Plain water rushes into the bloodstream but the body often sends it back out in urine. Drinks that combine electrolytes and glucose, such as skim milk or oral rehydration solutions, keep more fluid in the body over a longer period.
How the body handles fluids
The stomach empties liquid at different rates depending on its composition. Pure water leaves the stomach quickly, but its low osmolality can dilute blood plasma, prompting a diuretic response. A drink that matches blood osmolality or contains sodium and glucose slows gastric emptying and promotes retention.
Osmolality matters
Body fluids are about 285–295 mOsm/kg. Hypotonic drinks are less concentrated and are absorbed fast; isotonic drinks are similar to plasma and balance speed and retention; hypertonic drinks are more concentrated and can slow gastric emptying but may activate active transport pathways.
The beverage hydration index (BHI)
In a controlled study, participants drank one liter of various drinks and urine output was measured for two hours. Water was set at 1.0. Drinks scoring above 1.0 retained more fluid than water; below 1.0, less.
BHI results
- Water (still or sparkling): 1.0
- Standard sports drinks: ~1.0
- Coffee & tea: ~1.0 (caffeine’s diuretic effect is offset by water volume)
- Orange juice: slightly above 1.0 but higher in calories
- Skim milk & oral rehydration solutions: 1.5–1.6 (≈50% more fluid retained)
Why milk and ORS outperform water
Milk contains protein, fat, and sodium, which slow gastric emptying and provide a steady release of fluid. Oral rehydration solutions mix glucose and sodium in precise ratios that activate the SGLT‑1 transporter, pulling water into the bloodstream efficiently.
Choosing the right drink for you
If you need rapid hydration with minimal calories, plain water or a balanced sports drink is fine. For longer retention, especially after prolonged exercise or heat exposure, skim milk or a properly formulated oral rehydration solution is superior. Avoid high‑sugar drinks when fluid retention is the priority.