Does Milk Hydrate Better Than Water?
In a landmark randomized trial, skim milk and whole milk both retained significantly more fluid than plain water — because protein, lactose, fat and sodium slow how fast the drink leaves your stomach.
Quick answer
Does Milk Hydrate Better Than Water?
Yes. In the 2016 Beverage Hydration Index study, skim milk scored 1.58 and whole milk 1.50 at 2 hours, both significantly above water's reference 1.00 (Maughan 2016, Am J Clin Nutr). Over 4 hours, drinkers passed 339 g less urine after skim milk and 294 g less after whole milk than after water — milk's protein, lactose and sodium slow gastric emptying and aid retention.
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Yes — milk hydrates better than water, and the difference is measurable. In the 2016 Beverage Hydration Index (BHI) study, Maughan and colleagues had 72 euhydrated, fasted men each drink 1 litre of still water or one of 12 other beverages over 30 minutes, then collected their urine for 4 hours (Maughan 2016). Water was anchored at a BHI of 1.00; any drink above that retained more fluid than water. Skim milk scored 1.58 ± 0.60 at 2 hours and full-fat (whole) milk scored 1.50 ± 0.58 — both significantly higher than water (P < 0.05). In practical terms, participants produced 339 g less urine after skim milk and 294 g less after whole milk than after the same volume of water. Milk's protein, lactose, fat and sodium slow gastric emptying and electrolyte content aids fluid retention, so the water you drink stays in your body longer.
Milk vs Water: Beverage Hydration Index at 2 Hours
| Beverage | BHI at 2h | Urine vs water (4h) | Notes |
|---|---|---|---|
| Still water (reference) | 1.00 0 g (baseline) | Anchored at 1.00 — every other drink is measured against it. | 0 g (baseline) Source: Maughan 2016 (Am J Clin Nutr) |
| Skim (skimmed) milk | 1.58 ± 0.60 −339 g (95% CI 190–489) | Highest BHI of any milk; water-corrected difference vs water remained significant (P < 0.01). 350 kcal/L. | −339 g (95% CI 190–489) Source: Maughan 2016 (Am J Clin Nutr) |
| Full-fat (whole) milk | 1.50 ± 0.58 −294 g (95% CI 154–434) | Significantly better than water; water-corrected difference remained significant (P = 0.02). 640 kcal/L. | −294 g (95% CI 154–434) Source: Maughan 2016 (Am J Clin Nutr) |
| Oral rehydration solution (ORS) | 1.54 ± 0.74 −362 g (95% CI 222–505) | The only non-milk drink to beat water through 4 hours; water-corrected P = 0.01. | −362 g (95% CI 222–505) Source: Maughan 2016 (Am J Clin Nutr) |
Why Milk Out-Hydrates Water
Protein and lactose slow gastric emptying
Milk's protein, lactose and fat make the stomach release fluid into the gut more gradually than plain water. Slower emptying means a steadier, more sustained delivery of fluid — and less of a sudden bolus for the kidneys to flush out as urine (Maughan 2016).
Sodium and electrolytes aid retention
Milk naturally contains sodium and other electrolytes. These help the body hold onto the fluid you drink, which is the same principle behind oral rehydration solution (BHI 1.54) — the other drink that stayed ahead of water through 4 hours in the study (Maughan 2016).
Less urine, more fluid retained
Retention is what BHI actually measures. Over 4 hours, participants passed 339 g less urine after skim milk and 294 g less after whole milk than after the same 1 litre of water (Maughan 2016) — direct evidence that more of the milk stayed in the body.
Energy density is part of the mechanism
Whole milk carries 640 kcal/L and skim milk 350 kcal/L (Maughan 2016). That energy density — protein, sugar and (for whole milk) fat — is exactly what slows the drink down in the gut and stretches out fluid delivery, unlike calorie-free water.
Milk vs Plain Water for Hydration
| Feature | Milk | Plain Water |
|---|---|---|
| Beverage Hydration Index at 2h | Skim 1.58 / Whole 1.50 | 1.00 (reference) |
| Significantly better retention than water? | Yes — P < 0.05 at 2h | Baseline |
| 4h urine output vs water | −339 g (skim) / −294 g (whole) | Baseline |
| Water-corrected difference still significant? | Yes — skim P < 0.01, whole P = 0.02 | — |
| Electrolytes that aid retention | Naturally contains sodium | None |
Putting Milk's Hydration Edge to Use
- Reach for milk when you need fluid to last — its BHI of 1.50–1.58 at 2 hours means more is retained than the same volume of water (Maughan 2016).
- Skim milk edged out whole milk (1.58 vs 1.50) while carrying far fewer calories (350 vs 640 kcal/L), so it's the better choice if you want hydration without the energy load (Maughan 2016).
- Oral rehydration solution (BHI 1.54) is the non-dairy alternative that also beat water through 4 hours — useful if you can't or don't drink milk (Maughan 2016).
- Don't expect every drink to do this: cola, diet cola, tea, iced tea, coffee, lager, orange juice, sparkling water and the sports drink tested all matched water (BHI ≈ 1.0) over 4 hours (Maughan 2016).
- Plain water is still perfectly hydrating — no drink in the study scored significantly below water, so milk's advantage is about retaining fluid longer, not water being inadequate (Maughan 2016).
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- BHI 1.58 ± 0.60 vs water 1.00 — Skim milk had a mean Beverage Hydration Index of 1.58 ± 0.60 at 2 hours, significantly higher than water (set at 1.00).[1]
- BHI 1.50 ± 0.58 vs water 1.00 — Full-fat (whole) milk had a mean Beverage Hydration Index of 1.50 ± 0.58 at 2 hours, significantly higher than water.[1]
- P < 0.05 vs water at 2h — At 2 hours, full-fat milk, skim milk, ORS and orange juice all had significantly higher BHI than water (P < 0.05).[1]
- −294 g (whole); −339 g (skim) vs water — Over 4 hours, whole milk produced 294 g less urine than water (95% CI 154–434) and skim milk produced 339 g less (95% CI 190–489).[1]
- skim P < 0.01; whole P = 0.02 — Water-content-corrected 2 h BHI stayed significantly above water for skim milk (P < 0.01) and whole milk (P = 0.02).[1]
- ORS BHI 1.54; −362 g vs water — Oral rehydration solution scored a 2 h BHI of 1.54 ± 0.74, retaining 362 g less urine than water over 4 hours (95% CI 222–505).[1]
- whole 640 kcal/L; skim 350 kcal/L — Energy density of the milks: full-fat milk 640 kcal/L and skim milk 350 kcal/L.[1]
- n=72; 1 L; 30 min; 4 h collection; 13 beverages — Study design: 72 euhydrated, fasted healthy men each ingested 1 L of still water or another beverage over 30 minutes, with urine collected for 4 hours across a 13-beverage panel.[1]
- 0 beverages below water (n.s.) — No beverage tested scored significantly below water, so none caused net dehydration at the 1 L dose.[2]
- [1]Maughan 2016 (Am J Clin Nutr) — Maughan RJ, Watson P, Cordery PA, Walsh NP, Oliver SJ, Dolci A, Rodriguez-Sanchez N, Galloway SD. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. Am J Clin Nutr. 2016 Mar;103(3):717-23.PMID: 26702122DOI: 10.3945/ajcn.115.114769
- [2]American Council on Exercise (ACE) ProSource 2016 — American Council on Exercise (ACE) ProSource, April 2016. The Newest Index on the Block: The Hydration Index (summary of Maughan et al. 2016).
Last reviewed: 2026-06-20. Every figure on this page is sourced to the named primary references above.
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Frequently Asked Questions
Is milk more hydrating than water?
Yes. In the 2016 Beverage Hydration Index study, skim milk scored 1.58 and full-fat milk 1.50 at 2 hours, both significantly higher than water's reference value of 1.00 (Maughan 2016). Over 4 hours, drinkers produced 339 g less urine after skim milk and 294 g less after whole milk than after the same litre of water — meaning more of the milk stayed in the body.
Why does milk hydrate better than water?
Milk's protein, lactose and fat slow how quickly the stomach empties, so fluid is delivered to the body more gradually rather than as a sudden bolus the kidneys flush out. Milk also naturally contains sodium and electrolytes that help the body hold onto fluid. Together these slow fluid loss and improve retention, giving milk a Beverage Hydration Index of 1.50–1.58 versus water at 1.00 (Maughan 2016).
Which is more hydrating, skim milk or whole milk?
Both beat water, but skim milk had the slightly higher Beverage Hydration Index — 1.58 at 2 hours versus 1.50 for full-fat milk (Maughan 2016). Skim milk also led to more fluid retained, with 339 g less urine over 4 hours compared with 294 g less for whole milk. Skim milk delivers this with fewer calories too: 350 kcal/L versus 640 kcal/L for whole milk.
How much less urine do you produce after drinking milk?
Compared with drinking the same 1 litre of still water, participants produced 294 g less urine over 4 hours after whole milk (95% CI 154–434) and 339 g less after skim milk (95% CI 190–489) (Maughan 2016). Less urine output is the direct sign that more of the fluid was retained — which is exactly what the Beverage Hydration Index measures.
Does milk hydrate better than a sports drink?
Yes, in this study. Skim milk (1.58) and whole milk (1.50) hydrated significantly better than water, while the sports drink tested produced urine output not different from water, giving a Beverage Hydration Index of about 1.0 (Maughan 2016). Milk's protein, electrolytes and energy density (350–640 kcal/L) slow gastric emptying and improve fluid retention more than a typical sports drink.
What is the Beverage Hydration Index?
The Beverage Hydration Index (BHI), created by Maughan and colleagues in 2016, compares how well a drink is retained versus still water, which is anchored at 1.00. It is the urine output after water divided by the urine output after the test drink. A BHI above 1.0 means the beverage is retained better than plain water over the 4-hour measurement window (Maughan 2016).
Does this mean I should stop drinking water?
No. Water is still a fully effective way to hydrate, and no beverage in the study scored significantly below water (Maughan 2016). Milk's advantage is that it is retained longer, not that water fails. For most everyday hydration, water is convenient and calorie-free; milk and oral rehydration solution are useful when you specifically want fluid to stay in the body longer.
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