What does the research say? A succinct summary (Found by The Machine) of the research was provided by one of the members of Crossfit LA: The research seems to acknowledge that fluid loss and core temperature vary (pretty drastically) from person to person based on body size, clothing, amount of sweating, genetic predisposition, and metabolic rates so they can't make any quantitative recommendations. They define dehydration as losing 2% or more of one's body weight in sweat/fluids and showed that it causes cognitive decline as well as degrading aerobic exercise performance (via increased core body temperature, altered metabolic function, cadriovascular strain), but it has less of an effect on muscle strength (people losing up to 5% of their body weight in water maintained almost comparable strength abilities). This also varies widely from person to person (defined as dehydration tolerance). Prior to exercising they recommend people drink 5-7ml/kg (per kg body weight) for at least 4 hours prior to exercising. They also say eating small amounts of salted snacks 2-4 hours prior to exercising will help people retain water during their workout. During exercise they recommend replacing fluid to maintain less than a 2% loss of body weight in sweat. They have a chart with some examples, i.e. an average 70kg man running at 10km/hour needs to drink 800ml per hour to maintain an appropriate hydration. Carbohydrate consumption at a rate of 30-60g/hour has been shown to sustain glucose levels and sustain exercise performance levels. They recommend weighing oneself before and after a workout to determine the amount of fluid lost. After exercise they recommend drinking 1.5L of fluid for every kg of body weight lost, but it doesn't have to be all at once because you'll just pee it all out, so gradually is okay.