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Fluid balance testing

September 6, 2017

I am currently in Cairns attending the Mountain Bike World Championships. I am assisting two athletes with various aspects of nutrition and hydration in the lead up to their race. I will also be in the feed zone on race day.

One of the challenges up here is the heat. The race will take place at 12 noon, one of the hottest times of the day. We know that dehydration (in the order of ~2% body weight lost in the form of body fluid) can effect performance and so one of my main roles is to ensure that these athletes maintain a desirable hydration status.

Measuring fluid balance is one way we can keep an eye on this. Fluid balance testing involves weighing the athlete before training. We also weigh the drink bottles they take with them out on course. We take account of the weight of the bottle itself and any powder used (thereby ensuring we are measuring only the fluid being consumed and not other variables). When they return from training, they towel down (removing as much sweat from their body/clothing as possible) and we weigh them again. We also re-weigh their drink bottles. We then use these numbers to calculate how much fluid they have taken in and compare this to the weight they might have lost on the scale. From these numbers we can work out their sweat rate (sweat lost in mL or L per hour). We can also calculate the amount of fluid they need to consume post training in order to adequately rehydrate from that session (1.5 x the amount they have lost/not replaced while on course). Ideally this fluid replacement would contain some salt to aid in fluid retention.

Over time, these numbers help dictate how much fluid will be required on race day in order for them to stay within a safe level of dehydration that will not effect performance. The goal isn’t necessarily to keep them in exact fluid balance (no change on the scale) but to keep them within that 2% loss I mentioned earlier. We also don’t want to see any weight gain (fluid gain) after a session as this can have dangerous outcomes for the athlete if done in excess.

As an example, one rider lost 1.5L of fluid while out on course for 1hr 45 minutes yesterday. She replaced much of this while riding, coming back only 300g lighter. This equated to 1.3% of her body weight, which meant her performance should have been protected. Because she was still 300g down on the scale (300mL fluid), she required 450mL (1.5 x 300mL) in order to adequately rehydrate and recover from that ride.

If you have any questions regarding fluid balance or would like to book a fluid balance session, please contact me:


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