What Does Fasting Tell Your Body?
In the same way, the chemical changes your body undergoes when you’re fasting are telling your brain it isn’t smart enough. If you were smart enough, you would have enough food. The chemical changes in your body that result from the fact that you don’t have enough food stimulates your brain to grow new brain cells in a desperate attempt to help you survive. Think about it. If your condition continues for too much longer, you would be dead. So the brain is taking the data very seriously and will do anything it can to keep you alive.
Or at least that is a valid way to explain the finding that fasting causes new brain cells to form.
Here is Mark Mattson talking about how fasting stimulates the brain to generate new brain cells: Why Fasting Is So Good For Your Brain.