system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If you can survive weeks without food as long as you drink water, can obese people just starve their fat away?
gpt
In theory yes anyone can reduce their fat stores by simply not eating.

However the reality is that its not that simple. While your fat stores will keep you from starving they will not necessarily provide you with all of the necessary vitamins and minerals that food does. 

Also the "fuel" you run on when surviving off fat is not the same as your normal "fuel". The preferred fuel for your body is glucose which is mostly formed from carbohydrates. Fat is converted to ketones which your body then used for fuel.

Ketones however are not really the same as glucose. They cannot directly fuel your brain, they are generally just less efficient/effective than glucose. You basically will start to feel sort of like you do when you wake up in the morning all the time. 

Also not having any sugar intake can damage your bodies ability to manage sugar levels. Prolonged fasting can cause diabetes.

If you manage your fasting very carefully to ensure your get all necessary vitamins and minerals and also intake enough sugar in your water to stave off diabetes you could in theory "fast your fat away".