We’ve long been told that drinking eight glasses—or roughly 2.5 liters—of water a day is the gold standard for hydration. However, according to a surprising scientific study, water may not be the most effective drink for keeping you hydrated.
Researchers at the University of St. Andrews in Scotland, publishing in the American Journal of Clinical Nutrition, found that milk—both whole and skim—outperforms water in terms of hydration index. In a comparison of 13 different beverages, milk ranked higher than plain water in terms of how well the body retains fluids over time.
Why? It all comes down to milk’s nutritional profile. Unlike water, milk contains electrolytes, natural sugars, protein, and a small amount of fat. These nutrients slow gastric emptying and help the body absorb and retain fluids more efficiently. In other words, milk doesn’t just hydrate—it sticks around.
While water is still the best choice for daily hydration under most conditions, milk may offer an edge in certain scenarios:
- Post-exercise recovery: Replaces both fluids and nutrients lost through sweat.
- For children and older adults: Helps support hydration in groups that are more vulnerable to dehydration.
- During mild dehydration, it encourages fluid retention when the body needs it most.
That said, milk isn’t a one-size-fits-all solution. People with lactose intolerance or dairy allergies should steer clear. While plant-based alternatives like soy or oat milk offer some hydration, they don’t provide the same fluid-retention benefits.
Bottom line? Don’t ditch your water bottle—but don’t overlook milk as a powerful, underappreciated hydrator, especially in the sweltering heat of summer. Just be sure your body tolerates it well before chugging a glass for hydration’s sake.