Mistake #6: Overhydrating or drinking only water in the race.
Often athletes go into “must hydrate as much as possible” panic mode when it gets hot, but drinking too much (especially only plain water) is also a risk and can lead to hyponatremia, a condition resulting from too much fluid and not enough sodium in the blood. Hyponatremia can lead to, ironically, similar GI issues brought on by dehydration, such as nausea and cramping.
Sports dietitian Lauren Antonucci suggests taking in an average of 800–1000mg per hour of sodium during a race, using electrolyte beverages and/or salt tablets. Also, do an at-home sweat test in similar conditions to those you’ll be racing in. Weigh yourself nude. Work out at race pace for an hour. Empty bladder and weigh yourself nude again. If you lost two pounds (32 ounces), your sweat rate is 32 ounces per hour and you should aim to replace 60–80 percent of that.