Your body weight fluctuates naturally daily based on the number of calories you eat and how much water your body gains and loses through natural water loss and sweat loss each day. Some body weight fluctuation daily is normal and expected. However, drastic changes in body weight on a daily basis can be a sign of something more serious.
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Your body is fairly efficient at maintaining water balance between the amount of water you drink and the water your body excretes. Normal, healthy body weight changes due to fluctuations in total body water are fairly small -- about 0.2 percent of your total body weight per day, according to the University of Rochester Medical Center. For example, if you weigh 120 pounds, a healthy water-weight fluctuation is about 1/4 pound per day. If you're gaining or losing more than 1/4 pound of water weight daily, your diet may be unbalanced or you could be dehydrated.
If your body excretes too much water because of an unbalanced diet or excessive water losses -- which can occur due to prolonged exercise, high air temperatures, diarrhea, vomiting, high fever and trauma -- dehydration is a concern and can be dangerous. The University of Rochester Medical Center reports that if you lose 1 percent of your body weight, which is 1.2 pounds for a 120-pound adult, you will likely feel tired and your physical performance will be affected. If in a day you lose 2 to 4 percent of your body weight in water, equal to 2.4 to 4.8 pounds for a 120-pound person, mental functioning will decline.
Body Fat Changes
Body fat weight does not fluctuate as rapidly as water weight. One pound of body fat equals 3,500 calories, according to the Centers for Disease Control and Prevention. Therefore, to lose 1 pound of body fat in a day you'd have to burn off 3,500 more calories than you eat, which is nearly impossible -- and unhealthy. Many adults burn 13 to 18 calories per pound of their body weight daily, according to Harvard Medical School -- which equates to 1,560 to 2,160 calories daily for a 120-pound adult. Eating low-carb diets often causes water-weight loss, at least initially, which isn't a good indicator of fluctuations in body fat.
Safe Daily Weight Fluctuations
To lose -- or gain -- weight at safe rate, aim for a weekly fluctuation of 1 to 2 pounds per week, according to the Centers for Disease Control and Prevention and a 2008 issue of "Today's Dietitian." Therefore, a safe change in body weight is 0.14 to 0.29 pound daily plus or minus 0.25 pound of water-weight fluctuation. When you're eating a well-balanced diet and drinking plenty of water, look for weekly -- not daily -- weight-loss and weight-gain trends.