The absolute best replenishment drink is Shaklee's Performance. After reading this excerpt, please take a look at the bottom for research Performance has in it's corner.
Actually under exteme circumstances drinking water can actually cause death during long periods of strenuous exercise by diluting the sodium content withing the body. This is called Hyponatremia.
It's a relatively newly identified condition that is found predominantly in athletes who participate in endurance activities (distance running, cycling, adventure racing, triathlons, X-terras etc.).
Hyponatremia occurs as a result of over-hydrating (ie. drinking so much fluid that the athlete lowers the amount of sodium in the blood). Most at risk for hyponatremia are endurance athletes who participate in long and arduous bouts of exercise and those with a low body mass index.
Generally, slower athletes are more at risk because they take the time to stop at rest stops, at water stations etc. during races. Faster and more competitive athletes are usually the opposite - although they need to drink (because losing even 2% of your body weight from fluid loss can adversely affect athletic performance), they typically finish races somewhat dehydrated.
My guess is that this condition has finally hit the radar because of the large number of "regular folks" - slower runners and walkers - who have begun participating in marathons over the past few years.
Here's an article on the condition:
http://www.runnersworld.com/article...-0-7958,00.html
The only other thing I would add is that I don't want to scare people into not drinking during exercise. Hyponatremia exists, but dehydration is a MUCH more common and easily preventable condition - so drink.
In the mid 1980s, Shaklee was asked to help with the Daedalus Project, in which a cyclist would pedal a super-lightweight plane 115 km (72 miles) over the Sea of Crete. Originally, Shaklee was to provide nutritional counselling to the team of pilots during their intensive training. When it was discovered that no sports drink on the market (including Gatorade) could meet these athletes’ extreme energy and fluid needs, Shaklee scientists developed Performance®. As a result, the Daedalus pilot pedalled between the Greek islands of Crete and Santorini in less than four hours – a record that still stands today.
SHAKLEE PERFORMANCE®
The Physiology of Human-Powered Flight. Weiss. Science News 1990;137:140.
The Effects of Sodium: Carbohydrate (Na: Carb) Ratios in Rehydration beverages (RB) on Plasma (PG), Osmolality (PO), Volume (PV), and Subject Tolerance (ST). FASEB J 1990;4:A381 (abstract).
The Daedalus Project: Physiological Problems and Solutions. Nadel, Bussolari. Am Scientist 1988:76:351-60.
Glucose-Electrolyte Ingestion: Influence on Endurance Factors in Highly Trained Female Cyclists. Sirotniak, Foster. Med Sci Sports Exer Suppl 1992;24:S121 (abstract).
Vascular Uptake of Rehydration Fluids in Hypohydrated Men at Rest and Exercise. Greenleaf, Geelen, Jackson et al. NASA Technical Memorandum 1992:1039-42.
Vascular Uptake of Rehydration Fluids in Resting Hypohydrated Men. Greenleaf, Geelen, Saumet et al. FASEB J 1991;5:A1147 (abstract).
Hypervolemia in Men from Drinking Hyperhydration Fluids at Rest and During Exercise. Greenleaf, Looft-Wilson,Wisherd et al. NASA Technical Memorandum 1994:46-57.
Drink Composition and Cycle-Ergometer Endurance in Men: Carbohydrate, Na+, Osmolality. Greenleaf, Looft-Wilson, Wisherd et al. NASA Technical Memorandum 1994:46-50.
CD4+/CD8+ T-Lymphocyte Ratio: Effects of Rehydration Before Exercise in Dehydrated Men. Greenleaf, Jackson, Lawless. Med Sci Sports Exer 1995;27:194-9.
Drinking-Induced Plasma Vasopressin and Norepinephrine Changes in Dehydrated Humans. Geelen, Greenleaf, Keil. J Clin Endocrinol Metab 1996;81:2131-35.
Hypervolemia in Men from Fluid Ingestion at Rest and During Exercise. Greenleaf, Looft-Wilson, Wisherd et al. Aviat Space Environ Med 1998;69:374-86.