Skim milk is the most hydrating common beverage, retaining roughly 44% more fluid in your body than plain water over a few hours. That finding comes from the Beverage Hydration Index (BHI), a scoring system that uses still water as a baseline of 1.0 and measures how much fluid your body actually keeps after drinking. Skim milk scores 1.44, followed by oral rehydration solutions at 1.50 (a medical product, not a typical drink), full-fat milk at 1.32, and orange juice at 1.39. The answer surprises most people, but the reasons behind it are straightforward.
Why Milk Hydrates Better Than Water
Water moves through your stomach quickly. It’s absorbed fast, but it also triggers your kidneys to start producing urine relatively soon after you drink it. Milk slows that whole process down in two ways.
First, milk contains sodium and potassium, two electrolytes that help your body hold onto fluid instead of flushing it out. Sodium is the main electrolyte in the fluid outside your cells and plays the biggest role in regulating water balance. When you consume it alongside water, your kidneys get the signal to retain more of what you drank. Second, milk contains protein, fat, and a moderate amount of natural sugar (lactose). These nutrients slow gastric emptying, meaning the liquid sits in your stomach longer and trickles into your small intestine at a steadier pace. That gives your body more time to absorb the fluid instead of overwhelming your kidneys all at once.
A study published in the British Journal of Nutrition tested this directly after exercise. Participants who drank low-fat milk produced about half the urine (roughly 611 mL) over four hours compared to those who drank water or a sports drink (about 1,184 to 1,205 mL). That’s a dramatic difference in fluid retention from a drink most people already have in the fridge.
How the Beverage Hydration Index Works
The BHI doesn’t measure how fast a drink hydrates you. It measures how much of what you drank is still in your body a few hours later. Researchers give participants a set volume of a test beverage, then collect their urine for the next several hours and compare it to what happens after drinking the same amount of still water. A score above 1.0 means the drink keeps more fluid in your body than water does.
Here’s how common drinks rank after adjusting for water content:
- Oral rehydration solution: 1.50
- Skim milk: 1.44
- Full-fat milk: 1.32
- Orange juice: 1.39 (unadjusted; not statistically different from water after adjustment)
- Still water: 1.0 (baseline)
The key takeaway: water is not a poor hydrator. It’s the standard. Everything else is measured against it. Drinks that score higher simply stay in your body longer, which matters most when you’re already dehydrated or losing fluid through sweat.
Oral Rehydration Solutions: The Medical Option
Oral rehydration solutions (ORS) score highest on the hydration index and exist for exactly that purpose. The World Health Organization’s formula uses a precise 1:1 ratio of sodium to glucose, both at 75 millimoles per liter. This ratio works because glucose and sodium are absorbed together in your small intestine through the same transport system, and water follows them through osmosis. Without the glucose, your gut can’t absorb sodium efficiently at lower concentrations. Without the sodium, the fluid doesn’t stay in your body.
You can make a simple version at home: combine 4 cups of water with half a teaspoon of table salt and 2 tablespoons of sugar. This won’t taste great, but it’s effective when you’re dealing with illness, diarrhea, or significant dehydration. Commercial versions like Pedialyte follow the same principle with added flavoring.
What About Sports Drinks and Coconut Water?
Sports drinks performed no better than plain water in the British Journal of Nutrition rehydration study, producing nearly identical urine volumes. They contain sodium and carbohydrates, but typically not enough to meaningfully slow fluid loss the way milk does.
Coconut water is often marketed as nature’s sports drink, and it does contain impressive potassium levels: about 404 mg per cup compared to just 37 mg in a cup of Gatorade. But it’s lower in sodium (64 mg vs. 97 mg), and sodium is the electrolyte that matters most for fluid retention. One small study found coconut water matched a sports drink for maintaining hydration during prolonged cycling, but for serious rehydration needs, drinks with higher sodium and carbohydrate content tend to work faster.
Sugar Can Work Against Hydration
Not all sugary drinks are equal. When a beverage’s sugar concentration climbs above 8 to 10%, it actually slows gastric emptying to the point where it delays hydration rather than helping it. Worse, strongly sugary (hypertonic) drinks pull water from your bloodstream into your gut to dilute the sugar before it can be absorbed. This can cause bloating, cramping, and even diarrhea, which makes dehydration worse.
Fruit juices, sodas, and energy drinks often exceed that 8 to 10% sugar threshold. Orange juice scored reasonably well on the BHI before statistical adjustment, but after correction, it wasn’t significantly different from water. The lesson: a little sugar helps hydration by activating sodium transport, but a lot of sugar hurts it.
Coffee and Tea Are Not Dehydrating
The idea that caffeinated drinks dehydrate you is one of the most persistent hydration myths. Research consistently shows that caffeine in moderate doses, around 3 mg per kilogram of body weight (roughly 200 mg for a 150-pound person, or about two cups of coffee), does not disrupt fluid balance. At that dose, urine output is no different from drinking plain water.
The diuretic effect kicks in at higher doses, around 6 mg per kilogram of body weight or above 250 mg of caffeine. At that level, your kidneys do flush out more fluid. For context, that’s roughly three to four cups of strong coffee consumed in a short window. If you’re a regular coffee drinker, your body also adapts to caffeine over time, further blunting any diuretic effect. Your morning coffee counts toward your daily fluid intake.
Sparkling Water Hydrates the Same as Still
Carbonation doesn’t change how well water hydrates you. Research shows no significant difference in fluid retention or urine output between sparkling and still water. If you find it easier to drink enough water when it has bubbles, there’s no hydration penalty for choosing sparkling. The general daily target is roughly 11.5 to 15.5 cups of total fluid, depending on your size, activity level, and climate.
Choosing the Right Drink for the Situation
For everyday hydration when you’re not sick or heavily sweating, water is perfectly effective. It’s calorie-free, widely available, and your body handles it well. The BHI difference between water and milk matters most in specific situations: recovering from exercise, rehydrating after illness, or when you know you won’t be able to drink again for several hours and want what you consume to last.
If you’re recovering from a workout, a glass of milk does double duty by providing fluid retention plus protein for muscle recovery. If you’re dealing with a stomach bug or diarrhea, an oral rehydration solution is the strongest option. For a long meeting or a flight where bathroom access is limited, milk or a drink with some sodium and protein will stay with you longer than a bottle of water.
The most hydrating drink overall is the one that combines electrolytes (especially sodium), a small amount of sugar, and some protein or fat to slow absorption. Skim milk hits all three of those marks naturally. But the best hydrating drink on any given day is the one you’ll actually finish.