Water is by far the most common substance in the human body, making up around 60% of mass in the average lean adult. During exercise, or hotter temperatures, the rate of water loss to the atmosphere can be as high as 2L/h and can exceed total losses of 10-12L in prolonged exercise or heavy manual work in heat. This equates to a quarter to a third of the total water volume in the body.
Effects of dehydration are thirst, confusion, fatigue, cramp, and reduced performance/concentration. At more severe levels, heart rate can soar to dangerous levels and become arrhythmic, and nausea/vomiting/diarrhoea can occur.
Rehydrating with an electrolyte drink at the rate of 1-2L/hr, though mixing with powders tends to be better than commercially prepared drinks, as they are made too high in sugar and can cause rebound hypoglycaemia, further effecting performance. If “Store Bought” drinks are all that is available, dilute by about 2:1 with water for best results.
Make sure you keep drinking water during these hot conditions!