Yes, a bit. In a 2016 study both full-fat and skimmed milk were kept about 1.5 times as well as water over four hours. Hydra counts milk ×1.2: more than water, but not so much that a big latte inflates your day.
Does milk hydrate better than water? When I was choosing how much each drink should count in Hydra, milk was the one I spent the longest on. The science says milk beats water. But a number that's right in a lab isn't always the right number for an app you use every day with real coffee orders. Here's how both sides fit together.
What the study found
The research behind this is a 2016 study that built the beverage hydration index. Volunteers drank about a litre (34 oz) of one of 13 different drinks on separate days. Then the researchers measured how much of it the body kept over the next four hours, with still water as the reference.
Most drinks came out the same as water: sparkling water, cola, diet cola, tea, coffee, lager and a sports drink. Four came out better: an oral rehydration solution, orange juice, full-fat milk and skimmed milk. Both milks landed at about 1.5 times water, in the same range as the rehydration solution made for exactly this job.
So the honest answer is yes. Glass for glass, milk leaves more water behind in your body a few hours later than water does.
Why milk stays in your body longer
Milk isn't just water. It carries protein, sugar, sodium and potassium, and together they slow down how fast the water leaves your body. Plain water has nothing holding it back, so more of it is gone within the four hours the study measured.
One detail I like: full-fat and skimmed milk both did well. So it isn't the fat doing the work. Whichever milk you drink, the effect was there.
The same idea explains the oral rehydration solution's result, which I go into in whether electrolytes hydrate better than water. Drinks that carry salts and nutrients are held on to longer. Drinks that are mostly plain water, flavoured or not, behave like water.
Why Hydra counts milk ×1.2, not 1.5
This is the part only I can tell you, because it's a choice I made. The study measured milk at about 1.5 to 1.58. I could have put 1.5 in the app and called it science. I didn't, and here's why.
The study tested a litre of milk drunk on its own. In real life, most milk arrives in coffee. Picture someone who has a 400 ml (about 13.5 oz) latte every morning and logs it as milk:
- At ×1.5 it would add 600 ml, 200 ml more than they actually drank.
- At ×1.2 it adds 480 ml: still more than water, but only by a little.
Do that every day, add a cappuccino in the afternoon, and at 1.5 the milk bonus starts doing a real part of the work towards the goal. I'd rather the app slightly undercount than tell you you're done when you aren't. So milk counts ×1.2: it gets credit for being better than water, without inflating the day of someone who loves milky coffee.
Rehydration drinks were a different case, and they keep their measured ×1.5. You usually reach for one after a hard, sweaty session or on a hot day, exactly when the extra water staying in the body is the point.
Milk compared with other drinks
Here's how milk stacks up against the usual drinks of the day, and what a 250 ml glass (about 8.5 oz) adds in Hydra:
| Drink | In the 2016 study | Hydra factor | 250 ml adds |
|---|---|---|---|
| Water | The reference | ×1 | 250 ml |
| Coffee, tea | Same as water | ×1 | 250 ml |
| Cola, sports drink | Same as water | ×1 | 250 ml |
| Milk (full-fat or skimmed) | Kept better, about 1.5 | ×1.2 | 300 ml |
| Oral rehydration solution | Kept better, about 1.54 | ×1.5 | 375 ml |
Lattes, cappuccinos and milky coffee
Since most milk is drunk in coffee, the practical question is how to log it. My rule of thumb:
- Mostly milk (latte, flat white, cappuccino, milky tea): log it as Milk. It counts ×1.2.
- Mostly coffee (espresso, americano, filter coffee with a splash): log it as Coffee. It counts ×1.
You can't get this badly wrong. Coffee already counts like water, as I explain in does coffee dehydrate you, so the difference between the two choices on a 300 ml latte is 60 ml. That's a sip, not a glass.
Notice the screen above shows two numbers: what you drank and your net hydration. With milk, net hydration ends up a little higher than what you drank; with alcohol it ends up lower. The goal is checked against net hydration, because that's what your body keeps.
Should you drink milk instead of water?
Not for hydration's sake. The difference is real but modest, and milk is also food: it brings energy and nutrients that water doesn't. Whether that suits your diet is a separate question, and not one a hydration app should answer for you.
What milk does show is that a lot of what you drink, and even eat, adds water to your day. I wrote about that side in whether food counts towards your water intake. And if you'd like to find out how much you need overall, the water intake calculator works it out from your weight, sex, activity and climate with the same formula as the app.
If you simply find water boring, milk in your coffee is one of many small ways to get more fluid in. There are more ideas in habits that help you drink more water.
What to take from it
- Milk, full-fat or skimmed, was kept about 1.5 times as well as water in the study.
- Its protein, sugar, sodium and potassium slow down how fast the water leaves.
- Hydra counts milk ×1.2 on purpose, so a daily latte doesn't do your drinking for you.
- Log milky coffee as Milk and black coffee as Coffee. Either way, it counts.
Your latte counts. Hydra knows by how much.
Hydra counts milk, coffee, tea and every other drink by how much of it your body keeps, so your day adds up right. Free on iPhone and Apple Watch.
