Hydration: Milk is better than water, but not for everyone

We were told it repeatedly at school, at the doctor's office, and at the office between meetings: to stay healthy, we need to drink, ideally 6 to 8 glasses of water a day. Except that science sometimes throws a wrench into our plans. Several academic studies conducted since the late 2000s show that, under certain conditions, milk can maintain hydration longer than water, a surprising result because it challenges a simple, almost sacred notion: that water is the only answer to dehydration.

As early as 2007, dehydrated volunteers after physical exertion were used as test subjects: milk resulted in lower urine output than water and certain so-called energy drinks, and fluid balance remained positive for up to five hours after exercise, whereas it dropped to zero after about an hour with the other drinks tested. About ten years later, a team from the University of Limerick observed the same benefit in seven physically active young men. The signal is small but consistent, like a needle stubbornly returning to the same point.

When electrolytes act like sponges

The mechanism itself is less mysterious than it seems. The body doesn't just react to the volume ingested; it also reacts to its contents: sodium and potassium retain water, while calories, protein, and fat slow gastric emptying, thus slowing the rapid elimination of fluids. A 2016 study of 72 men, comparing a range of beverages from water and soda to coffee, beer, and orange juice, ranked whole milk, skim milk, and an oral rehydration solution as the most effective at promoting water retention over four hours. More recent research, published in 2020, confirms the advantage of milk over water for short-term hydration.

The crucial point is to clarify the distinctions that prevent misunderstandings, and that's important. These studies often involve small groups and short durations, a few hours at most, which limits the applicability of the conclusions to real-life situations, such as a full day with meals, travel, and temperature variations. And above all, "more hydrating" doesn't mean "drink without restraint": milk provides calories, rehydration solutions contain salt, and other drinks contain sugar. In public health recommendations, water therefore remains the go-to beverage for daily consumption, with a simple underlying principle: adapt intake according to physical exertion, heat, age, and health status—a topic that always resurfaces when the temperature rises and the question of prevention becomes very real.