You've probably made a similar evaluation during oppressively warm weather: "It's not so much the heat, but the humidity." The statement, while almost a cliché, recognizes that the temperature reading on a thermostat isn't the only measure of how you feel when Mother Nature bears down. The heat index is a more scientific and precise accounting of the forces that create discomfort or, alternatively, conditions that feel pleasant to the human body.
The heat index uses 25 percent humidity as a baseline. If, for example, it's 100 degrees Fahrenheit (37.78 degrees Celsius) outside and humidity is at 100 percent, you're experiencing the equivalent of a 195 degree day (or 90.56 degrees Celsius) at 25 percent humidity. That's a statistic that gets people's attention and helps them appreciate the dangers of exertion in such an environment -- which is the intent of the heat index.
Hot weather forces your body to work harder and burn more calories [sources: Mirkin; Popke]. Your heart is increasingly called upon to distribute overheated blood to the skin for cooling, the liver and kidneys are taxed to a greater degree, and vital water -- even your muscles are made of more than 70 percent water -- is lost through sweating at a much faster rate [sources: Ryan; Mirkin; Popke].
All of these factors can lead to everything from cramping and nausea to the shutdown of organs, heatstroke and even death [sources: Popke; Weill Cornell Medical College]. Undertrained athletes and the elderly are at particular risk, but the threat of sickness and death is very real, even for fit, elite and professional athletes [sources: Popke; Weill Cornell Medical College].
Click ahead to learn how you can alter your nutrition for optimum health and safe performance in less-than-ideal conditions.