Nutrition

How Much Does Pre-Race Hydration Affect Performance?

by Pip Taylor

According to a study done at Ironman Louisville 2008 – 2010, proper pre-race hydration could get you to the finish line faster. The study, done by body composition scales manufacturer Tanita, provides a telling insight into hydration levels and the implications for race-day performance.

The study measured 5740 participants’ hydration levels during the pre-race check-in and correlated this with their finishing time. A resoundingly clear trend emerged over the three years of measurements, which highlights just how crucial being hydrated is for race day.

What do the results mean?

The study showed that the majority of participants started the even with optimal or adequate levels of hydration (55-65 percent is considered adequate), which is good news. Further analysis shows that if all else is equal, then potentially for every 1 percent increase in hydration, men’s finish times will decrease (be faster) by 16 minutes and women’s by 12 minutes.

How is hydration measured?

The Tanita scales measure body composition through bioelectrical impedance analysis (BIA). This involves sending a low electrical signal – which is carried by fluids that are within the body – up one leg and down the other. Since fat is much more dense than muscle (and subsequently contains less fluid), the impedance information is used to calculate the reading including the amount of lean and fat tissue within the body, bone mass and hydration. Hydration readings are an estimate of total body water, both intracellular and extracellular given as a percentage of total body weight.

Keep this in mind

There are a lot of things that go into an Ironman performance, not just hydration. And of course you can also over-drink, which will only lead to flushing of valuable electrolytes. Consider these study limitations:

The scale measures total body fluid. What it cannot tell you is whether this is extracellular or intracellular, which is important. Is there a lot of fluid in your stomach and/or bladder (that will excreted and hence useless) or is it stored in your muscle tissue? Hydration is also about proper electrolyte balance, something that is not included in the measurements.
The scale readings are calculated from formulas based on gender, height, etc., so there is always a margin of error which makes it harder to compare one individual to another as opposed to the same individual on different days.
If you have high levels of body fat then your hydration level is likely going to be lower due to formulaic calculations. Logic would suggest that the heaviest participants are unlikely to be the fastest – so how does this factor into the interpretation of the results?
Other data that would have been interesting to know: Post-race hydration and how that compared to pre-race readings and race performance; and the numbers of participants in each hydration percentage category.

The take-home message

Pay attention to your hydration! Failing to start the race adequately hydrated will lead to a significant decrease in performance. A body composition scale, such as the Tanita BD-350 ($229.99, Tanita.com), can be a valuable, cost-effective tool to help establish your optimal level of hydration during training. You can also use these other cues to guide you:

Urine color (aim for pale yellow, although this can be greatly affected by specific foods/drinks – check out the hydration app iPee Daily on iTunes)
Frequency and volume of urination (you should be going every couple of hours)
Thirst signals. Be smart and listen to them!