It’s not uncommon to gain 5 pounds in a day, particularly if you’ve done a heavy workout or consumed a lot of fluids and salty foods. This weight is usually water weight and will fluctuate throughout the day.
A common fear among people who are trying to lose weight is that they’ve gained fat overnight when they step on the scale and see a sudden increase. However, it’s important to understand that this is generally not the case and there are many factors that can contribute to such fluctuations.
It is highly unlikely you will gain 5 pounds of fat in one day. In order to gain 5 pounds of fat, you would need to consume approximately 18,000 extra calories (1). Most people are unable to consume this amount in a single day.
And even so, gaining fat isn’t an instant process. It takes time for your body to break down and store excess calories as fat.
So, while it may seem you’ve gained 5 pounds of fat overnight, it’s more likely just a temporary change in water weight. For the same reason, you may weigh less in the morning some days.