Does Milk Hydrate Better Than Water? What the Beverage Hydration Index Actually Found

Yes, milk hydrates you—and in the one well-known controlled trial that measured it, a liter of milk kept more fluid in the body over the following four hours than a liter of plain water did. That finding is real, it is often quoted, and it is also routinely stretched past what the study actually showed. Milk counts fully toward your daily fluid intake and holds on to water a bit better than plain water does in the short term; it is not a reason to swap your water bottle for a milk jug.

What the Beverage Hydration Index Study Did

In 2016, researchers led by Ronald Maughan published a randomized trial in the American Journal of Clinical Nutrition that set out to rank common drinks by how much of them the body retained. Seventy-two healthy men each drank one liter of a test beverage over 30 minutes, then had their urine collected for four hours. Each drink's total urine output was compared with the output after a liter of still water, and the result was expressed as a “beverage hydration index” (BHI): a score above 1.0 means less urine, and therefore more fluid retained, than water.

Thirteen drinks were tested. Cola, diet cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water, and a sports drink all produced urine outputs that were not different from still water—in other words, they hydrated about as well as water did, no better and no worse. Three drinks did better: an oral rehydration solution (BHI about 1.54), full-fat milk (about 1.50), and skimmed milk (about 1.58). Total urine over four hours after either milk was roughly a liter, compared with more than that after water.

Why Milk Holds Water Better

The explanation is not mysterious. Milk carries sodium and potassium, which the kidneys use as a signal to hold on to water rather than flush it, and it delivers protein, fat, and lactose that slow how quickly the drink leaves the stomach. A liter of plain water on an empty stomach empties fast, gets absorbed fast, dilutes the blood slightly, and triggers the kidneys to shed the excess promptly. Milk trickles in and gets held longer. That is the same mechanism that put the oral rehydration solution at the top of the list, and it is why skimmed milk scored as well as whole milk: the electrolytes and protein are doing the work, not the fat.

What the Study Does Not Say

It measured resting, already-hydrated men over four hours. The trial was designed to compare drinks under one controlled condition. It did not test people who were dehydrated, sweating through a workout, or drinking over a whole day, and the authors were careful to frame the index as a short-term measure.

“Retains more” is not “hydrates better for you.” A liter of whole milk is around 600 calories and a liter of skim about 340; a liter of water is zero. The National Academies' adequate-intake reference for total water is about 3.7 liters a day for men and 2.7 for women, with roughly 80 percent coming from beverages. Meeting that with milk would be an enormous calorie load. Meeting it with water costs nothing.

The ordinary drinks were not worse than water. The headline is usually “milk beats water,” but the quieter finding is arguably more useful: coffee, tea, juice, and sparkling water all held fluid about as well as still water. Moderate coffee is not dehydrating you, and neither is your sparkling water.

How to Use This

Count milk toward your daily fluid: a glass with breakfast or after a workout is real hydration, and after exercise its protein and electrolytes are a bonus rather than a problem. For the rest of the day, plain water remains the sensible default—calorie-free, cheap, and, per this same study, on par with almost every other common drink for how much of it you keep. The practical routine is a glass of milk where it fits your meals and a refillable bottle for everything in between.

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This post is for informational purposes only and does not constitute medical or nutritional advice. People with lactose intolerance, milk allergy, or kidney conditions should follow their clinician's guidance. Individual hydration needs vary.

Sources: Maughan et al., A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index, American Journal of Clinical Nutrition 103(3):717–723 (2016) — 72 men, 1 L over 30 min, 4-h urine; BHI ORS 1.54, full-fat milk 1.50, skimmed milk 1.58; cola, diet cola, tea, coffee, lager, orange juice, sparkling water, sports drink not different from water (pubmed.ncbi.nlm.nih.gov/26702122) • National Academies, Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005) — AI 3.7 L/day men, 2.7 L/day women, ~80% from beverages (nationalacademies.org) • NuRich product specifications