What keeps you hydrated the longest?

October 21, 2025 •

5 min read

According to research comparing beverage hydration, plain water is not the most hydrating fluid for long-term retention. This unexpected finding reveals that certain drinks with specific nutrient compositions can actually keep you hydrated the longest, making your beverage choice a key factor in sustained hydration.

Quick Summary

This article explores the science of long-lasting hydration, explaining why beverages like milk and oral rehydration solutions are more effective than water at retaining fluid in the body over time. The nutrient content and how the body processes various drinks are key to optimizing hydration.

Key Points

Milk is more hydrating than water: Studies using the Beverage Hydration Index show that both skim and whole milk lead to longer fluid retention than plain water.
Nutrient content is key: The electrolytes, carbohydrates (lactose), protein, and fat in milk slow down gastric emptying, allowing the body to absorb and retain fluid for a longer duration.
Oral Rehydration Solutions are highly effective: Precise mixes of electrolytes and glucose make ORS superior to water for rapid and sustained rehydration, especially after significant fluid loss from exercise or illness.
Water-rich foods aid hydration: A significant portion of your daily fluid intake can come from high water-content fruits and vegetables like watermelon, cucumbers, and lettuce.
Avoid excessive diuretics: Drinks with high levels of alcohol or caffeine can increase urine output and counteract hydration efforts. Limit these, especially when rehydrating.
Check your urine color: Monitoring urine color is a simple way to track your hydration level. Aim for pale, clear urine to indicate you are well-hydrated.

The Science Behind Long-Lasting Hydration

While water is the fundamental fluid for human survival, its hydrating capacity over time is not always superior to other drinks. The difference lies in how the body processes and retains fluid. Researchers at Scotland's St. Andrews University developed a metric called the Beverage Hydration Index (BHI) to measure the body's fluid retention relative to plain water over a four-hour period. The higher the BHI score, the better the drink is at keeping you hydrated for a sustained period.

The BHI revealed that several beverages, particularly milk and oral rehydration solutions (ORS), surpassed water's hydrating effects. The primary reasons for this superior performance are a drink's nutrient composition and its effect on gastric emptying and sodium retention. The presence of electrolytes like sodium and potassium, carbohydrates, and protein in a drink helps to slow down the rate at which fluids leave the stomach and enter the bloodstream. This slower absorption means less fluid is excreted as urine in the short term, leading to more prolonged hydration. Furthermore, electrolytes like sodium act as sponges, helping the body hold onto water more effectively.

Why Milk Often Outperforms Water

Milk consistently ranks as one of the most hydrating beverages, with studies showing both skim and whole milk outperforming plain water in terms of fluid retention over several hours. This is due to several key factors:

Electrolyte Content: Milk naturally contains sodium and potassium, crucial electrolytes that aid in fluid balance and retention within the body.
Carbohydrates and Protein: The presence of lactose (a natural sugar), protein, and fat in milk slows down the rate of gastric emptying. This means the fluid and its accompanying nutrients are absorbed more gradually, providing a longer-lasting hydrating effect. This is particularly beneficial after a workout when the body needs both rehydration and nutrient replenishment for recovery.
Natural Sodium: The sodium in milk acts to hold water in the body, which reduces urine production.

For athletes or individuals recovering from significant fluid loss due to illness, the combination of water, electrolytes, and macronutrients makes milk an excellent rehydration choice.

Top Contenders for Sustained Hydration

While milk is a top performer, several other beverages and foods offer excellent, long-lasting hydration. Understanding the benefits of each can help you make informed choices for your daily fluid intake.

Here are some of the most hydrating options, ranked highly for their ability to promote fluid retention:

Oral Rehydration Solutions (ORS): Formulated with a precise balance of water, glucose, and electrolytes, ORS are designed for rapid and sustained rehydration, especially after severe fluid loss from diarrhea or intense exercise.
Milk (Skim and Whole): As discussed, the combination of protein, carbs, fat, and electrolytes makes it highly effective for long-term fluid retention.
Fruit and Vegetable Juices (100%): Juices provide water along with vitamins, but the higher sugar concentration can be less effective than milk or ORS due to potential osmotic effects. Diluting with water can be a better option.
Coconut Water: Known for its high potassium content and other electrolytes, coconut water is a natural option for replenishing lost minerals after sweating.
Water-Rich Foods: Eating fruits and vegetables can contribute significantly to your daily fluid intake, providing slow-release hydration along with vitamins and minerals.

Comparing Hydrating Beverages


Beverage	Key Hydrating Components	Hydration Index (BHI)	Best For
Plain Water	H₂O, trace minerals	1.0 (Baseline)	General, everyday hydration for most activities.
Skim Milk	Water, electrolytes (sodium, potassium), protein, carbs	~1.44	Post-exercise rehydration, sustained hydration due to slower gastric emptying.
Oral Rehydration Solution (ORS)	Water, electrolytes, glucose	~1.50	Rapid and effective rehydration after illness or intense fluid loss.
Coconut Water	Water, electrolytes (high potassium)	Similar to or slightly higher than water	Replenishing fluids and minerals after mild-to-moderate sweating.

The Role of Water-Rich Foods

Approximately 20% of your daily fluid intake comes from foods. Incorporating water-dense fruits and vegetables can be a delicious and effective way to boost your hydration levels. Unlike simply drinking liquids, consuming foods provides fluid along with fiber and other nutrients, which contributes to overall health and sustained hydration.

Fruits: Watermelon (92%), strawberries (91%), cantaloupe (90%), peaches, oranges, and grapefruit are all excellent sources of water and vitamins.
Vegetables: Cucumbers (96%), lettuce (95%), celery (95%), tomatoes (94%), bell peppers (94%), and zucchini are among the most hydrating vegetables.
Soups and Broths: These warm, comforting foods are mostly water and also contain sodium, which helps with fluid retention.

Optimizing Your Hydration Strategy

To ensure you stay properly hydrated for longer periods, consider these strategies:

Hydrate Strategically: Don't wait until you are thirsty, as thirst is a lagging indicator of dehydration. Make drinking fluids a regular habit throughout the day.
Choose Wisely Post-Workout: After intense exercise, consider a drink with electrolytes and macronutrients like milk or an ORS to more effectively restore fluid balance and aid recovery.
Monitor Your Urine: A simple and effective way to gauge hydration is by checking your urine color. Pale, clear urine indicates good hydration, while dark yellow suggests you need more fluids.
Be Mindful of Diuretics: Beverages like alcohol and those high in caffeine can increase urine production, causing you to lose fluids faster. Moderate your intake of these to avoid counteracting your hydration efforts.
Carry a Reusable Water Bottle: Having water readily available can increase overall fluid intake. For flavor without sugar, add slices of fruit or herbs.

For more detailed information on healthy fluid intake, consult authoritative health sources such as the Centers for Disease Control and Prevention (CDC) about water and healthier drink options.

Conclusion

While plain water is an essential part of daily fluid intake, it is not the most effective option for sustained hydration, especially after significant fluid loss. The science of the Beverage Hydration Index proves that beverages with a balance of electrolytes, protein, and carbohydrates, most notably milk and oral rehydration solutions, provide longer-lasting hydration by slowing gastric emptying and promoting fluid retention. Incorporating water-rich foods also plays a vital role in meeting your daily fluid needs. By understanding the specific hydrating properties of different drinks, you can optimize your strategy for effective and lasting hydration based on your activity level and individual needs.

Note: If you are experiencing severe dehydration symptoms, such as dizziness, confusion, or inability to keep fluids down, seek immediate medical attention.

Frequently Asked Questions

Milk is more hydrating than water because it contains a balanced mix of electrolytes (like sodium and potassium), carbohydrates (lactose), protein, and fat. These components slow down the emptying of fluid from your stomach, allowing your body to absorb it over a longer period. The sodium also helps your body retain the fluid more effectively.

Yes, ORS are more hydrating than water, particularly after significant fluid loss from intense exercise or illness. They are specifically formulated with an optimal ratio of electrolytes and sugars to accelerate and maximize fluid absorption and retention.

Caffeine is a mild diuretic, meaning it can cause you to urinate more frequently, potentially leading to dehydration. However, moderate consumption (2-4 cups of coffee) has a minimal effect for most people. Large quantities, particularly in individuals not accustomed to caffeine, can increase fluid loss.

A reliable method for checking your hydration level is to monitor the color of your urine. If it is a pale, straw-like color, you are likely well-hydrated. Dark yellow urine is a sign that you need to increase your fluid intake.

For short-duration or low-intensity exercise, water is sufficient for hydration. However, for intense or prolonged exercise (over 60 minutes), sports drinks can be more effective. They replenish both fluids and electrolytes lost through sweat, and provide carbohydrates for energy.

Absolutely. Foods with high water content, such as watermelon, cucumber, lettuce, and soups, can contribute significantly to your daily fluid intake. Eating these foods provides hydration along with essential vitamins and fiber.

Yes, coconut water is a natural source of electrolytes, particularly potassium. It is an excellent choice for replenishing lost fluids and minerals after exercise, though some people may find it causes bloating.

For most people engaging in everyday activities, plain water is the best drink for daily hydration. It is calorie-free and effective at quenching thirst. However, after intense exercise or for long-lasting effects, milk or an ORS can offer superior rehydration.