mission is to finish the bike portion of the IRONMAN 'not too dehydrated'.  That would be about 1% loss of fluids. We then have a platform of available fluid loss that we can lose, for cooling effect, throughout the run.

If it were only a bike race, hydration would still be a factor, as we are riding for 4, 5, 6 hours. The difference is that we wouldn't need to finish at ~ 1% loss. We could afford to finish at or around 4% loss without fear of performance decline. I should note that there plenty of stories of elite athletes completing races in greater fluid loss than 4%, but the 4% general rule would be well suited to follow here.  

What this all means is that we hydrate on the bike with consistency in mind, and a plan to intake fluid closely related to your normal body chemistry, and to finish 'not too dehydrated'. You can then launch into the run with room to spare, knowing that while you will become more dehydrated, it should not come at the expense of performance.

Cheers,

Matt