Physiology and Performance Benefits of Optimal Hydration

Edward F. Coyle, Ph.D., The University of Texas at Austin

During vigorous physical activity in a warm environment, people can lose sweat at astonishing rates, often in excess of 1 - 2 quarts per hour. Losing even a small amount of fluid makes it difficult for the body to maintain cardiovascular function and temperature control. Dehydration is always a risk for anyone who is physically active and the negative consequences to both health and performance are unavoidable, even for the fittest athletes.This review includes the latest research on dehydration and provides practical, everyday tips for staying well hydrated.

KEY POINTS

  1. There is not a critical amount of dehydration which can be tolerated without physiological consequence.
    - the smallest amount of dehydration will have adverse effects on health and performance.
  2. During prolonged exercise in the heat, people can become dehydrated at a rate of 1,000 mL to 2,000 mL every hour (about 2-4 pounds of body weight loss per hour).
    - you lose a lot more fluid during a long match than you think.
  3. Every 1,000 mL loss of water (i.e. 2.2 lbs) will cause:
    a) the heart rate to increase by 8 beats per minute
    b) one's cardiac output to decrease by 1 liter per minute
    c) your core temperature to increase by 0.3°C when exercising in the heat
    - this excessive fluid loss will result in a faster heart rate while less blood will be pumped by the heart and the athlete's core temperature will rise substantially.
  4. Perceived exertion is not a good indicator of the need for fluid replacement because people do not voluntarily replace more than 50% of their fluid losses during exercise.
    - an athlete will typically replace only 50% of fluid lost during exercise
  5. Hyperthermia is the most serious health hazard of dehydration.
    - overheating is the worst result of dehydration
  6. Dehydration during exercise causes large reductions in stroke volume as well as reductions in skin blood flow. The latter may be related to the large stress hormone response to dehydration (i.e. increases plasma epinephrine levels).
    - dehydration causes the heart to pump less blood to the body and less of this blood goes to the skin...a stress response to conserve body fluid
  7. Dehydration induced reductions in skin blood flow and sweating rate cause hyperthermia.
    - the decrease in blood flow to the skin and therefore reduced sweating resulting in overheating
  8. Ingestion of a large fluid volume during one hour of intense exercise in a warm environment slightly attenuates the hyperthermia and increase in heart rate which occurs when only a small volume of fluid is ingested. Furthermore, both fluid replacement and carbohydrate ingestion equally improve high-intensity cycling performance and their beneficial effects are addivite.
    - drinking a lot of water during exercise only slightly helps to reduce body temperature and heart function (pumping blood to muscles) as compared to drinking a small volume of water. Drinking a carbohydrate drink will have an additive effect on reducing dehydration and overheating.
  9. During prolonged moderate-intensity exercise in a warm environment, ingestion of water alone attenuates the decline in maximal "spinting" power. Furthermore, ingestion of water plus carbohydrate attenuates the decline in maximal power more than water alone, whereas ingestion of carbohydrate alone does not attnuate the decline in maximal power compared to placebo.
    - the effects of drinking a carbohydrate drink (in reducing dehydration/overheating) can also be related toward maximal power.
  10. American football players display maximal power at the end of practice when fluid replacement is actively encouraged and dehydration prevented. Furthermore, the addition of carbohydrate to fluid replacement beverages attenuates the decline in maximal power at the end of practice during repeated bouts performed with little time for recovery.
    - drinking a carbohydrate drink is especially beneficial during exercise involving repeated bouts with short rest intervals (such as racquetball)