When we feel thirsty, our first instinct is to reach for a glass of water. It’s often considered the gold standard of hydration—but is it actually the best drink to keep us hydrated? According to a fascinating study by researchers at Scotland’s St. Andrews University, the answer may surprise you.The 2016 study, published in The American Journal of Clinical Nutrition, found that while plain water—both still and sparkling—is effective at hydrating the body quickly, it's not necessarily the most effective at keeping us hydrated over time. Beverages that contain a mix of nutrients—like milk or oral rehydration solutions—may do a better job.Why Some Drinks Hydrate Better Than OthersHydration isn’t just about how quickly fluid enters your system. It also depends on how long that fluid stays in your body. Professor Ronald Maughan, who led the study at St. Andrews’ School of Medicine, explained that two key factors influence how hydrating a beverage is: volume and nutrient composition.First, the volume of a drink plays a crucial role in how fast it empties from the stomach into the bloodstream, where it can start rehydrating body tissues. The more fluid you drink, the quicker it’s absorbed—up to a point.Second, drinks that contain small amounts of sugar, fat, protein, or electrolytes can actually slow down gastric emptying. That might sound like a bad thing, but in terms of hydration, it’s beneficial. It means fluids stay in your system longer, which helps you retain water and stay hydrated for extended periods.Milk Tops the ListSurprisingly, skim milk was ranked as the most hydrating beverage in the study. Its mix of lactose (a natural sugar), protein, and fat slows the rate at which fluid leaves the stomach. Additionally, milk contains sodium, an electrolyte that helps the body retain fluid and reduces urine output.“Milk’s nutrient profile essentially acts like a hydration booster,” said registered dietitian Melissa Majumdar, who was not involved in the study but reviewed its findings. “It offers a balance of electrolytes and macronutrients that support fluid retention.”Hydration Rankings: What the Study FoundThe research team tested 13 common beverages to determine how well they hydrated the body over a four-hour period. Here’s the ranking, from most to least hydrating:Skim milkOral rehydration solutions (e.g., Pedialyte, Liquid I.V.)Whole milkOrange juiceColaDiet colaCold teaTeaSports drinkStill waterSparkling waterLagerCoffeeWhile oral rehydration solutions are designed for medical use—such as rehydrating patients with diarrhea—they include optimal amounts of sodium, potassium, and sugar to help the body absorb water efficiently.Not All Sugars Are Created EqualIt might be tempting to think fruit juices and sodas are good hydrating options due to their high liquid content, but the reality is more complex. While these beverages may linger in the stomach slightly longer than water, their high sugar concentrations can pull water into the intestine via osmosis. That can actually reduce your body’s net fluid gain.And unlike solid foods, sugary beverages add calories without satiety. “Juices and colas might quench your thirst briefly, but they don’t support long-term hydration and contribute excess sugars,” Majumdar warned.What About Coffee and Alcohol?Contrary to popular belief, coffee can contribute to your daily hydration—up to a point. A cup of coffee with around 80 milligrams of caffeine hydrates you about as well as water. However, drinking more than 300 milligrams (roughly 2–4 cups) could tip the scale, causing mild fluid loss, especially in those unaccustomed to caffeine.Alcohol, on the other hand, is not your hydration friend. Strong alcoholic beverages like whiskey act as diuretics, increasing fluid loss. But a lower-alcohol beverage like beer may have a less dehydrating effect, primarily because it contains more water. “Beer would result in less water loss than whiskey,” Maughan explained. “Strong alcoholic drinks will dehydrate, dilute alcoholic drinks will not.”Do You Need to Rethink Your Beverage Choices?In everyday life, most people can rely on thirst cues to guide their fluid intake. However, for certain groups—like athletes training in the heat or individuals working long hours without access to fluids—strategic hydration becomes crucial.While water remains an excellent and essential drink, it’s worth considering what else you’re sipping. Milk, rehydration solutions, and even some teas might offer better hydration under certain circumstances. But the takeaway is clear: hydration is about more than just drinking water—it's about how your body processes what you drink.