Your body’s hydration needs change constantly.
Your level of food consumption changes your body’s water composition due to glycogen storage.
Inflammation comes from many sources and can cause fluid retention. Illness, exercise, injury, inflammation, menstrual cycles, etc. all can contribute to water weight. The intestines recruit water for processing fiber and waste through the gut.
A gallon of water weighs about 8 pounds, so even minor changes in fluid levels can cause large fluctuations in daily weight, and that’s why it’s talked about so much, and what makes weight better to achieve within a range instead of a single constant value.
When people say “water weight” they can mean a lot of things. I most often hear it to describe anything that can cause your weight to fluctuate without changing the makeup of your body’s tissues.
Sometimes it’s food in your digestive tract. Sometimes it water in your bladder. Sometimes it’s liquid outside of your cells (called lymph). But you get the biggest variability from how much water is in each of your individual cells. How much water a cell holds on to depends on a lot of things including hydration, electrolyte levels, and hormones.
Water weight is literally the weight of the water in your body at any given time, which includes the water taken in by the food and beverages you consume as well as waste products.
If you were to weigh yourself on a Tuesday morning immediately after going to the toilet and before drinking anything and then next weighed yourself on a Friday night after drinking a few beers and having not had a bowel movement since Wednesday, the scale might show substantial weight gain when in reality you did not gain and may have even lost.
If you want to accurately keep track of your weight and not let water weight skew your numbers, you must weigh yourself in consistent circumstances. The easiest way to do that is to wake up in the morning, go to the toilet, and then step on the scale naked before eating or drinking anything.