Gut content is the food you have already eaten but not yet excreted. It sits in your digestive tract, gets weighed along with you, and yet is not the same as body mass. Together with water, salt and glycogen, it is one of the main reasons your daily weight fluctuates.
How long food weighs in
For most people, 24 to 72 hours pass between a meal and its excretion. During that time, it counts on the scale. How much that is depends on how much and what you eat, for example how much fiber. Irregular digestion, say on vacation, also reliably causes spikes on the scale.
Example: If you eat and drink two or three kilograms more than usual on a cheat day, you still carry part of it the next morning, even though your body has not used it yet. The numbers describe an example, not a measurement.
Gut content and fat
One extra kilogram the morning after a big meal is therefore not a kilogram of fat. For one kilogram of real tissue gain you would need roughly 7,700 kcal of surplus. The gap between what the scale shows and what can be tissue is made up of gut content, glycogen and water. If you go back to eating normally, most of it is usually gone within two to four days.
What to do
Always weigh yourself under the same conditions: in the morning, after your first bathroom visit, before breakfast. That does not remove the fluctuation, but it keeps the conditions the same. Your trend weight handles the rest by smoothing out single high readings. Make calorie decisions based on your trend over two to three weeks, not on a daily reading.