Not all changes in the scale are caused by fat loss or gain. It’s normal for weight to fluctuate from day to day—or even on the same day—due to changes in fluid retention (commonly referred to as “water weight”).
Below are a few of the most common causes of major weight fluctuations:
Carbohydrate intake
There are two ways that eating more carbs can cause an increase in water retention:
When you increase your carb intake, odds are that you’ll store at least some of these carbs in the form of glycogen. For every gram of glycogen your body stores, you’ll store an additional three grams of water. This water will show up on the scale as extra weight.
When you increase your carb intake drastically, the production of a hormone called insulin can increase. While insulin is helpful in that it will help pull carbs into cells so that they can be processed for energy, insulin can also cause salt and water retention, leading to a temporary increase in scale weight.
When you drastically decrease carbs, insulin levels will fall and you’ll burn through much of your glycogen, which is why those starting a low-carb diet usually see a dramatic drop in weight during their first week. Alternatively, if you’re just coming off a low-calorie or carb diet or have eaten a lot of extra carbs on your refeed day, you may see a temporary jump in the scale. Not to worry, this will level out after a day or two!
Dehydration
Ironically, not drinking enough water can actually cause your body to retain more water to compensate, which could cause a temporary increase in scale weight.
Sodium balance
A drastic increase in salt intake above usual levels can cause your kidneys to retain more water. Some people are more sensitive to this than others, but if you plan on going to a sodium-laden Chinese buffet at some point in the near future, you’d better be prepared to see a small jump in the scale the next day.
Menstrual cycle
Because of hormonal changes around that time of the month, you may retain water—particularly in the week or so leading up to your period. Some women are more prone to such fluctuation than others and will notice that weight rises as menstruation draws nearer, and then drastically declines within a few days of starting their period.
If that time of the month is artificially increasing your weight, let the system know at weigh-in.
Stress levels
Stress may increase the hormone cortisol, which can lead to the retention of salt and water and an increase in scale weight.
Sleep
Poor quality sleep and sleep deprivation can lead to disturbances in hormones (cortisol and aldosterone) that affect electrolyte and water balance, in turn causing water retention and a temporary jump in scale weight.
Timing of last meal
If a large meal is eaten unusually late at night, some people might see a slightly higher scale weight the next day.
The time you weigh-in
Weight is usually lower first thing in the morning than it is later at night. Your body may retain fluid during the day due to circadian rhythm-mediated alterations in hormones.
Bowel movements
If you’re not having normal bowel movements or are feeling constipated, this can reflect on the scale.
Alcohol consumption
Alcohol has a dehydrating effect, so you may expect to see a transient drop in body weight the day following a margarita drinking rendezvous.