Milk More Hydrating Than Water? Experts Weigh In

A glass of milk can keep your body holding onto fluid longer than a glass of plain water, according to several controlled studies that measured urine output after each drink.

Quick Take

A 2015 randomized trial found milk and an oral rehydration solution beat still water at reducing urine loss over several hours.
Researchers say milk’s natural sugar, protein, fat, and sodium slow how fast fluid leaves the stomach and body.
The findings come from small, short-term lab studies, not everyday drinking habits or long-term health outcomes.
Experts still recommend water as the default daily drink, with milk as a helpful option after exercise or heat exposure.

What the Landmark Study Found

The core evidence comes from a 2015 trial that created something called a hydration index. Researchers gave volunteers different drinks, then tracked how much urine they produced over several hours. Oral rehydration solution scored 1.53, full-fat milk scored 1.32, and skimmed milk scored 1.44, all beating plain water’s baseline score of 1.0. A separate 2016 study in The American Journal of Clinical Nutrition backed this up, showing skim milk, whole milk, and orange juice all outperformed water in the same kind of test.

A 2007 study out of the United Kingdom pointed in the same direction years earlier. Eleven volunteers exercised until they were noticeably dehydrated, then rehydrated with different drinks on separate days. Researchers tracked their urine output for the next five hours. Milk consistently led to less fluid loss than water did, giving early support to what later, larger trials would confirm.

Why Milk Holds Onto Water Longer

The explanation boils down to what’s actually in the glass. Milk carries lactose, a small amount of protein, some fat, and sodium. Together, these slow how quickly the stomach empties fluid into the bloodstream, stretching out the hydration effect over a longer window. Sodium in particular acts almost like a sponge, helping the body hang onto water instead of flushing it out through urine right away.

A newer study on a milk-based hydration drink found similar results. Researchers gave people a milk permeate solution and compared it to water and a standard sports drink. The milk-based option produced significantly less cumulative urine output over four hours than both other drinks, and it performed close to a dedicated rehydration solution. That consistency across different research teams and different milk products is what makes the finding hard to dismiss outright.

The Catch Researchers Want You to Remember

None of this means water is a bad choice or that milk should replace it at every meal. Harvard Health notes that even with these results, milk’s taste, calories, and texture make it a poor everyday swap for most people. Critics have also pointed out that some of the underlying research involved small, narrow groups of participants, which limits how confidently the results apply to everyone.

There’s also a fair distinction between “retained more fluid” and “hydrated better” in a meaningful, everyday sense. One nutrition-focused critique argued that researchers measured fluid retention but couldn’t always confirm that fluid was purely water rather than other substances tied to digestion. That’s a reasonable caveat, not a takedown of the underlying data, and it’s worth keeping in mind before anyone swaps their water bottle for a milk carton.

What This Means for Your Daily Routine

The practical takeaway is simpler than the debate makes it sound. Water remains the smart default for daily hydration because it’s calorie-free, widely available, and easy on the stomach. Milk earns its place as a smart add-on after workouts, in hot weather, or when someone needs extra recovery support, since its nutrients appear to extend hydration rather than just quench thirst for a moment.

For parents packing lunches, athletes finishing practice, or anyone recovering from a long day in the sun, the research offers a useful reminder. Reaching for milk isn’t a gimmick pushed by dairy marketing. It’s backed by repeated, peer-reviewed testing showing real physiological effects, even if it will never fully replace the simple, reliable glass of water most Americans already trust.