Water and sports drink are thought off as drinks with great hydration properties whereas caffeine containing beverages such as tea and coffee and alcohol containing beverages such as beer are thought of as dehydrating. But how do these drinks really stack up? The volume and composition of ingested drinks have a strong influence on how rapidly they will leave the stomach and are absorbed in the small intestine. Of course the faster they are emptied from the stomach and the faster they are absorbed, the faster fluids will enter the body. However, it is not just about the speed at which fluids enter the body because if you pee out the same amount, the net effect is no fluid retention. The contents of a drink will modify both absorption and excretion. Some drinks will slow down the delivery of fluid, but may also slow down the excretion. Perhaps surprisingly no an had ever compared some of the most commonly consumed drinks.
Professor Ron Maughan and colleagues set out to compare a number of different drinks and capture the hydrating properties in what they called the beverage hydration index (BHI). Essentially, the index compares how much of a drink is retained 2 hours after consumption compared to the same amount of water. The higher the index the more fluid is retained in the body. The full paper is available online, but in brief, they compared 13 beverages. Seventy two volunteers consumed 1 liter of each of these drinks. For the next 2 hours urine was collected. 2 hours is a practical time because it is probably similar to normal drink consumptions patters (as opposed to 4 hours which was used in many previous studies).
The results demonstrated that some drinks had better hydrating properties than water. Perhaps not surprising is the fact that oral rehydration solutions scored the highest value. These heavily researched drinks deliver fluid fast and the high electrolyte content is responsible for fluid retention. But skimmed milk, whole milk, and orange juice also scored well. These drinks are higher in calories and have more ingredients that may slow down gastric emptying and absorption but result in an overall greater fluid retention.
The other effect that may be somewhat surprising to some was that beer (lager), coffee and tea had scores that were very similar to water. They certainly did not display the dehydrating properties that are often talked about. The difference between beer, coffee, tea and water was not statistically significant. It is likely that the dehydrating properties of alcohol and caffeine were counterbalanced by the fluid retaining properties of the other ingredients. It is also possible that alcohol and caffeine in very small amounts do not have diuretic effects. The early studies that demonstrated these properties were done with much larger amounts of alcohol or caffeine.
We can further learn ffrom this study that a number of beverages were pretty similar to water (sports drinks, tea, cola, diet cola).
So although some drinks are better than others at retaining fluids, most drinks will contribute to daily fluid requirements.