To examine this myth regarding the hydrating effect of soda, you have to begin with analyzing what makes up soda. Essentially, soda is almost entirely simply carbonated water. So this begs the question: Does carbonated water hydrate equally or nearly equally to water? Could this be the source of some dehydration?
The simple act of adding carbonation to water is merely aesthetic, and does not effect how the water is processed in your body. Your body easy removes the carbonation, and is left with just water.
So what else is in soda? The next big ingredient in sodas is sugar. Unlike carbonation, high sugar content does slow hydration. Your body needs more time to process these sugars, and therefore devotes some water away from other places in your body to break down the sugar. That being said, the effect is rather minimal. Take a look at Gatorade for instance. It prides itself as being one of the best ways to hydrate, as well as add electrolytes back into your body. However, Gatorade also has very high sugar content. Depending on the brand, Gatorade can be just as sugary as soda. But for the sake of argument, let’s consider diet soda. There is no sugar in diet sodas, only chemical sweeteners which have been shown to have no effect on hydration (showed through studies of low-calorie and zero-calorie flavored water).
Could dehydration stem from the sodium concentration in soda? The answer here is a simple no. Sodium helps your body retain water, and stay hydrated. However, sodas are very weak sources of sodium, so any effect either way is practically non-existent.
But there’s still a big elephant in the room when it comes to soda, and that’s the caffeine content. For years, experts were saying that caffeine acted as a diuretic in the body, causing you to urinate more fluid and therefore become less hydrated. However, real world studies have shown that caffeine has only a mild diuretic effect, similar to that of just plain water.
But logically, it is obvious that soda wouldn’t dehydrate you. For a fluid to dehydrate you, it would have to remove more from you then you add by actually drinking the fluid. So let’s look at diet coke for example. It has 45 mg of caffeine in it per 12 ounces. For diet coke to have a dehydrating effect, 45 mg of caffeine would have to remove more than 12 ounces of water from your body. This is obviously incorrect. To show this, let’s look at a 12 ounce Starbucks coffee, which contains 260 mg of caffeine, or about 6 times as much as a same size diet coke. If the amount of caffeine in a diet coke was capable of dehydrating you, imagine what the coffee would do to you. If this were true, then you would lose 60 ounces of water per cup of coffee. That means for every two coffees you drink, the caffeine would remove a gallon of water from your body. Everyone knows that person who drinks over 10 cups of coffee a day, yet you don’t see them peeing out over 5 gallons of water. Obviously the argument that soda actually dehydrates you is ridiculous.
I believe what people mean to say is that soda simply doesn’t hydrate you as much as pure water. This is probably true. The caffeine content combined with the sugar could somewhat lower the hydrating effect. That being said, it is ignorant to claim that soda dehydrates you. In reality, the hydrating effect of diet soda compared with that of water is so minimally different that there is no point in actually distinguishing the two. In fact, because people are more likely to drink larger amounts of soda that water due to its good taste, soda often allows people to hydrate better than pure water.