Science Explains Why You Lose Water Weight

How Water Weight Loss Works

When you cut your calorie intake or reduce carbs, your body uses glycogen. Glycogen is attached to water, so when it's burned you lose the extra water.
When you cut your calorie intake or reduce carbs, your body uses glycogen. Glycogen is attached to water, so when it's burned you lose the extra water. Jamie Grill / Getty Images

New dieters, especially if they're eating a low carb diet, see a dramatic initial weight loss ranging from four to 12 pounds in the first week. The initial loss is exciting, but it quickly slows to one or two pounds per week. You've probably heard this early weight loss is water weight, rather than fat. Where does water weight come from and why does it drop before fat? Here's the scientific explanation.

The Source of Water Weight

The early weight loss from a diet can be partly fat, especially if you're exercising and reducing calories, but if you're using more energy than you're replacing as food and drink, the first weight you'll lose will be water. Why? It's because the energy source your body turns to once it runs out of its relatively small store ​​of carbohydrates (sugars) is glycogen. Glycogen is a large molecule made up of a protein core surrounded by glucose subunits. It's stored in the liver and muscles for use during energy-intensive activities, like running away from danger and supporting the brain when food is scarce. Glycogen can be quickly metabolized to meet the body's need for glucose, but each gram of glycogen is bound to three to four grams of water. So, if you use up your body's glycogen stores (as when dieting or with prolonged exercising), a lot of water is released over a short amount of time.

It only takes a few days of dieting for glycogen to be expended, so the initial weight loss is dramatic. Loss of water can lead to loss of inches! However, as soon as you eat enough carbohydrates (sugars or starches), your body readily replaces its glycogen stores. This is one reason people often see an initial weight gain immediately after going off a diet, particularly if it was one that restricted carbohydrates.

It's not the fat coming back, but you can expect all the water you lost the first couple of days of a diet to return.

Other Causes of Water Weight Changes

There are many biochemical reactions in the body that affect how much water is stored or released. Natural hormonal fluctuations can have a big impact on water storage. Since the body maintains stable electrolyte levels, losing too much of an electrolyte can leave you dehydrated, while too high of an intake can cause you to retain water.

Diuretics are chemicals that prompt the release of water. Natural diuretics include any stimulant, such as coffee or tea. These chemicals temporarily alter the natural set point for water retention, causing slight dehydration. Alcohol also acts as a diuretic, potentially causing much greater dehydration because additional water is used to metabolize ethanol.

Eating too much sodium (as from salt) leads to water retention because water is needed to dilute the high level of the electrolyte. Low potassium, another electrolyte, can also cause fluid retention because potassium is used in the mechanism that releases water.

Many medications also affect water homeostasis, potentially leading to water weight gain or loss.

So do some supplements. For example, dandelion and stinging nettle are natural diuretic herbs.

Because water is used for thermoregulation, heavy perspiration, whether it be from exertion or sweating in a sauna, can produce temporary weight loss from dehydration. This weight is immediately replaced after drinking water or other beverages or eating foods that contain water.

A surprising cause of water retention is mild dehydration. Because water is critical to so many processes, when it's not being replenished at a fast enough rate, conservation mechanisms kick in. Water weight won't be lost until adequate water is being consumed and normal hydration is achieved. After that point, research indicates drinking more water doesn't aid weight loss. Nutrition expert Beth Kitchen (University of Alabama at Birmingham) conducted research that concluding drinking more water does burn a few more calories, but it wasn't a significant number.

Her research also indicated drinking ice-cold water as opposed to room temperature water resulting in an insignificant difference in calories burned and weight lost.