Dehydration in sport causes fatigue- no question. Fatigue toward the end of a prolonged sporting event may result as much from dehydration as fuel depletion! 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).
Triathletes and all endurance athletes are particularly susceptible to the effects of dehydration. In a long event, over 3 hours, one cannot actually drink sufficient fluid to keep up with the fluid losses. So in these events, it is more a matter of staving off the inevitable dehydration and resulting fatigue as long as possible. Athletes who travel to hot climates to compete are also at high risk of being dehydrated before and during their event.
Even in normal temperatures, maximal aerobic power (VO2max) decreases by about 5% when you experience fluid losses equivalent to 3% of body mass or more (Pinchan et al. 1988). The endurance capacity during incremental exercise is decreased by marginal dehydration (fluid loss of 1% to 2% of body weight). Endurance capacity is impaired much more in hot environments than in cool conditions.
Sawka, Young, Francescone, et al. 1985 did a study investigating time to fatigue at different levels of dehydration. They found even at low intensity exercise like walking, dehydration increases the incidence of exhaustion. The dehydrated group, could not finish the task.
The main reasons dehydration in sport reduces exercise performance in triathletes:
- Reduction in blood volume
- Decreased skin blood flow
- Decreased sweat rate
- Decreased heat dissipation
- Increased core temperature
- Increased rate of muscle glycogen use
A reduced maximal cardiac output is the most likely physiologic mechanism where dehydration will decrease your VO2max and impair work capacity. Dehydration also impairs your ability to lose heat. Both sweat rate and skin blood flow are lower at the same core temperature for the dehydrated compared with the normal state (Nadel et al. 1979 1980; Sawka and Wenger 1988). Body temperature rises faster during exercise when the body is dehydrated. This critical temperature is reached more quickly when dehydrated.
Dehydration in sport not only elevates core temperature responses but also negates the thermoregulatory advantages conferred by high aerobic fitness and heat acclimatisation(Pinchan et al. 1988).
How To Prevent Dehydration in Sport
An athlete can adapt to altitude, heat, higher work capacity but CANNOT adapt to dehydration. Internally, you’re about 65% water. Without water, you can only survive a few days. Dehydration makes your blood thicker, increasing your heart rate and decreasing the amount of blood your heart can pump with one beat and causing your blood pressure to fall. Dehydration makes it harder for fat to get into your muscles to be used for fuel, so your muscles burn the limited sugars (glycogen) already there. Since your brain is about 85% water, even mild dehydration can bring on changes in your mood and a decline in your concentration and alertness.
From a sports perspective, losing as little as 2% of your body weight in fluids can cause measurable decreases in performance. Dehydration of more than 3% of your body weight is serious, increasing the possibility of heat exhaustion and heat stroke in warm and/or humid conditions as discussed above. Since athletes can sweat out 6% to 10% of their body weight during competition, rehydration is critical.
Thirst is not a reliable indicator of dehydration. If you wait to drink until you are thirsty and stop drinking when your thirst is satisfied, you’ll remain 25% to 50% dehydrated. Ensure you are sipping water throughout the day. Check the colour of your urine to ensure it is a light, straw like colour, not dark.
The longer your workout or competition, the more dehydration will hurt your performance. A review of scientific studies showed that endurance athletes like triathletes and marathoners had a performance drop of 7% to 60% when dehydrated. Athletes requiring muscle strength, like bodybuilders and Amercian football linemen, saw their power reduced when their sweat loss was as low as 3% of their body weight.
Be objective about measuring your sweat rate so you know how much you need to drink in training and racing and not rely on guess work.(This is discussed in detail in my latest book).
It’s pretty easy to figure out. Before you exercise, weigh yourself nude. Then exercise for 1 hour at 75% heart rate max. Drink 16 oz over the hour. Then weigh yourself again at the end- nude. Weight before- weight after= net weight lost. Add back 1 pound ( for the 16oz fluid drunk). The remaining will tell you your sweat rate. So if you lost 8oz despite drinking 16oz during exercise- you will need to drink 24 oz during exercise to remain neutral.
Remember in a long event you will be sweating salt and electrolytes as well as water so be sure to have sports drink rather than just water alone.If your event is 60 minutes or less, water is just fine.
In cycling it is easy to take your water bottles in bottle cages on the bike. In swimming be sure to have a drink bottle at the end of the lane for sipping during sets. In running either carry a water bottle or better still, use a running hydration belt so you do not have to carry it. 
Having to carry water bottle over long distances can alter your gait and cause your neck to tighten up on that side.
Typical signs of dehydration in sport
Many triathletes have upset stomachs in race week and especially on race morning. Be very aware of this as vomiting and diarrhea are a major cause of dehydration. Continue to sip fluid if this happens to you.
Typical symptoms of mild dehydration are not just thirst- watch out for these signs:
- Dry, sticky mouth
- Sleepiness or tiredness
- Thirst
- Decreased urine output
- Dry skin
- Headache
- Constipation
- Dizziness or lightheadedness
Hunger can also be confused with dehydration. Sometimes when you have the afternoon slump and feel you have the munchies, it is actually your brain telling you that you need to drink more.
Everyone is at risk of dehydration. Many “normal”people are actually chronically dehydrated. Athletes are a greater risk than most as it is hard to drink enough during hard, long training sessions to replace the fluid lost. This is why we must be particularly vigilant.
Happy Training- have a sip of water now :)
Dan
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