Water weight refers to water in your cells and the spaces between cells that your body retains due to the presence of excess salt and sugar. When people begin to diet they tend to cut down on salt and sugar, which leads to a rapid loss of weight, but it's easy come, easy go. This isn't a loss of fat tissue, it's a loss of retained water, so it isn't what people usually mean by "losing weight" which to most means "burning fat."

Water weight is exactly that. More water in your body. Consuming a lot of salt or sugar can cause your body to retain extra water.

The human body is about 80% water. And an adult male can have their body weight fluctuate up to 5% completely safely just based on their water weight. If you eat a big salty meal and drink a lot of water, you'll gain a lot of weight pretty immediately. But as the salt works it's way out of your system, so too will the excess water. And you'll drop a lot of weight. By mildly dehydrating yourself you can lose a lot of weight.

Personally my weight can fluctuate by about 10 lbs. Just based on how hydrated I am.

Your body stores sugar for quick use as glycogen. And glycogen binds to 3-4x its weight in water. Diets that restrict your carb intake cause you to use glycogen and not replenish it, which releases a lot of water.

The human body is like 60% water by weight, and the difference between being well hydrated and dehydrated is like 5 lbs. So, you can easily lose “water weight” by exercising hard without proper hydration. You really do lose weight if you get on a scale, but it’s highly impermanent because you’ll get the weight back as soon as you drink enough water.

You stop eating for a day. Total caloric deficit.

Your day to day energy is stored in glycogen a kind of easy to access sugar that is stored for a bit but easier to metabolize than body fat. This is replenished from food you eat. So if you don't eat then your body will start pulling energy from your fat stores. This small amount of energy is basically packaged with a bunch of water for important chemical reasons but the water makes it heavy. Your body also keeps water based on your current salt content.

So what has happened at the end of your one day fast it that you have used up all of your glycogen releasing all that heavy water it was tied too and your salt content has dropped because you haven't eaten anything to replace it. So you'll pee out more water so you body keeps the salt concentration it likes.

Your day of burning calories burned some fat depending on your level of activity. But less than a pound. For most people significantly less than a pound of fat per day of complete non consumption.

But when you step on the scale you and most other people will see that you have lost somewhere between 2 and 5 pounds. Because you pissed them out for reasons only tangentially related to your goal.

That's water weight. Your body will hold on to that same amount of water the very next day when you celebrate with one slice of salty pizza and make it look like you've lost all your progress. Thus why understanding water weight can help someone trying to slim down because it can be really demoralizing not knowing why daily results seem to bounce around so much.

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