So obviously we’re made up of a lot of water. There are a couple of reasons why that amount of water fluctuates. The primary one for weight lose is glycogen. The body doesn’t just store energy as fat, it also puts some essentially sugar water away for itself in the liver and muscles. Each gram of glycogen also needs three grams of water.
The average adult has a couple pounds of this sugar water being stored. When we change our diets and start to lose weight we use up this glycogen and the body passes that now excess water it doesn’t need.
> Are people drinking less? … If you fast but drink lots of water, will your initial losses still be water weight?
Actually I think you’re thinking of it the opposite way from how it works. The more water you drink, the less your body holds onto. Since you have more access to it, your body doesn’t feel the need to hang onto it. This has to do with salt balance. If you eat salty things or don’t drink much water, your body retains water to keep the correct levels. If you don’t eat much salt or if you drink a lot of water, your body flushes water out of your system instead.
An interesting example of this is weigh ins for wrestlers. Their weight class will be like 175 but they’ll actually weigh 190. Many of them can drop 15-20 lbs just by strategically drinking a ton of water and not eating any salt (DO NOT TRY THIS UNSUPERVISED). Then between the weigh in and the fight, they regain it. Their muscle and fat reserves don’t change during that process.
Glycogen stores like others mentioned are in addition to this. So if you start out bloated, by drinking more water, eating less salty foods, and staying in a caloric deficit, someone might see 10+ lbs of weight loss that’s completely water weight. The reverse of this, of course, is that you could, for example, over the course of a couple weeks, lose 3 lbs of body fat but also change your salt/water intake and look like you’ve stagnated or even gained, when really you’ve lost fat. That’s why you have to look at trends instead of exact numbers.
It should really be called fluid retention or fluid weight. In addition to the glycogen issue already discussed by others, chronically eating at or above one’s energy needs is going to cause swelling, inflammation, and constant digestive activity. Many people see reductions in swelling of lower extremities within just a week or so of being in an ongoing calorie deficit. Then, with weight loss and more activity,, fluid stops building up because of improved blood circulation.
Overeating is also correlated with high levels of sodium intake, which also results in a higher capacity for fluid retention.
Stated another way, a healthy, active person in an ongoing calorie deficit (and not already dehydrated) can drink tons of water and it’s not going to cause noticable water weight. They’ll just urinate more often.
On the other hand, intense exercise can cause fluid retention as muscles heal.
Glycogen (from carb intake or lack therof) is the biggest hitter for water weight gain/loss, but sodium intake can also cause a temporary increase in water weight. Ironically, the best way to mitigate that (to avoid seeing the non-fat scale weight gain) is to drink extra water, to “dilute” the salt and you’ll pee it out.