Daily fluctuations in your weight on the scale are completely normal. What you ate, your hormones, how hard you worked out, your hydration levels -- all influence what the number reveals. Your weight can fluctuate by as much as five pounds in either direction from one day to the next. If you weigh yourself two days in a row, it could show that you've lost a pound or two; however, it's unlikely that pound came entirely from fat.
Water Weight Loss in One Day
Your weight often fluctuates within 24 hours due to changes in the amount of fluid your body carries. A salty meal and menstrual hormones can cause you to retain water. Ironically, being dehydrated can cause water gain too, as your body struggles to hold onto the fluids it does have.
When you reverse any of these actions -- such as drinking enough water so that you urinate more, eating a diet low in salt or experiencing a hormonal shift -- you may lose a pound or two on the scale. You haven't suddenly dropped a pound of fat, you're simply retaining less fluid.
Often, when you first start a diet plan, your body will release water weight as you make healthier food choices and reduce portion sizes. Losing a pound overnight sure feels good and can be a great incentive to keep with your plan, but it takes longer than one day or night to lose an actual pound of fat.
What It Takes to Lose One Pound of Fat
A pound of fat is equal to 3,500 calories. To lose one pound, you must eat 3,500 calories fewer than you burn in that day. Since most adults, on average, burn between 1,600 and 3,000 calories daily -- depending on size, age, gender and activity level -- it's nearly impossible to create a 3,500-calorie deficit in a day, even if you don't eat a thing.
If you do burn more than 3,500 calories per day, you could technically drop a pound of fat in one day on a starvation or near-starvation diet, but it's not recommended. The calorie deprivation could leave you nutritionally deficient. Losing at this fast rate for a consistent period of time also puts you at risk of gallstones.
A healthy, sustainable rate of loss for the average person is just one to two pounds per week -- not per day. Even morbidly obese patients who are prescribed a very-low calorie diet, which consists of meal supplements of 800 or fewer calories per day, do not lose a pound of fat per day; instead, they usually lose an average of three to five pounds per week. They're put on these diets to address immediate health concerns related to their size but are kept on them for no more than 12 weeks because of the potential health risks of losing weight too quickly.
Exercise, Dehydration and Pounds Lost
A particularly hard workout could also make it seem like you lost one pound or more in one day -- or even a matter of hours. In one football game or practice, for example, a running back may drop between four or five pounds, depending on his size and intensity level. This isn't fat weight, however. Sweat and respiration lead to excessive fluid loss, which shows up as weight loss on the scale.
The average person -- not just professional athletes -- also sweats out water weight when working out. Take an intense indoor cycling class or run a marathon in hot conditions, and you may lose a pound or more on the scale. Rehydrate after your workout with 16 to 24 ounces of fluids for each pound you lose during the session to bring your body back into balance. If you don't rehydrate adequately after a workout, you may seem to weigh a pound -- or more -- less the next day. Dehydration isn't a healthy way to get a lower number on the scale. It can negatively affect sports performance, energy levels and organ health.
Digestion and Weight Changes
The scale measures fat along with bone, muscle, fluids, organs, connective tissue and any food you've yet to digest. If you weigh yourself the night after a large meal and then again the next morning, for example, it's possible the morning number will be lower. The higher weight in the evening after your meal included the actual weight of the food you ate. Once that food's been processed and waste eliminated, you'll likely weigh less.
Inefficient food processing can also artificially inflate the scale. Constipation means you may be holding onto extra weight due to compacted feces. If you've been backed up for a couple of days, when you do finally get relief, it could show up as a pound lost on the scale. Eating more fiber and drinking enough water could help you have more regular bowel movements and avoid weight fluctuations due to irregularity.