Before we proceed, RunSociety would like to thank Hammer Nutrition for being so kind as to allow us to republish part of their extensive library of research material in the broad area of nutrition. The following is an article written by Steve Born who’s had nearly 15 years of nutritional fuelling and supplementation research under his belt. Here’s what he says about hydration.

Keep Fluid Intake during Exercise between 470 to 830 ml per Hour

There’s probably more misinformation on the subject of hydration than any other aspect of fuelling, which is really bad because over hydration also presents the most serious physiological consequences of any fuelling issue. Acute over hydration can cause hyponatremic (low sodium) induced coma and death.

In general, most athletes, under most conditions, will satisfy hydration needs with a fluid intake in the range of 590 to 740 ml/hour – roughly the equivalent of a standard sized small or large water bottle. Lighter athletes and/or athletes exercising in cool weather conditions may only require an intake of 470 to 530 ml/hour. Larger athletes and/or athletes exercising under very hot and humid conditions are the ones that can consider fluid intakes at the high end of that range (830 ml/hour), perhaps even upwards of up to 890 ml/hour on occasion. Sure, you can sweat more than that, but you cannot physiologically replace it ounce-for-ounce.

Regular fluid intake over 890 to 1005 ml hourly really increases the potential for serious performance and health problems, so keep that in mind before you indiscriminately gulp down excessive amounts of fluid. If you override your internal mechanisms, you’ll find out the hard way how your body deals with excess water intake during intense exercise. Unless you enjoy nausea, bloating, and DNFs, forget advice like “drink to replace” or “drink even when you’re not thirsty” – it’s just plain wrong.