Some sports drinks, and even vegetables claim to be 'more hydrating than water'. What does this even mean? How is 'hydration' even measured? If I ate enough cucumbers could I survive without ever drinking anything at all?
They seem to measure hydration by seeing how much of the fluid stays in the body instead of being secreted.
For milk there a study with describes the process:
Urine samples were collected before and for 5 h after exercise to assess fluid balance. Urine excretion over the recovery period did not change during the milk trials whereas there was a marked increase in output between 1 and 2 h after drinking water and the sports drink. Cumulative urine output was less after the milk drinks were consumed (611 (sd 207) and 550 (sd 141) ml for milk and milk with added sodium, respectively, compared to 1184 (sd 321) and 1205 (sd 142) ml for the water and sports drink; P < 0·001). Subjects remained in net positive fluid balance or euhydrated throughout the recovery period after drinking the milk drinks but returned to net negative fluid balance 1 h after drinking the other drinks.