We Americans, never without our water bottles, are probably the most hydrated population on earth. But how much of what we drink is retained and how much is just as quickly passed through our pipes?

All liquids are not created equal. Diuretics, such as coffee, by their nature increase urine output and so reduce fluid retention. This can be a good thing if you are inordinately fearful of feeling bloated, but less beneficial if you are running a marathon and trying to avoid frequent bathroom breaks.

A group of British scientists decided to figure out specifically which drinks we were retaining the longest, and which were most likely ending up in the porta-potties along the side of the race courses. They published their results in the American Journal of Clinical Nutrition.

The experiment compared how well each of ten drinks were retained in the two to four hours immediately after consumption. Volunteers downed one liter of fluid and then had their urine output monitored to indicate how much had been retained.

In order to properly communicate results, the team had to first create a new metric, which they called the “Beverage Hydration Index.” Baseline for the index was still, “non-fizzy” water, and was assigned a BHI score of 1. A drink with a BHI score of 1.4 would indicate that 40 percent more of it was retained in the body two hours after consumption than when drinking the same amount of water.

Triathlete and hydration expert Andy Blow analyzed the results and noted that the top performer in terms of fluid retention was oral hydration solution (as you might expect, as it is engineered specifically for this purpose). The place and show positions may come as a surprise to you: milk was number two, followed by orange juice. These were the only drinks to perform significantly better than water, with oral hydration solution and milk approaching a 1.5 BHI score, meaning 50 percent more fluid was retained in the body two hours after consumption when compared with water alone.

What's the science? Blow speculates that the likely reason for these drinks' longer laps through your body's systems is due to the significant amounts of calories and/or electrolytes they contain. That’s especially in the case of oral hydration solution, as it has over 1300mg sodium per liter as well as some glucose. Blow recommends sports drinks with large amounts of electrolytes, up to 1500mg per liter, for “preloading” before a big event, as the extra sodium will help you retain more of the fluid you take in for longer.