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These 10 signs of dehydration show up long before your mouth feels dry, from headaches and cramps to slower digestion and darker urine
Thirst is the body's last resort, not its first warning. By the time the brain registers a dry mouth and an urge to drink, several other systems have often been signaling a fluid shortfall for hours. Blood volume dips slightly, the kidneys start conserving water, muscles lose some of the electrolytes they need to fire correctly, and the brain itself can lose a small amount of fluid volume long before thirst kicks in. That delay is partly biological: the thirst response is triggered by a rise in blood sodium concentration and a drop in blood volume that only becomes noticeable after a measurable amount of fluid has already been lost.
Older adults, people on certain medications, and anyone distracted by exercise, heat, illness, or a busy workday often miss the thirst cue entirely, or feel it far later than the body would prefer. As people age, the thirst reflex itself weakens, which is part of why dehydration is more common and more dangerous in older adults even when they do not report feeling thirsty at all.
