Could Milk Be a Superior Hydration Choice Compared to Water?

A recent billboard claiming “Milk hydrates better than water” grabbed the attention of a Harvard Medical School physician, sparking renewed interest in the evidence behind this statement. Though the advertisement was funded by the dairy industry, there is scientific backing for the claim that milk can maintain body hydration for longer periods than plain water, albeit with important considerations for everyday hydration habits.

The advantage of milk is most noticeable in the initial hours after drinking. In a 2016 publication in The American Journal of Clinical Nutrition, researchers introduced the Beverage Hydration Index to compare hydration effects of various drinks. Over a four-hour period, both skim and whole milk generated significantly less urine than water, resulting in a higher hydration score, comparable to specially designed oral rehydration solutions used medically to restore fluids lost through diarrhea.

Milk retains fluid longer than water thanks to its electrolytes and key nutrients. Image credit: Shutterstock

This is largely due to milk’s distinctive nutritional makeup. Unlike water, which quickly empties from the stomach, milk's combination of lactose, protein, and fat acts to slow gastric emptying. This gradual release allows the body to absorb fluids over an extended period, preventing rapid excretion through urine.

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The Role of Sodium in Fluid Retention

Electrolytes also play a crucial role. Milk naturally contains sodium and potassium, minerals essential for holding water within the body’s tissues and bloodstream. Sodium functions somewhat like a sponge, minimizing urine production by the kidneys. According to Melissa Majumdar, a registered dietitian representing the Academy of Nutrition and Dietetics, interviewed by CBS News, “Electrolytes such as sodium and potassium enhance hydration, while the calories in beverages slow gastric emptying, delaying the urge to urinate.”

These physiological effects are supported by a series of small trials. For example, a study led by Susan Shirreffs in 2007, published in the British Journal of Nutrition, showed that people who rehydrated with low-fat milk or sodium-enriched milk after dehydration maintained a positive fluid balance for five hours. Conversely, those drinking water or sports drinks reverted to a negative fluid balance within an hour.

Sodium retains water in tissues, lowering kidney urine output and extending hydration time. Image credit: Shutterstock

Similarly, a 2016 investigation from the University of Limerick confirmed that milk consumption after exercise-induced dehydration restored whole-body fluid status more effectively than carbohydrate-electrolyte beverages or water. The study concluded that the combination of nutrients in milk enhanced fluid retention efficiency.

Researchers from Pennsylvania State University in a 2020 study published in Nutrients evaluated milk permeate, a filtered milk product containing milk’s carbohydrates and minerals but excluding fat and proteins. It achieved a significantly higher Beverage Hydration Index than water and sports drinks after four hours, with the lowest urine volume recorded, underscoring the critical role of milk’s electrolyte content in maintaining hydration.

Calories Present a Challenge for Daily Hydration

Despite these promising findings, specialists from Harvard Health and other authorities advise caution when applying these results to routine hydration. Most studies feature limited participants and observe effects over short intervals, so it is uncertain if milk’s benefits would remain significant over an entire day or yield notable health gains for non-dehydrated individuals.

A major limitation is the caloric content. The quantities of milk used in trials to achieve better hydration often contain more than 1,000 calories. While such calories can be helpful for endurance athletes expending large amounts of energy, they become a burdensome addition for typical adults engaging in moderate physical activity or sedentary tasks, possibly hindering weight control and general wellness.

The volume of milk needed for hydration benefits packs over 1,000 calories, making it impractical for everyday consumption. Image credit: Shutterstock

Given these points, health experts still advocate water as the ideal drink for regular hydration. Relying on thirst cues is an effective approach for most healthy individuals. The body’s innate thirst mechanism reliably signals fluid requirements except in extreme situations like prolonged heat exposure or illness causing severe fluid loss.

Milk has potential as a post-exercise recovery beverage to replenish both fluids and nutrients after intensive activity leading to dehydration. For the general population, however, water remains the calorie-free, accessible choice that supports kidney function, joint health, and cell maintenance without added sugars or fats. Harvard Health emphasizes that tap water is free and does not introduce extra caloric intake, unlike milk.