What is Water Weight?

If you gain or lose three or four pounds overnight, most of that is probably due to water weight. But how and why do we gain and lose so much water?

Monica Reinagel, MS, LD/N, CNS ,
March 6, 2018
Episode #468

image of a water bottle with weights and exercise equipment

Hai-Ting asks: “What exactly is water weight? It’s a phrase that gets thrown around a lot. I would love to know what we are talking about. How and why do we gain and lose so much water?”

If you are in the habit of getting on the scale every morning (a practice which is linked with healthier body weights over the long-term) you’ve probably noticed that your weight can vary wildly from day to dayin ways that sometimes seem unfair. “How could I have gained two pounds overnight? I had a salad for lunch and skipped dessert at dinner. I should weigh two pounds less!”

It’s tempting to think whatever we did or didn’t eat yesterday should show up the next day on the scale. But it doesn’t work that way. As Dan Ariely pointed out in our recent interview, it takes a lot longer for dietary changes to result in fat loss (or gain). If you gain or lose three or four pounds overnight, most of that is probably due to water weight.

Where is Water Weight Stored

Our bodies contain a lot of water, but we’re not just giant water balloons. Water is held in a variety of places in the body. Some of these don’t change very much from day to day. Our bones, for example, are about 30% water but that stays relatively constant.

Other compartments, such as our stomachs and bladders, can hold relatively large amounts of water, but only on a very temporary basis. And here’s the thing: Water is heavy. A pint of water (or, for that matter, beer) weighs about one pound.

The amount of fluid in your stomach and bladder have a profound effect on your body weight at any given moment.

If you were to weigh yourself, drink 16 ounces of fluids and immediately weigh yourself again, you’d have “gained” one pound in about 15 seconds. A couple of hours later, a lot of that water will be collected in your bladder. Weigh yourself before and after visiting the bathroom and you can enjoy the thrill of “losing” up to a pound in 15 seconds.

Obviously, the amount of fluid in your stomach and bladder have a fairly profound (yet meaningless) effect on your body weight at any given moment. But that’s not the only thing we’re talking about when we mention water weight.

In between the stomach and the bladder, things get a bit more complex. As it passes through the small intestines, the water we consume is absorbed into the bloodstream and distributed throughout the body’s organs and tissues, where it is used for all kinds of things, everything from lubricating our  membranes to metabolizing nutrients to maintaining electrolyte balance. All of these activities affect the amount of water retained in our bodies.

Water is a major component of perspiration, for example. Heavy sweating can cause us to lose a lot of water weight in a relatively short period of time. Run a few miles on a hot day and jump on the scale before rehydrating and you’ll see how much water you’ve lost. (You probably won’t, however, see how much fat you burned.)

Water is also involved in lots of different chemical reactions. Some reactionssuch as converting carbohydrates into glycogenrequire water. Otherssuch as breaking down proteinsrelease water.

Sudden changes in the protein or carbohydrate content of your diet can either lock up or mobilize relatively large amounts of water.

How Diet Affects Water Weight

If you dramatically reduce the amount of carbohydrates you are eating, your body will be forced to dip into the glycogen stored in your muscles and liver for energy. As the glycogen is used for energy, a lot of water is released into the bloodstream and routed to the kidneys for elimination. Obviously, this has nothing to do with fat loss but it does give low-carb dieters a big sense of accomplishment during the first few days of their diet.

Even if you don’t go low carb, suddenly reducing your calorie intake will also force the body to use its glycogen stores and result in some water loss. The effect is just not quite as dramatic as it is with a low carb diet.  

This effect is also completely reversible. As soon as you start eating more calories or carbohydrates, your body will use some of those carbs plus some water to restock its depleted glycogen energy stores, leading to an increase in water weight.


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