How Many Pounds of Water Weight Can I Lose in a Week?

Feet on scale over green lawn and wet pebble.
Feet on a scale (Image: kyolshin/iStock/Getty Images)

Your body is composed of numerous substances, but nothing is more fundamental and pervasive as water. It plays a role in many of the body’s basic functions. The percentage of water in your body changes daily, confusing many people who are trying to lose weight. Big drops on the scale during the first week or two of a diet are due in large part to the loss of water. After initial weight loss, water loss slows down.

Water Composition

Water exists throughout your body, from your brain to your toes and everywhere in between. According to Dr. Jeffrey Utz, a neuroscientist and pediatrician from Allegheny University, about 60 percent of an adult male body and 55 percent of an adult female body is water. Women generally carry more weight as fat -- which is 50 percent water, compared to muscle, which is 75 percent water.

Why Water Matters

The amount of water in your body fluctuates throughout the day, depending upon consumption and excretion through urine and perspiration. You need to consume at least 6 to 7 cups -- 48 to 56 ounces -- of water a day to carry out the integral functions of the body. Water helps transport molecules through the blood, lubricates joints, regulates body temperature, protects tissues and aids in waste and toxin elimination. In addition, water is the final byproduct of cellular respiration.

Water Loss

It’s possible to lose as much as 20 pounds of body weight during one week, most of which is water. This water weight loss, however, will only occur during the first week or two of a new diet or exercise regimen. You may lose very little water in subsequent weeks -- a few pounds at most -- and you may even gain some of that water back.

Water Stores

Water loss is accelerated at the beginning of a diet because the body sheds excess stored water, for example, in glycogen. Glycogen is a carbohydrate in the liver and muscles. When the body needs a quick infusion of energy, it turns to these stores. The human body can store about 400 grams of glycogen, but for each gram of glycogen, there are three additional grams of water stored with it. This represents 1,600 combined grams of glycogen and water, which equates 3.5 pounds.

Knowing What to Expect

Many people confuse fat loss with weight loss. After a fast jump start during the first week of a diet, when water weight is quickly shed, it can be frustrating when weight loss slows down. But due to the constant fluctuation of water, it is actually possible to lose fat but gain weight. Expect weight loss to slow down. Just remember that after the initial phase of a diet, further losses represent more fat loss.

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