Exercise Hydration

Written By Josh LipkaCycling

Dehydration is one of the most reliable ways to make yourself slow down on a bicycle. Losing just 1% of your body weight during a ride will start to lower your power output. When you reach 4% of your body weight your power output can drop by 20-30%. Four percent may seem like a rare occasion but it is more attainable than you might think. Your body can lose on average 34 ounces (and up to 3x that in extreme heat and humidity) of water through sweating per hour. Unfortunately, our body can only absorb 20-25 ounces of water per hour. So if we kept exercising long enough, dehydration would be inevitable. That is why it is so important to optimize our hydration because it is a losing battle even when done perfectly!

Here is an example to show how easy it could be to reach 4% dehydration:

149 lb person

4 hour ride = 136 ounces lost

2 bottles consumed = ~40 ounces

96 ounce deficit = ~6 lbs = ~4% body weight loss due to dehydration

I feel like four hour rides with two bottles happen pretty often. So, in general we can absorb 20-25 ounces of fluid per hour. The hammer nutrition article below suggests if you’re a smaller rider you may be okay to consume as little as 16-18 ounces of water in cool conditions (March CRCA brr) and if you’re a bigger rider in warmer conditions you may stretch your consumption to 28 ounces per hour. This range is basically a bottle per hour. If you want to get a little more personalized keep reading.

More Personalized

This is so simple it just might work. Weigh yourself before your ride. Weigh yourself after your ride. Almost all of the weight you lost came from fluid loss. If you lost a pound then you had a net loss of 16 ounces of fluid (1 pound = 16 ounces). So if you lost a pound of weight but drank less than 20-25 ounces per hour then bump your consumption up next time and see if you lose less weight during your ride. If you consumed the 20-25 ounces per hour then you may experiment with different electrolytes/sports drinks.

You may have drank enough and had the perfect beverage and still lost water weight. Remember that is possible since we can sweat more than we can absorb. At this point it would be beneficial to look at the other side of the coin. Can we prevent any sweat loss? A fan/fans while training inside, different clothing options, unzipping your jersey, a helmet with more ventilation, an ice sock down your jersey, an ice vest before your event (there is research that shows runners who warm up with an ice vest have significantly lower core body temperatures at the end of their race compared to runners who did not use an ice vest in their warm up), training in the heat to acclimate, and I’m sure the list continues.

So weigh yourself pre and post ride. Remember, if we are not hydrating optimally the worst case here is that death can occur with too little or too much hydration (google hyponatremia). The best case is we lose a little performance. Either way it’s important to us. Try to consume 20-25 ounces, maybe a little more or a little less depending on the temperature and your size. Try different drink mixes and electrolytes. Acclimate to the heat. Try to figure out ways to sweat less. The goal over time will be to end the ride weighing as close as we can to our pre ride weight!

Another interesting way you can tell if you dehydrated yourself on a ride

I like to train on the mega pretzel on Zwift. I go really hard and I treat it as a time trial. When the road goes up I try to hold sweet spot or lactate threshold. When the road is flat or downhill I hold tempo. It takes me three hours. Sometimes I don’t start the training hydrated enough and sometimes it’s too hot in my room and I sweat much more. Here is a time where both of those things happened. Check out my power and heart rate for the first half hour and the last half hour.

First half hour - 310 watts - 140 bpm

Last half hour - 313 - 162 bpm

If you notice that your heart rate is way higher for the same power then chances are most of the reason is because of dehydration. If you notice this during the ride, it’s already too late. Our muscles need oxygen. Our heart pumps the oxygen saturated blood to the muscles. When we get dehydrated we lose blood volume and our heart has to beat faster to pump the same total amount of blood/oxygen to our muscles. Just another neat way to catch dehydration in the act. Bottoms up!

Josh LipkaCycling
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