Your brain needs glucose, not calories to function. Yes, we can get glucose from the calories we eat, but that's not the only way. Thanks to insulin, extra gluc

Your brain needs glucose, not calories to function. Yes, we can get glucose from the calories we eat, but that's not the only way. Thanks to insulin, extra glucose can be stored in the liver and muscle tissues as glycogen. When those stores are full, it gets converted to fat and stored in your adipose tissue. This is lipogenesis. When you're in a deficit, your body keeps blood glucose levels stable (and your ability to think and work constant) by tapping into glycogen stores from the liver. When your liver is out of glycogen, the body can convert glycerol, a component of fat molecules stored in your adipose tissue, into glucose through a process called gluconeogenesis. While this process works, it's not always comfortable. However you can become more adapted to it and get to the point where your comfortable doing light exercise and heavy thinking in a fasted state.