Our body has a set of complex thirst signals that include blood pressure, hormones – especially angiotensin II – and areas of the brain including the lamina terminalis. Some neurons of the lamina terminalis that are outside of the blood-brain barrier, monitor the fluid in the third ventricle “to determine its osmolality and the amount of sodium present” – which provides a snapshot of whole body hydration.
“Changes in blood osmolality correlate well with the subjective feeling of thirst in humans, and increased blood osmolality is probably the most important homeostatic signal for drinking in everyday life.”
Osmolality is the relative concentration of chemical particles found in fluid. Blood osmolality, the balance of fluid and charged minerals or electrolytes, is key to hydration. Electrolytes including sodium, calcium, potassium, magnesium, chloride, in certain concentrations inside and outside of cells are essential for the flow of water and other molecules, nutrients and waste.
The water-mineral balance is dynamic! Too much water may dilute electrolytes and, ironically, cause dehydration. Exercise, sweat, activity, and illness will all affect the balance as well as coffee or tea, and other known diuretics. Sports drinks may provide minerals, but the sugar will deplete them. Certain water filtration systems, such as reverse osmosis, may strip the electrolytes/minerals from the water. Unless re-mineralized, RO water may actually pull minerals from the body.
Some common signs of adult dehydration include headache, dark urine, muscle cramps, palpitations, lightheadedness, skin ‘tenting’ and more. Depending on the situation, a serum osmolality test may be useful. A 2015 Cochrane review used “serum osmolality of greater than 294 mOSm/kg to define dehydration.”