Hydration

As the heat is on us, and I have had problems in the past, I'm starting to investigate the necessary fluid intake. During my 43 mile bike ride today, I lost 3.5 pounds (1.7% of body weight) in spite of ingesting 36 ounces of fluid during the nearly 2.5 hour ride in today's weather. Not enough fluid intake. I'd like to keep weight loss during long training runs or rides at around 1% of body weight. So I should have had double the amount of fluid since the weight of 36 ounces of water is 2.35 pounds.

You might want to check out the attached article from the Journal of Sports Sciences, 2004, 22, 39–55, whose abstract I have reprinted.

Fluid and fuel intake during exercise

EDWARD F. COYLE
The Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, TX 78712, USA
Accepted 7 August 2003

"The amounts of water, carbohydrate and salt that athletes are advised to ingest during exercise are based upon their effectiveness in attenuating both fatigue as well as illness due to hyperthermia, dehydration or hyperhydration. When possible, fluid should be ingested at rates that most closely match sweating rate. When that is not possible or practical or sufficiently ergogenic, some athletes might tolerate body water losses amounting to 2% of body weight without significant risk to physical well-being or performance when the environment is cold (e.g. 5–10C) or temperate (e.g. 21–22C). However, when exercising in a hot environment(>30C), dehydration by 2% of body weight impairs absolute power production and predisposes individuals to heat injury. Fluid should not be ingested at rates in excess of sweating rate and thus body water and weight should not increase during exercise. Fatigue can be reduced by adding carbohydrate to the fluids consumed so that 30–60 g of rapidly absorbed carbohydrate are ingested throughout each hour of an athletic event.

Furthermore, sodium should be included in fluids consumed during exercise lasting longer than 2 h or by individuals during any event that stimulates heavy sodium loss (more than 3–4 g of sodium). Athletes do not benefit by ingesting glycerol, amino acids or alleged precursors of neurotransmitter. Ingestion of other substances during exercise, with the possible exception of caffeine, is discouraged. Athletes will benefit the most by tailoring their individual needs for water, carbohydrate and salt to the specific challenges of their sport, especially considering the environment’s impact on sweating and heat stress."