A lot of questions have been asked and answered about hydration. How much fluid do we need daily? It depends on the person and the activity level, but generally 2.7 liters for women and 3.7 liters for men. Does fluid in food count towards our necessary daily intake? Yes! Do caffeinated beverages count? Yes, unless they are hyper-caffeinated. Do alcoholic beverages count? Only beer does, because most other alcoholic drinks do not have enough volume of fluid to counteract the dehydrating effects of alcohol. Despite all the talk, it turns out we are generally able to maintain a state of euhydration (optimal water content in our bodies as regulated by our brains) without a lot of effort.

But let’s face it, the heat index is rising. This is the hottest summer on record. Many of us work outside or workout outside, without steady access to bathrooms and hydration. In these situations, how you hydrate really matters. And it turns out, water may not be your best choice.

Fast and Refreshing Versus Slow and Steady

When we look at effective hydration, we have to consider how our bodies respond to beverages. When we drink, the beverage goes through our stomach and into our lower digestive tract. Once the fluid is emptied from the stomach it can begin to be absorbed into our bloodstream, where it dilutes the fluid in the body and hydrates us. Water (both still and sparkling) is a super-efficient hydrator because it goes straight through your stomach and straight into your bloodstream.

But when you are dealing with extreme heat, fast and refreshing may not be the way you want to hydrate. Researchers at St. Andrews School of Medicine in Scotland compared hydration responses to several different drinks and found that when it comes to sustained hydration, milk is a better hydrator than water. Milk was found to be more hydrating because it contains lactose (a sugar), some protein and some fat, all of which help to slow the emptying of fluid from the stomach and keep us hydrated for a longer period of time.

This study investigated the effects of 13 different commonly consumed drinks on fluid balance and urine output and came up with a “beverage hydration index,” establishing a beverage hydration index (BHI). Each beverage was measured against the “hydration standard,” which was still water.

How Did Common Beverages Stack Up?

As it turned out, sparkling water, cola, diet cola, orange juice, hot tea, iced tea, coffee, beer and a sports drink were eliminated from the body and had a hydration index very similar to still water. The drinks that sustained hydration better than water were whole milk, skim milk and the oral rehydration solution (ORS). Oral rehydration solutions are water mixed with some salt , sugar and electrolytes. Oral hydration salts are available in small packets that can be mixed with water. In this study, whole milk had the best hydration index, followed by skim milk and the ORS.

These results are particularly compelling because sports drinks are generally considered to be the gold standard when it comes to powerful rehydration. But they may not have the longer- term benefits of milk or oral hydration solutions.

The Bottom Line:

Having water handy is generally an effective way to make sure you stay hydrated throughout the day. But in extreme heat, or during times when you need longer term hydration you may get more benefit from milk. Another excellent idea is to carry oral rehydration packets, which you can mix with water if you begin to feel very dehydrated.