For Maximum Hydration, Stick To Plain Milk
Feeling thirsty? Common sense might dictate that you reach for a glass of water, but some of the science out there actually suggests that you might be better off grabbing a carton of milk as it might be more hydrating.
A 2015 study published in the American Journal of Clinical Nutrition found that unflavored milk (both skimmed and full fat) had a higher beverage hydration index (BHI). A higher BHI score means that the drink should help you to retain more water, and the two varieties of milk both came out on top of water in their BHI score — although skimmed milk was rated as better for hydration than full fat. Orange juice also scored above water, as did oral rehydration solution (the powdered stuff that you might dissolve in water and drink to help rehydrate if you're suffering from diarrhea).
Other drinks like coffee and sugary sodas rated below water when it came to hydration — while sparkling water was a fraction less hydrating than its still counterpart.
What makes milk hydrating?
The chemical makeup of milk is what makes it so hydrating. It has just the right combination of lactose (a sugar), sodium, fat, and proteins to enable your body and stomach to retain water. Plus, there's water in milk too: the study lists full-fat milk as about 88% water and skimmed milk as 91% water.
But it's all about moderation, particularly when it comes to the sugars in milk and other drinks. This is because if your body is flooded with excess sugar, it'll work to flush it out in the form of urine. So, while this study didn't include sweetened milk drinks, common sense dictates that you won't want to reach for a beverage like chocolate milk if you're feeling all dried up.
As for milk alternatives like oat or almond milk, the jury is still out. They may have slightly more water content (particularly in the case of almond milk) but could be missing some of the other nutrients that make cow's milk hydrating. Plus — and perhaps most importantly — the hydration impacts of non-dairy milks just haven't been studied much.
The science might not be totally settled
While there is scientific evidence for the hydration benefits of milk, you may not want to go jogging with a gallon of it — not only because it might get all heated up and nasty as you sweat. Harvard Medical School doctor Robert H. Schmerling has raised interesting questions about the idea of milk being more hydrating than water. He notes that the studies had fairly limited numbers of participants and that the reported hydration difference between milk and water may be minuscule. On top of that, chugging milk to recover from exercise will pump calories back into your body, potentially offsetting the benefits of exercising in the first place — though runners know that a small glass of chocolate milk often makes a great recovery drink (per Runner's World).
While it can't hurt to experiment with trying a glass of milk when you're thirsty, more research is needed before coming to a definitive conclusion on whether it's truly better than water. In any case, just don't reach for super sweet milk if you're looking to quench a serious thirst.