Dehydration can derail even the most accomplished endurance athlete. I’ve learned the effects of dehydration the hard way. During a half-Ironman distance race a few years ago, I became crippled on the run because I could not adequately hydrate. But it’s a lot more complicated than just flooding your system with water or sports drinks.
Hyponatremia
The old mantra for athletes was drink early, drink often. But it’s a little more complicated than than. It’s also important to ensure that you ingest the right amount of electrolytes. In short, drinking too much water can lead to a condition called hypornatremia, or water intoxication. The longer the event or training session, the higher the risk of hyponatremia. Symptoms of hyponatremia include nausea, muscle cramps, disorientation, slurred speech, and confusion. Because many of these symptoms are very similar to dehydration, many athletes drink more water because they think they are dehydrated.
Read more about hyponatremia and endurance athletes
Dehydration
An athlete’s water needs can vary dramatically based on effort levels and conditions. Simply put, an athlete does not sweat at the same rate all the time. According to a report published on HumanKinetics.com, exercise performance is impaired when an individual is dehydrated by as little as 2% of body weight. Losses in excess of 5% of body weight can decrease the capacity for work by about 30% (Armstrong et al. 1985; Craig and Cummings 1966; Maughan 1991; Sawka and Pandolf 1990).
The main reasons why dehydration affects an endurance athlete’s performance:
- reduced blood volume
- decreased skin blood flow
- decreased sweat rate
- decreased heat dissipation
- increased core temperature
- increased rate of muscle glycogen use
So the body is working harder in an attempt to cool itself, and is using more fuel in its attempts to do so.
The key to preventing dehydration is understanding your sweat rate. Luckily, there is a simple method for determining your fluid loss through sweat. Here is the process that involves exercising for an hour at your target effort level. Ensure you are well hydrated before you begin.
- Weigh yourself naked prior to your workout.
- Run for an hour and make note of the conditions terrain you are running on. Also pay attention to how you feel and at what time in the workout that you felt better or worse. If you consume fluids on your run, take note of the amount of fluid you intake.
- When you return home, immediately weigh yourself naked again.
- Subtract your post-run weight from your pre-run weight, then multiply that number by 16.
- If you drank during the run, add the number of ounces to the number above. This total is the amount of fluid in ounces you lost during your run.
Example, or you can use this sweat rate spreadsheet to do a fluid loss tests to see how your sweat rate varies.
Pre-run weight: 165
Post-run weight: 160
Pounds lost: 5
Drank 12 ounces of water
(165-160) x 16 = 80
80 + 12 (ounces of water)
Total fluids lost = 92
**Try to replace 92 ounces of fluid each hour. When replacing fluids, It is best if you drink fluids every 15 to 20 minutes.