A few beverages actually hydrate better than plain water, and the answer comes down to what your body holds onto rather than what you swallow. In a landmark hydration study from the American Journal of Clinical Nutrition, researchers tested 13 common drinks and found that oral rehydration solutions and milk, both skim and full-fat, kept people hydrated significantly longer than water. The key is fluid retention: drinks with the right balance of electrolytes, protein, or fat slow down how quickly your kidneys flush liquid out.
The Beverage Hydration Index
Researchers developed something called the Beverage Hydration Index (BHI) to measure how well different drinks keep you hydrated compared to still water, which serves as the baseline at 1.0. They had participants drink one liter of each beverage, then measured urine output over four hours. The results were surprisingly clear-cut.
Skim milk scored a BHI of 1.58, full-fat milk came in at 1.50, and oral rehydration solutions scored 1.54. In practical terms, your body retained roughly 50% more fluid from these drinks than from the same volume of water. Meanwhile, cola, diet cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water, and sports drinks all performed no differently from plain water at the four-hour mark.
Why Milk Outperforms Water
Milk’s advantage comes from multiple mechanisms working together. Its protein, fat, and natural sugar (lactose) all slow the rate at which your stomach empties liquid into the intestines. High-calorie beverages empty from the stomach considerably slower than low-calorie ones, and that slower drip gives your intestines more time to absorb the fluid rather than sending it straight to your bladder.
Milk proteins, particularly casein, appear to play a special role. Casein forms a soft clot in the stomach, which further slows absorption and leads to greater fluid retention. There’s also evidence that milk proteins stimulate a hormone that tells your kidneys to hold onto water rather than producing urine. On top of all this, milk naturally contains sodium, potassium, and lactose, which help pull water across the intestinal wall and into your bloodstream. For people with lactose intolerance, though, this benefit disappears quickly since undigested lactose can cause digestive problems that work against hydration.
How Oral Rehydration Solutions Work
Oral rehydration solutions (ORS) are the gold standard for treating dehydration in clinical settings worldwide. They work by exploiting a specific feature of your small intestine: sodium absorption increases dramatically when glucose is present, and water follows sodium. The WHO formula pairs glucose and sodium in roughly equal concentrations at a low overall osmolality (around 235 mOsm/L), which maximizes fluid absorption.
You can find commercial versions at most pharmacies, often marketed for children or illness recovery. These aren’t the same as sports drinks. Sports drinks contain more sugar and fewer electrolytes, which is why they didn’t outperform water in the hydration index study. ORS is specifically engineered to get fluid into your bloodstream as efficiently as possible.
Why Sugary Drinks Can Work Against You
There’s a tipping point where sugar stops helping and starts hurting hydration. When a beverage’s carbohydrate concentration exceeds about 6%, it actually pulls water from your body into your intestinal tract through osmosis. This means your body temporarily loses water to dilute the sugar sitting in your gut. Fruit juices, regular sodas, and many sweetened beverages cross this threshold easily. A typical soda contains around 10-11% sugar.
This is why fruit juice and cola scored no better than water despite containing electrolytes and calories. Their high sugar content offsets any absorption advantage. If you’re drinking juice for hydration, diluting it with water brings the sugar concentration down and improves fluid delivery.
Coffee and Tea Don’t Dehydrate You
The idea that coffee and tea dehydrate you is one of the most persistent hydration myths. A controlled crossover study found that men who drank four cups of coffee per day (a moderate dose) showed no difference in total body water, urine volume, or any blood or urine hydration marker compared to when they drank the same volume of plain water. Their 24-hour urine output was virtually identical: 2,409 mL on coffee versus 2,428 mL on water.
Caffeine is a mild diuretic, meaning it does make your kidneys produce slightly more urine. But the volume of water in the coffee or tea more than compensates for that small loss. The net effect is hydration, not dehydration. This holds true for people who regularly consume caffeine. If you rarely drink it, the diuretic effect may be slightly more noticeable, but you’re still gaining fluid overall.
Coconut Water and Sports Drinks
Coconut water contains potassium, sodium, and manganese, and some evidence suggests it performs comparably to a sports drink. But according to the Mayo Clinic, coconut water is no more hydrating than plain water. It’s a fine choice if you enjoy it, but it’s not the upgrade many people assume.
Sports drinks occupy a similar middle ground. They don’t hydrate better than water for most daily activities. Their real value shows up during prolonged exercise lasting more than one hour, especially in hot and humid conditions. During those sessions, your body loses enough electrolytes and burns enough stored energy that the sodium, potassium, and carbohydrates in a sports drink genuinely improve endurance and fluid balance. For a 30-minute gym session or a regular day at work, water does the job just as well.
When You Actually Need More Than Water
For everyday hydration, water is perfectly adequate. The beverages that outperform it matter most in specific situations: recovering from illness with vomiting or diarrhea (where oral rehydration solutions are clearly superior), rehydrating after intense or prolonged exercise, or when you know you won’t be able to drink again for several hours and need your body to retain what you take in.
People with high sweat rates, above roughly 2 liters per hour during exercise, face a physical limit: the stomach can only absorb about 1.2 liters per hour. In that scenario, choosing a drink your body retains more efficiently can make a real difference. People who notice white salt stains on their workout clothes (a sign of high sodium loss in sweat) benefit from adding extra sodium through food or an electrolyte drink.
For most people on most days, the best hydrating drink is whichever one you’ll actually consume enough of. Milk at meals, coffee in the morning, and water throughout the day all contribute to your fluid balance. The hydration differences between beverages matter at the margins. Drinking enough total fluid matters far more.