Understanding What's the Most Hydrating Thing in the World

October 7, 2025 •

5 min read

While most of us reach for a glass of water to hydrate, a 2016 study published in the American Journal of Clinical Nutrition revealed that beverages with a blend of nutrients and electrolytes can be more hydrating over a longer period than plain water itself. This surprising finding redefines what's the most hydrating thing in the world, shifting the focus from simple water content to a more nuanced understanding of fluid retention.

Quick Summary

This article explores the Beverage Hydration Index (BHI) and explains why certain beverages like milk and oral rehydration solutions (ORS) are more effective than water for sustained fluid retention. It details the role of electrolytes, carbohydrates, and protein in maximizing hydration and offers practical advice for choosing the right fluids for different situations.

Key Points

Milk is more hydrating than water: Studies using the Beverage Hydration Index (BHI) show that skim and full-fat milk retain fluids longer than water due to their content of protein, carbs, and electrolytes.
ORS is extremely effective: Oral Rehydration Solutions are scientifically formulated to replenish fluids and lost electrolytes rapidly, making them ideal for recovery from illness or intense exercise.
Electrolytes are crucial for fluid retention: The minerals found in milk and ORS, especially sodium and potassium, help the body absorb and hold onto water, reducing urine output and maximizing hydration.
Highly hydrating foods boost fluid intake: Fruits and vegetables with high water content, such as cucumbers, watermelon, and lettuce, can contribute significantly to your daily fluid needs.
Plain water is best for daily needs: For general hydration and moderate activity, plain water is still the best choice as it provides hydration without extra calories, sugar, or additives.
Context matters for hydration choices: The ideal hydrating beverage depends on your situation; choose plain water for daily needs, milk or ORS for post-exercise recovery or illness, and water-rich foods for a nutritional boost.
Caffeine in moderation is fine: Contrary to some myths, caffeinated drinks like tea and coffee don't significantly dehydrate you when consumed in moderation, but they are not as effective for prolonged hydration as milk or ORS.

The Science Behind Superior Hydration: The Beverage Hydration Index (BHI)

For decades, water has been the unquestioned king of hydration. However, scientific research, particularly the creation of the Beverage Hydration Index (BHI) in 2016, has provided a more sophisticated metric for assessing a drink's hydration potential. The BHI measures the amount of fluid the body retains from a beverage over a four-hour period, relative to plain water.

Unlike the simple water content of a drink, the BHI takes into account several physiological factors that influence fluid retention, including:

Electrolyte Content: Electrolytes, especially sodium and potassium, play a critical role in fluid balance. They help slow the rate of fluid excretion via urination, allowing the body to absorb and retain more water. This is why an oral rehydration solution (ORS), which is packed with electrolytes, ranks so highly.
Carbohydrates and Protein: The presence of sugars (carbohydrates like lactose) and protein can slow down the stomach's emptying time. This slower digestion and absorption process allows the body to utilize the fluid more efficiently, leading to better long-term hydration compared to plain water, which is absorbed quickly and excreted just as fast.
Nutrient Profile: The combination of nutrients, not just the water, influences how the body processes the liquid. This synergistic effect is why beverages like milk and ORS have a higher BHI than water, even though they aren't 100% water by volume.

Why Skim Milk and ORS Outperform Water

Studies have consistently shown that skimmed milk and oral rehydration solutions like Dioralyte are among the most hydrating beverages available. For example, one study found that skimmed milk had a BHI of 1.58, meaning it provided significantly more long-term hydration than plain water, which has a baseline BHI of 1.0.

Reasons for their superior performance include:

Nutrient Density: Milk contains a combination of lactose (a natural sugar), whey and casein protein, and key electrolytes like sodium and potassium. These components work together to slow gastric emptying, ensuring a more gradual release of fluid into the bloodstream.
Strategic Formulation: Oral Rehydration Solutions are scientifically designed to treat and prevent dehydration, especially following illness. They contain precise ratios of electrolytes and glucose that facilitate rapid fluid and nutrient absorption by the intestines, making them exceptionally effective.

The Most Hydrating Foods: A Delicious Alternative

For those who prefer to eat their hydration, many fruits and vegetables offer a high water content along with beneficial vitamins and minerals.

Cucumber (96% water): Composed of nearly all water, cucumbers are also a source of potassium and magnesium.
Watermelon (92% water): This summer fruit is packed with antioxidants and vitamins A and C, along with significant water content.
Lettuce (95% water): Varieties like iceberg and romaine are excellent sources of water and vitamins A and K.
Celery (95% water): Low in calories and high in fiber, celery also provides folate, potassium, and vitamin K.
Tomatoes (95% water): A great source of fiber, vitamin C, and potassium, tomatoes are a versatile hydrating food.
Cantaloupe (90% water): This melon provides a boost of vitamin C, potassium, and fiber.

Choosing the Right Drink for the Right Situation

The most hydrating thing in the world isn't a one-size-fits-all solution; the best choice depends on the context. Your hydration needs differ between everyday life, intense physical activity, and illness.

Comparison Table: Hydration Options


Beverage	Best For	Pros	Cons
Plain Water	Daily Hydration	Zero calories, readily available, sufficient for general needs.	Faster absorption leads to quicker excretion; less effective for sustained rehydration after heavy fluid loss.
Skim Milk	Post-Exercise Recovery	Superior fluid retention due to electrolytes, protein, and carbs.	Higher calorie count than water, unsuitable for individuals with lactose intolerance.
Oral Rehydration Solution (ORS)	Illness (Diarrhea/Vomiting)	Specifically formulated for rapid, efficient rehydration and electrolyte balance.	Unnecessary for everyday use; often higher in sugar than plain water.
Coconut Water	Mild Exercise/General Hydration	Contains natural sugars and potassium, lower in sodium than sports drinks.	Can cause bloating; less effective than milk for long-term retention due to lower sodium content.
Sports Drinks	Intense, Long-Duration Exercise	Provides carbs and electrolytes to replenish energy and fluids lost through heavy sweating.	Often high in added sugar, not necessary for moderate activity.

The Role of Electrolytes and Nutrients

The reason milk and ORS are considered more hydrating over time stems from their ability to help the body retain fluid. Electrolytes, particularly sodium, are key to this process. When you sweat or are ill, your body loses electrolytes along with water. Replenishing these salts is crucial for restoring proper fluid balance.

In addition to electrolytes, the carbohydrates and protein in beverages like milk slow down the digestive process. This ensures that the fluid is released into the bloodstream more gradually, prolonging the hydration effect and reducing the need for frequent urination. For general daily hydration, however, these added nutrients are not necessary and can add excess calories or sugar to your diet.

Making Smart Hydration Choices

To make informed decisions about your hydration, consider the following points:

Assess the Need: For typical daily activities and moderate exercise, plain water is the best and most calorie-conscious choice.
Prioritize Recovery: If you've just completed an intense workout or are recovering from an illness with significant fluid loss, consider milk or an ORS to maximize fluid and electrolyte replenishment.
Eat Your Water: Incorporate plenty of water-rich fruits and vegetables into your diet, especially during warmer months, to boost your hydration naturally.
Avoid Over-reliance on Sugary Drinks: While sports drinks have a place in an athlete's routine, they are often high in added sugars and are not ideal for general hydration. Be mindful of the sugar content in fruit juices and opt for 100% juice diluted with water to mitigate sugar intake.

Conclusion: The Hydration Index's Lesson

While water remains the most fundamental and universally accessible source of hydration, modern science, via the Beverage Hydration Index, teaches us that the story is more complex. What's the most hydrating thing in the world isn't a single liquid but rather an informed choice based on our physiological needs at any given moment. For sustained fluid retention, milk and Oral Rehydration Solutions top the list, but for daily needs, water is still the champion. Understanding this allows us to make more strategic choices for better health and recovery, whether after a tough workout or a bout of illness.

For more detailed information on hydration and optimal fluid intake, explore studies from the National Institutes of Health.

Frequently Asked Questions

Yes, according to research using the Beverage Hydration Index (BHI), milk is more hydrating than plain water over a sustained period of a few hours. This is because its natural sugars (lactose), protein, and electrolytes slow down the rate at which fluids are passed from the body.

ORS is highly effective because it contains a specific balance of water, glucose, and electrolytes (sodium, potassium). This formulation facilitates the rapid absorption of fluid and electrolytes by the intestines, making it ideal for cases of dehydration caused by illness like diarrhea.

The BHI is a scientific tool that compares the short-term hydration potential of various beverages against still water, which is assigned a baseline index of 1.0. A beverage with a higher BHI is more effective at promoting long-term fluid retention in the body.

In moderation, drinks like coffee and tea are not significantly dehydrating. However, their diuretic effect means they are less effective than beverages with electrolytes and nutrients for sustained hydration. For daily fluid intake, they still count toward your total.

Many fruits and vegetables are excellent for hydration due to their high water content. Some top choices include cucumbers (96% water), lettuce (95% water), watermelon (92% water), and celery (95% water).

For moderate exercise, water is sufficient for rehydration. However, for intense, long-duration workouts, especially in hot weather, a sports drink can help replenish lost electrolytes and carbohydrates more effectively than water alone.

For most people with a balanced diet, it is possible to get all necessary electrolytes from food and water alone. A balanced diet and adequate water intake are usually sufficient to maintain electrolyte levels, and specialized drinks are not typically needed.