Why milk can outlast water in the body
Plain water is absorbed quickly, but it can also leave the body relatively quickly as urine. Milk supplies fluid along with small amounts of electrolytes—minerals such as sodium and potassium that help the body manage water—and nutrients including protein and lactose, its natural sugar.
That mixture can slow the movement of fluid out of the stomach and help the body retain more of it for longer. The effect is modest and depends on the person and circumstances; it does not mean water is inadequate or that milk is always the best drink.
What a hydration index measures
A beverage hydration index compares how much fluid people retain over a set period after drinking a beverage with how much they retain after drinking the same amount of water. Milk can score well because it replenishes fluid while its nutrients and electrolytes influence absorption and urine production.
The practical tradeoff is that milk also provides calories and may not suit everyone. For routine thirst, water remains a simple choice. Milk’s advantage is most relevant when the goal is to keep fluid in the body longer—not to make every glass of water less effective.
