On a warm summer afternoon, when your throat runs dry, reaching for a glass of water might feel like the most natural thing in the world. But according to researchers in the UK, that classic glass of H₂O might not be your best option for hydration after all.

The researchers set out to find out which drinks are best at keeping the body hydrated over time. Their results reveal that while water quenches thirst quickly, it’s not always the most effective at keeping fluids locked in the body.

Their work led to the creation of a new tool: the Beverage Hydration Index, or BHI. Like the glycemic index for food, the BHI ranks drinks based on how much liquid remains in the body hours after ingestion. And water, it turns out, doesn’t always take the top spot. Skimmed milk, full-fat milk, and oral rehydration solutions were all more hydrating than water.

Milk, Not Water, Tops the Charts

The study enrolled 72 healthy men, who arrived at the lab fasted and euhydrated — that is, with normal body water levels. Each drank a liter of still (uncarbonated) water or one of 13 other popular beverages, from cola to coffee to full-fat milk. Over the next four hours, researchers measured their urine output to see how much fluid the body retained.

Water, both still and sparkling, served as the baseline. But drinks like milk and oral rehydration solutions (ORS) outperformed it in terms of hydration.

At the two-hour mark, both skimmed and full-fat milk had significantly higher BHI scores than water, with skimmed milk topping the list. Skimmed milk retained roughly 339 grams more fluid than water.

Why milk, though?

“Milk was found to be even more hydrating than plain water because it contains the sugar lactose, some protein and some fat,” explained Dr. Ronald Maughan, the lead author of the study. These components slow the emptying of the stomach, keeping fluids in the body longer. Milk also contains sodium, which helps the body retain water by reducing urine production.

The same mechanism applies to oral rehydration solutions like Dioralyte, which are designed for medical use for cases of dehydration. These solutions contain small amounts of sugar, sodium, and potassium, creating a balance that enhances water retention in the body.

What About Juice, Soda, or Coffee?

Orange juice showed a moderate hydrating benefit. At two hours, it had a higher BHI than water, though when researchers corrected for water content, the difference became statistically insignificant. It turns out that too much sugar can become a drawback.

That’s because concentrated sugars in juice or cola can trigger a process called osmosis in the small intestine. When this happens, water is pulled from the body into the gut to dilute the sugar, effectively reversing hydration. Technically, anything inside the intestine is outside your body.

Sodas, both regular and diet, were no better than water. They didn’t retain more water, and neither did sparkling water (no surprise there).

Caffeinated drinks like coffee and tea often get a bad rap for being dehydrating. But a standard 12-ounce cup of coffee contains around 80 milligrams of caffeine, not enough to trigger a diuretic effect in regular consumers. “A regular coffee would be pretty much as hydrating as water,” said Maughan.

However, at doses above 300 mg of caffeine — around 3 or 4 cups — the risk of fluid loss increases, particularly for those unaccustomed to caffeine. Even so, the effect is short-lived and mild, and adding milk to coffee can help offset it by slowing gastric emptying.

Alcohol, on the other hand, is more complicated. The study found that lager beer, which contains about 4% alcohol, did not significantly increase urine output compared to water. “Beer would result in less water loss than whiskey, because you are ingesting more fluid with beer,” Maughan explained. The total water volume seems to matter more than the alcohol content alone.

A Tool for the Thirsty

The Beverage Hydration Index could become a helpful guide for certain groups, such as athletes, outdoor workers, or healthcare workers who don’t have easy access to fluids or restrooms. “An appreciation of the BHI has relevance for individuals for whom long-term maintenance of fluid balance is important,” the study authors wrote.

But for the average person, obsessing over your drink’s hydration rating isn’t necessary. In most circumstances, the body knows what it needs.

“If you’re thirsty, your body will tell you to drink more,” Maughan reminded.

The findings appeared in The American Journal of Clinical Nutrition.