Workout: A 21-mile run at Ironman race pace.
Hydration—Part I: Drank fluid every 10 to 11 minutes for the first hour and 41 minutes of the run.
Effect on temperature: Body temperature stayed at 100.7 degrees, the same temperature as day one, as long as Alexander kept drinking.
Hydration—Part II: Stopped drinking fluid for the next 42 minutes.
Effect on temperature: Body rose to 101.3 degrees.
Analysis: Although Steinmetz didn’t measure weight during this session, he thinks the run’s temperature data has real meaning and consequences for Alexander’s race in Kona. No matter how good he feels during the race, Alexander must keep drinking regularly or else his body temperature will increase the instant he loses 2 percent of his body mass—and he won’t be able to get it under control. A 42-minute lapse in attention—like he had during the training run—will start Alexander on an irreversible spiral of dehydration and overheating that will sap his strength.
In short, the first two tests showed that Alexander’s body is strong and stable at 100.7 degrees. However, losing about 2 percent of his body mass kicks his core temperature up to 101.3 degrees, and it might continue to rise as dehydration worsens.