Which hydrates you better, water or milk? A Scientific Comparison

November 14, 2025 •

4 min read

According to a groundbreaking 2016 study published in the American Journal of Clinical Nutrition, milk was found to be more hydrating than water over a four-hour period. This surprising finding begs the question: which hydrates you better, water or milk, and under what circumstances should you choose one over the other for optimal fluid balance?

Quick Summary

A comparison using the Beverage Hydration Index shows milk can retain fluids longer than plain water due to its fat, protein, and electrolytes. Water, however, remains a calorie-free, universal choice for general hydration. For post-exercise recovery, milk offers additional nutritional benefits beyond simple fluid replacement.

Key Points

Milk's Sustained Hydration: A 2016 study found milk hydrates better than water over a four-hour period due to its fat, protein, and lactose, which slow fluid release.
Role of Electrolytes: Milk's natural electrolytes like sodium and potassium help the body retain fluid more effectively than plain water.
Water's Role: Water remains the best choice for general, calorie-free, and rapid rehydration for most people throughout the day.
Optimal Post-Workout Recovery: Milk serves as an excellent post-exercise recovery drink, offering protein for muscle repair, carbohydrates for energy, and electrolytes for rehydration.
Considerations for Diet: Individuals on a low-calorie diet or with lactose intolerance may prefer water, as milk is higher in calories and requires proper digestion.
Individual Needs Matter: The better choice depends on your specific goals, whether that's general daily hydration (water) or enhanced post-activity recovery (milk).

A 2016 study from Scotland's St. Andrews University introduced the Beverage Hydration Index (BHI), a scientific metric designed to compare the hydration effectiveness of various drinks relative to plain water. The findings revealed that milk, both skim and whole, performed better than water at keeping participants hydrated for a longer period. While this doesn't mean you should abandon water, it highlights that the "best" hydrating drink depends on the situation and your body's specific needs.

The Science Behind Milk's Superior Hydration

The reason milk outperforms water in long-term hydration lies in its unique nutritional makeup. The components that many people see as drawbacks—its caloric density and macronutrient content—are precisely what make it effective for sustained hydration.

The Slow-Release Hydration Formula

Macronutrient Content: Milk contains natural sugars (lactose), protein, and fat. These macronutrients slow down the rate of gastric emptying, meaning the fluid stays in the stomach and is released into the intestines more slowly. This prolonged absorption prevents the body from rapidly processing and excreting the fluid, leading to less urine output and longer-lasting hydration compared to water.
Electrolyte Balance: Milk is a natural source of electrolytes such as sodium and potassium. These minerals play a crucial role in maintaining fluid balance within the body's cells and bloodstream. The sodium in milk acts like a sponge, helping the body retain the fluid it absorbs. This mechanism is similar to how oral rehydration solutions work, which are specifically designed to treat dehydration.
Balanced Osmolality: Osmolality is the concentration of solutes in a fluid. Milk has a balanced osmolality that promotes efficient water absorption by the body's cells. In contrast, drinks with a very high concentration of sugar, like soda, can actually pull water from the body into the intestine to dilute the sugar content, which is counterproductive to hydration.

Water vs. Milk: A Direct Comparison

The best choice depends on the context. Here's a comparative overview:


Feature	Water	Milk (Cow's)
Primary Function	Quick, calorie-free fluid replacement.	Sustained fluid retention with added nutrients.
Hydration Speed	Fastest initial rehydration.	Slower, more prolonged absorption.
Hydration Duration	Shorter-term due to rapid gastric emptying and excretion.	Longer-term due to slower digestion of fats, protein, and sugar.
Nutrient Profile	Contains no calories, protein, or fat; may contain trace minerals depending on source.	Contains protein, fat, carbohydrates (lactose), electrolytes (sodium, potassium), and vitamins (A, D, B12).
Calories	0 calories.	Varies by fat content; typically 90-150 kcal per cup.
Best For	General daily hydration, low-calorie diets, immediate thirst quenching.	Post-exercise recovery, sustained hydration, supplementing nutrients.

When Water is Still the Undisputed Winner

While milk's properties for sustained hydration are impressive, water remains the gold standard for everyday fluid needs for several reasons:

Calorie-Free: For individuals managing their weight, water is the obvious choice. Milk's caloric content, even in skim varieties, adds up over the course of a day.
Universal Applicability: Water is suitable for everyone, regardless of dietary restrictions like lactose intolerance or dairy allergies. Plain water is also much more affordable and readily available than milk or other specialty drinks.
Immediate Quenching: For quick rehydration, such as quenching a sudden thirst or when it's particularly hot, water is absorbed quickly and is highly effective.

Milk for Post-Workout Recovery: A Specialized Hydration Role

After an intense workout, the body loses not only fluid but also electrolytes, and it needs carbohydrates to replenish glycogen stores and protein to repair muscles. This is where milk shines as a superior post-exercise beverage.

Complete Recovery Package: Milk provides the perfect combination of protein, carbohydrates, fluid, and electrolytes needed for a comprehensive recovery. The whey protein is absorbed quickly for muscle repair, while the casein protein provides a sustained release of amino acids.
Superior to Water and Sports Drinks: Some studies have shown milk, including chocolate milk, to be more effective than water or many commercial sports drinks for post-exercise rehydration. Its nutrient density helps maintain a positive fluid balance throughout the recovery period.

Conclusion: Choosing Your Hydration Strategy

The question of which hydrates you better, water or milk, has a nuanced answer. While plain water is excellent for immediate, everyday hydration and maintaining a zero-calorie intake, milk offers a more sustained hydration effect due to its protein, fat, and electrolyte content. For athletes or individuals recovering from significant fluid loss through sweat, milk provides a more complete recovery package that aids in fluid retention, muscle repair, and nutrient replenishment. For most daily scenarios, relying on water is a healthy and calorie-conscious choice. However, incorporating milk strategically, especially after a workout, can optimize your body's recovery and hydration status.

Milk as an effective post-exercise rehydration drink - PubMed

Frequently Asked Questions

Milk is considered more hydrating in certain studies because its protein, fat, and lactose content slow down the stomach's emptying process. This leads to a slower and more sustained release of fluids into the body, causing less urine to be produced over a longer period.

The best time to choose milk for hydration is after a strenuous workout or during mild dehydration, as it not only replenishes fluids but also provides electrolytes, protein, and carbohydrates needed for muscle repair and recovery.

Yes, water is absolutely necessary. It's the most straightforward, calorie-free way to stay hydrated throughout the day and should be your primary beverage for fluid intake.

Yes, the fat content affects digestion speed. Higher fat content, along with protein and lactose, contributes to a slower gastric emptying process, which can lead to longer-lasting hydration compared to skim milk or water.

Chocolate milk is often recommended for post-workout recovery due to its balanced carb-to-protein ratio. Plant-based milks vary in composition; many have fewer electrolytes and nutrients than cow's milk and may contain added sugars, so checking labels is important.

For individuals with lactose intolerance, consuming dairy milk can cause digestive upset and may not be an appropriate hydrating choice. Lactose-free milk could be an alternative, but water or other beverages are often better.

Yes, it is possible. Because milk contains calories, protein, and fat, excessive consumption can lead to unintended weight gain. For general hydration, water is the more practical and healthier choice.