Does milk hydrate you as much as water does? The surprising science of milk vs. water for fluid balance

October 11, 2025 •

4 min read

According to research using the Beverage Hydration Index (BHI), both skim and whole milk were found to be more hydrating than plain water over a four-hour period. This surprising finding challenges the notion that water is always the optimal choice for staying hydrated and prompts a closer look at the key question: Does milk hydrate you as much as water does?

Quick Summary

Milk can be more effective for rehydration than water in certain scenarios due to its unique blend of electrolytes, carbohydrates, and proteins, which aid fluid retention. For general, daily needs, however, water remains the best option.

Key Points

Milk contains electrolytes and macronutrients: The presence of sodium, potassium, protein, and carbohydrates in milk significantly aids fluid retention by slowing digestion and urine output.
Water is best for everyday hydration: For daily fluid needs and weight management, plain water is the ideal, zero-calorie choice.
Milk excels for post-exercise recovery: Following intense physical activity, milk helps replace lost fluids and electrolytes while providing carbs and protein for muscle repair.
The Beverage Hydration Index ranks milk highly: Studies using the BHI have shown that both whole and skim milk are more effective than water at retaining fluid over a four-hour period.
Plant-based milks have varying hydrating properties: Alternative milks, like almond milk, have less protein and fewer electrolytes than cow's milk, making them potentially less effective for fluid retention.
Moderation is key: Replacing all daily fluid intake with milk would lead to excessive calorie consumption. Use milk strategically for specific hydration needs.
Hydration needs depend on context: The best choice between milk and water depends on factors like activity level, dietary goals, and overall nutritional needs.

The Science Behind Hydration

At its core, hydration is about maintaining the body's fluid balance. The water content of any beverage is essential, but other factors, including electrolytes and macronutrients, play a crucial role in how effectively the body absorbs and retains that fluid. When you drink, the fluid passes through your digestive system and is absorbed into the bloodstream. The speed of this process and the subsequent rate of urine excretion determine a beverage's effectiveness for hydration.

The Beverage Hydration Index (BHI)

To objectively compare the hydrating properties of different drinks, researchers developed the Beverage Hydration Index (BHI). This index measures fluid retention by comparing urine output after consuming a beverage to the urine output after drinking an equal volume of plain water. A beverage with a BHI greater than 1.0 means it leads to less urine excretion and greater fluid retention than water over a set period.

Studies utilizing the BHI have consistently shown milk scoring higher than water. The reason lies in milk's distinct nutritional makeup, which influences how your body processes it.

Why Milk Retains Fluid Better Than Water

Milk's superior fluid retention is not a myth; it's a physiological fact driven by its composition. Several key components contribute to this effect:

Electrolytes: Milk is a natural source of key electrolytes like sodium and potassium. When you sweat, you lose these electrolytes. The sodium in milk helps the body hold onto fluid by slowing down the rate at which you urinate.
Protein and Carbohydrates: The combination of protein and natural sugars (lactose) in milk slows down the digestive process. This delayed gastric emptying means fluids are released into the bloodstream more slowly and over a longer period, preventing a rapid rush of fluid that would otherwise trigger the kidneys to increase urine production.
Macronutrient Synergy: The interplay between milk's macronutrients—carbohydrates, fat, and protein—creates a balanced solution that promotes water absorption more efficiently than plain water. This synergy ensures that the fluid is utilized by the body for a longer duration before being excreted.

Milk vs. Water: The Best Time to Choose Each

Despite milk's impressive fluid-retaining properties, it's not a direct replacement for water in all situations. The ideal choice depends heavily on your specific hydration needs and the context in which you are drinking.

Comparison Table: Milk vs. Water for Hydration


Feature	Milk (especially low-fat)	Plain Water
Best for...	Post-exercise recovery, replacing lost fluids and nutrients	General, everyday hydration, staying refreshed
Calorie Content	Higher (contains calories from fat, protein, carbs)	Zero calories
Nutrient Profile	Rich in electrolytes (sodium, potassium, calcium), protein, carbs, vitamins	Minimal mineral content (depending on source)
Fluid Retention	High (delays gastric emptying and slows urine output)	Efficient for immediate hydration, but less long-lasting
Cost	More expensive per liter	Generally free or very low cost
Digestion	Longer digestion time due to macronutrients	Absorbed rapidly into the body

When is Milk the Superior Choice?

Milk excels in scenarios where you need more than just fluids. These situations include:

Post-Workout Rehydration: After an intense exercise session, the body needs to replenish lost fluids, electrolytes, and energy stores. Milk, particularly chocolate milk, offers an ideal ratio of carbohydrates and protein to aid in muscle recovery while restoring fluid balance.
Mild Dehydration in Specific Populations: For children or the elderly, who may be at higher risk of dehydration and can benefit from the added nutrients, milk can be a good option for more complete hydration.

When is Water Still the Best Choice?

For the majority of daily fluid intake, water is the clear winner. Here are the reasons why:

Everyday Hydration: For ongoing hydration throughout the day, water is the most practical and calorie-free choice. It prevents you from consuming excess calories or fat, which is important for weight management.
Optimal for Weight Management: If you are monitoring your caloric intake, relying on milk for all your hydration needs can lead to a significant increase in daily calories. Water allows you to hydrate without compromising your diet.
Quick Fluid Absorption: While milk's slower absorption is beneficial for fluid retention, water is absorbed faster, making it ideal for immediate thirst-quenching without the added nutrients that may not be necessary.

The Verdict: Context is Everything

The debate of whether milk hydrates you as much as water does isn't a simple yes or no. The answer depends on your specific needs. The scientific evidence shows that milk is highly effective for fluid retention, particularly for rehydrating after intense physical activity, due to its electrolytes, carbohydrates, and protein. However, water remains the best, most practical, and healthiest option for meeting your daily fluid requirements.

Ultimately, a balanced approach is key. Drink water regularly throughout the day, and consider milk as a strategic, post-workout recovery beverage to optimize rehydration and muscle repair.

Conclusion

The idea that milk could be more hydrating than water is counterintuitive, but it's grounded in science. The presence of electrolytes, protein, and carbohydrates in milk causes slower gastric emptying and reduced urine output, allowing the body to retain fluids for a longer period. While this makes milk an excellent choice for rehydration after exercise, especially when nutrient replenishment is needed, it doesn't dethrone water as the champion of daily hydration. Water's zero-calorie profile and rapid absorption make it the most suitable beverage for consistently meeting your fluid needs without adding unnecessary calories. Incorporating both into your diet—water for everyday sipping and milk for targeted recovery—is the most effective strategy for optimal hydration and overall health.

Frequently Asked Questions

Yes, studies using the Beverage Hydration Index (BHI) have shown that skim milk is more effective for short-term fluid retention than plain water. Its combination of electrolytes, protein, and natural sugars helps the body retain fluid for longer.

After a workout, you lose fluids and electrolytes through sweat. Milk replaces these electrolytes and provides carbohydrates to replenish energy stores and protein to aid muscle repair. The macronutrients in milk also slow gastric emptying, helping the body retain fluids longer.

No, it's not recommended. While milk is hydrating, it is also higher in calories and fat (depending on the type). Replacing all your daily water with milk would significantly increase your caloric intake, which is not suitable for general hydration.

The Beverage Hydration Index (BHI) is a tool used by researchers to rank beverages based on how well they maintain hydration status relative to plain water. A higher BHI means a beverage promotes less urine output and better fluid retention over a set period.

Not typically. Plant-based milks often contain fewer electrolytes and protein than cow's milk. While they still contribute to overall fluid intake, they generally don't offer the same fluid-retention benefits as dairy milk.

You should choose water for general, everyday hydration, especially if you are not exercising or if you are managing your caloric intake. Water is the most efficient, calorie-free way to stay hydrated.

Yes, to an extent. Both whole milk and skim milk have been shown to be more hydrating than water due to their nutrient profiles. The fat and protein in whole milk further slow gastric emptying, which can contribute to slightly longer fluid retention, but both types are effective.