You’ve probably been told since kindergarten that water is the absolute king of hydration. Drink eight glasses. Carry a gallon jug. If your pee isn’t clear, you’re basically a desert.
Honestly? It's not that simple.
While water is obviously essential, it turns out it’s not actually the most hydrating drink you can put in your body. If you’re just trying to survive a desk job, water is fine. But if you’re looking for what actually stays in your system the longest, science points toward a different fridge staple.
The Beverage Hydration Index (BHI) Explained
Back in 2016, a group of researchers at Scotland’s St. Andrews University decided to settle the "what is the most hydrating drink" debate once and for all. Led by Professor Ronald Maughan, they tested 13 common beverages on 72 men.
The goal wasn't just to see how much they drank. It was to see how much they kept.
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They created something called the Beverage Hydration Index (BHI). Think of it like a fuel efficiency rating for your body. Still water was given a baseline score of 1.0. If a drink scored higher, it meant the body retained more of that fluid after two to four hours compared to plain old H2O.
The Shocking Winner
When the results came in, the top spot didn't go to water. It didn't even go to the neon-colored sports drinks we see on every sideline.
The most hydrating drink—the one that stayed in the body the longest—was skim milk.
Following closely behind were oral rehydration solutions (like Pedialyte) and full-fat milk. Water, both still and sparkling, landed significantly further down the list.
Why Milk Beats Water
It sounds weird. We’ve been conditioned to think "thin, clear liquid equals hydration." But your body doesn't see it that way.
When you chug a glass of plain water, your stomach empties it almost immediately. It hits your bloodstream, your kidneys see the sudden influx, and they go, "Whoa, too much at once." Then, you’re running to the bathroom 20 minutes later.
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Milk is different.
Because milk contains a mix of lactose (sugar), protein, and a bit of fat, it stays in your stomach longer. This is a process called "gastric emptying." Since the liquid is moving through your system slowly, it’s released into the blood over a longer period.
But the real secret sauce is the sodium.
Sodium acts like a sponge. It helps your body hold onto water and reduces the amount of urine your kidneys produce. Milk naturally has high levels of sodium and potassium, making it a physiological powerhouse for fluid retention.
What About Sports Drinks and Soda?
You might think orange juice or soda would rank high because they also have sugar and calories to slow things down.
Sorta. But not really.
The St. Andrews study found that while sugary drinks like orange juice or full-sugar cola stay in the stomach longer, they have a major downside once they hit the small intestine. Because they are so concentrated with sugar, a process called osmosis occurs. Basically, the high sugar concentration pulls water out of your body's cells and into the gut to help dilute the sugar.
This can actually leave you slightly more dehydrated in the short term.
As for sports drinks? They performed better than plain water for long-term retention, but they didn't blow it out of the park. Most are designed for athletes losing massive amounts of salt through sweat, not for the average person sitting in a climate-controlled room.
The Coffee Myth
We’ve all heard that coffee is a diuretic and will "dry you out."
The science says that’s mostly a myth for regular drinkers. While caffeine can have a mild diuretic effect, the water content in a standard cup of coffee or tea usually offsets the fluid loss. In the BHI study, coffee and tea performed almost identically to plain water.
You aren't necessarily "losing" hydration by drinking coffee; you just aren't gaining as much as you would with milk.
Real-World Nuance: Is Milk Always Better?
Before you start carrying a carton of 2% on your five-mile run, let’s be real.
"Most hydrating" is a technical term for fluid retention. It doesn't mean "best for every situation." There are some very obvious reasons why water is still the daily go-to for most of us:
- Calories: Milk has them. Water doesn't. If you swapped all your water for milk to "maximize hydration," you’d be consuming hundreds of extra calories a day.
- Digestion: Gulping down milk in 90-degree heat while exercising is a recipe for a stomach ache.
- Lactose Intolerance: For a huge chunk of the population, milk causes more problems than it solves.
The Hydration Hierarchy
If we rank these based on the 2016 AJCN study and follow-up research, the list of the most hydrating drinks (specifically for fluid retention over 4 hours) looks like this:
- Oral Rehydration Solutions: The gold standard for medical-grade recovery.
- Skim Milk: The surprising champion for everyday availability.
- Whole Milk: High fat slows absorption even more, but lower water percentage than skim.
- Orange Juice: High nutrients, but the sugar can be a double-edged sword.
- Still/Sparkling Water: Great for immediate thirst, but exits the system fast.
- Coffee/Tea: Effectively the same as water for the average consumer.
- Beer: Surprisingly hydrating at low ABV, but the alcohol eventually triggers fluid loss.
Practical Steps for Staying Hydrated
Knowing that milk is technically the most hydrating drink is a cool party trick, but how do you use this in real life?
First, don't stop drinking water. It’s still the most efficient way to clear toxins and keep your kidneys happy without adding sugar or fat to your diet.
However, if you know you’re going to be in a situation where you can’t get to a bathroom for a long time—like a long flight, a marathon meeting, or a road trip—drinking something with a bit of electrolyte and protein (like a latte or a small glass of milk) might actually keep you from having to pee every thirty minutes.
If you're recovering from an illness or a truly grueling workout where you've sweat through your shirt, skip the plain water. Reach for an oral rehydration solution or, if your stomach can handle it, a glass of cold milk.
To stay properly hydrated day-to-day:
- Monitor your urine color. Aim for a pale straw color. If it's clear, you might actually be over-hydrating and flushing out electrolytes.
- Eat your water. Foods like cucumber, watermelon, and celery are about 90% water and come with natural salts that help with retention.
- Salt your food. If you drink tons of water but eat a zero-sodium diet, the water will just pass through you. You need that "sponge" effect from salt to actually utilize the fluid.
Focus on the quality of your hydration, not just the quantity. Sometimes, a single glass of milk is worth more to your cells than a liter of the "purest" bottled water.