Which of the following is a health risk when endurance training in the heat?

Hyperthermia is a significant health risk associated with endurance training in hot conditions. During prolonged physical activity in the heat, the body's temperature regulation becomes stressed as it is challenged to dissipate heat effectively. The primary mechanism for cooling the body is through sweat production; however, excessive heat can overwhelm this system, leading to an increase in core body temperature. This can result in serious conditions such as heat exhaustion or, in severe cases, heat stroke.

The other choices also relate to the physiological responses to heat during endurance training. An increase in heart rate is a normal response to both heat and exercise, as the body works harder to supply oxygen and remove waste products. Increased sweat production is a critical response to heat exposure, allowing for cooling through evaporation. Hypothermia, however, is unrelated to endurance training in the heat, as it occurs in cold environments when the body loses heat faster than it can produce it. Therefore, hyperthermia stands out as the primary health risk in this context.