As a coach, I get asked this question a lot. The relationship between dehydration and performance is well-documented. Studies show that loss of fluid of 2% or more of body weight is enough to cause a detectable decrease in performance (that’s only a 1.4 kg loss in a 70 kg athlete). Dehydration of greater than 2% loss of body weight increases the risk of nausea, vomiting, diarrhoea, and other gastro-intestinal problems during exercise. Unfortunately, we don’t usually notice thirst until we’ve lost 1-2% of our body water - this is particularly so during a race when lots of other things are going on. So it’s really important to have an idea of how much fluid you need to replace per hour and have a plan to follow to make sure you do.
What I usually advise athletes is to test out how much fluid they are losing during training. The best way to do this is as follows:
• Weigh yourself before a training session (ideally without clothes and after emptying bladder). This is your STARTING WEIGHT
• Add up how much fluid you are taking with you (e.g. 2 x 750 ml bottles = 1.5l = 1.5 kg). This is your STARTING FLUID
• Do your training session (preferably at race pace) and record the length of time of the session (in hours). This is your SESSION TIME
• Immediately after the session, weigh yourself without clothes and after emptying bladder. This is your FINISHING WEIGHT
• Add up how much fluid you have left over (e.g. 0.5 x 750 ml bottles = 375 ml = 0.375 kg). This is your FINISHING FLUID
• Then calculate STARTING WEIGHT + STARTING FLUID – FINISHING WEIGHT – FINISHING FLUID and divide the result by SESSION TIME
• This gives your sweat rate per hour
• E.g. for a 72 kg person starting with 2 x 750 mL bottles of fluid, going on a 2 hour ride, and finishing with a weight of 69 kg and 200 ml of fluid, the sweat rate is (72 + 1.5 – 69 – 0.2)/2 = 2.15 l/hr.
It’s best to do this test in a variety of conditions and closest to anticipated race conditions as possible.
Once you have an idea of your sweat rate, you should incorporate how to replace that much fluid into your race plan. Don’t rely on thirst - follow your pre-determined hydration plan.
What about electrolyte replacement?
As most of us know, we lose electrolytes when we sweat, the main one being sodium. When sodium runs low, we can run into problems similar to dehydration, such as nausea, vomiting, cramps, fatigue, etc. Ideally, sodium should be replaced at the same rate it is being lost, which is the rate you sweat it out. Unfortunately, the concentration of sodium in sweat varies widely – to be sure of your sweat rate, you really need to have a sweat analysis done. If you’ve had a sweat analysis it’s relatively easy to calculate the sodium you need to replace. You simply use a product such as SIS Go Hydro and add enough sodium to your water bottles to achieve the same concentration as in your sweat. Then drink the mixture according to your hydration plan as described above. However, if you haven’t had a sweat concentration done, it’s best to add say one tablet of SIS Go Hydro to every 500 ml (1.5 tabs to a 750 ml bottle). Although it won’t be accurate, it will probably provide enough sodium replacement to keep you out of trouble.