People underestimate dehydration. The symptoms are extremely subtle. Combined with any other form of physical exhaustion, dehydration can trigger extreme responses in the body. The US military has had to deal with 20 year olds collapsing from what is sometimes called heat stroke, but heat stroke often has a large degree of dehydration attached to it.

If you get older and start developing the slightest bit of elevated glucose and/or glucose resistance (which can start years prior to developing diabetes) then even mild dehydration can lead to the body mishandling blood sugar levels.