The Science Behind Dehydration and Rehydration
For most people, plain water is the go-to choice for staying hydrated. While it does an excellent job of rapidly hydrating the body, particularly when you're thirsty, studies show it is quickly processed and expelled by the kidneys. This rapid turnover means water isn't the most effective drink for long-term hydration. The key to maintaining hydration lies in how long fluids are retained in the body, which is influenced by a drink's composition.
The Role of Electrolytes and Nutrients
The reason certain drinks, such as milk, are more hydrating than water comes down to their nutritional content. Factors like the presence of sugar, fat, and protein slow down the process of gastric emptying, meaning fluids and nutrients are released into the bloodstream more gradually. Key electrolytes like sodium and potassium also play a crucial role. Sodium, in particular, acts like a sponge, helping the body hold on to water and reducing the amount of urine produced.
The Beverage Hydration Index (BHI): Comparing Drinks
Researchers at St. Andrews University developed a scientific metric called the Beverage Hydration Index (BHI) to compare how different drinks affect hydration status over a four-hour period. Still water was used as the control, with a baseline score of 1.0. A drink with a score higher than 1.0 indicates that more fluid was retained in the body than with still water. The results were surprising to many.
Ranking the Most Hydrating Drinks
While water sits in the middle of the pack, the drinks at the top of the index included skim milk, oral rehydration solutions (ORS), and full-fat milk. Oral rehydration solutions, formulated with specific ratios of water, electrolytes, and carbohydrates, are scientifically designed for maximum fluid retention and absorption, similar to milk's effect. Sugary drinks like soda and juice, however, ranked lower on the hydration scale because their high sugar concentration can pull water from the body into the intestine through osmosis, potentially counteracting the hydrating effects.
Comparison of Hydration Index Scores
The following table summarizes the findings from the St. Andrews study, showing how different common beverages compare against plain water based on the BHI. The index represents the ratio of fluid retained compared to still water after a fixed period.
| Beverage | Beverage Hydration Index (BHI) |
|---|---|
| Skim Milk | ~1.58 |
| Oral Rehydration Solution (ORS) | ~1.54 |
| Whole Milk | ~1.50 |
| Orange Juice | ~1.39 |
| Cola | ~1.23 |
| Cold Tea | ~1.02 |
| Sports Drink | ~1.02 |
| Still Water | 1.00 |
| Sparkling Water | ~1.00 |
| Coffee | ~0.91 |
| Lager Beer | ~0.90 |
How the Composition Affects Hydration
Here are some of the key factors that explain why some drinks are more hydrating than others:
- Electrolyte Content: Sodium and potassium help the body retain fluids by regulating the fluid balance in and around cells.
- Carbohydrates: The presence of sugars like lactose in milk, or specific carbs in ORS, aids in the absorption of fluids and electrolytes from the small intestine.
- Macronutrients: Fat and protein found in milk delay the rate at which fluid leaves the stomach, allowing for a slower, more prolonged absorption into the bloodstream.
- Osmolality: The concentration of a beverage's particles affects how quickly fluid is absorbed. Oral rehydration solutions are designed with a specific osmolality to maximize water uptake.
When is Plain Water Enough?
For most people in day-to-day life, plain water is perfectly sufficient for staying hydrated. It is calorie-free, inexpensive, and readily available. The more advanced hydration benefits of drinks like milk or ORS are typically only necessary in specific situations, such as recovering from intense, prolonged exercise, illness involving fluid loss (like diarrhea or vomiting), or when experiencing significant dehydration in hot conditions.
For example, athletes who engage in strenuous, prolonged training with high sweat loss might benefit from a drink with added electrolytes and carbohydrates to aid in faster and more complete recovery. Likewise, ORS is the gold standard for treating moderate dehydration caused by illness, where water and electrolytes have been severely depleted.
Practical Takeaways for Everyday Hydration
Understanding the science of hydration doesn't mean you should abandon plain water. Instead, it offers a more nuanced perspective on how different beverages serve different purposes. For standard thirst-quenching and daily needs, water is ideal. When you need a boost or are recovering from significant fluid loss, opting for a drink with electrolytes, and potentially some protein and fat, can be more effective for maintaining optimal hydration levels over time.
One common and effective post-workout drink is chocolate milk, which provides a great mix of protein, carbs, and electrolytes to help with both rehydration and muscle recovery. For general health, it's also important to consider your entire diet, as many water-rich fruits and vegetables also contribute significantly to your overall fluid intake.
Conclusion
While water is the undisputed king of general hydration, scientific evidence shows that for longer-lasting fluid retention, certain beverages like skim milk and oral rehydration solutions are superior. The key difference lies in their composition of electrolytes, carbohydrates, and proteins, which slows down fluid absorption and reduces urine output. For most daily situations, plain water is still the best choice. However, for those needing more efficient and sustained rehydration—like athletes or the moderately ill—reaching for milk or an ORS can provide a more significant benefit. By understanding the science behind the Beverage Hydration Index, we can make more informed choices to meet our specific hydration needs.
Read more about the scientific basis of the Beverage Hydration Index in this study from the National Institutes of Health.
