I worked in a neurogenesis lab for three years, so my attempt:
The cells in our body are continuously replaced throughout our life, with the exception of our brain cells. For many many years we believed that mammals didn't create new brains cells in adulthood. You were born, your brain grew and developed as you became an adult, and then you were done.
About 20 years ago we realized that that was mostly true, but small parts of the brain continued to produce neurons through adulthood. This happens in two important parts of the brain:
- the hippocampus, which is associated with memory
- the olfactory bulb, which is associated with sense of smell (and memories related to it).
Humans have a relatively weak sense of smell, and if we create new neurons in the olfactory bulb it's much less than other mammals.
The process is convoluted, but small regions of the brain create those new neurons and they migrate through fluid-filled holes in your brain - most die, but a small % survive and incorporate themselves into that brain structure. The key thing to realize is that neurogenesis accounts for a tiny fraction of the neurons in our brain, and yet it's still an important process.
So why does adult neurogenesis matter, and what does it do for us? Neurogenesis is tied to our ability to learn and form new memories. So, if it's impaired, our ability to learn is reduced. There are tons of things that impact it - age, sleep levels, exercise, among others.
In the lab I worked at, we used to induce a form of brain damage to rats which greatly impaired neurogenesis, then run them through mazes, which they did quite poorly at.
We took some of those same brain-damaged rats and had them run in exercise wheels for two weeks, and they were suddenly doing just fine at maze tasks. When we stained and looked at their brain, their neurogenesis rate was back up to (or even above) normal levels.
We've known for a long time that exercise is great for your health, and there's a growing body of evidence that the same is true for intermittent fasting. What's interesting is that it's not just good for your overall body health, but both can help keep your brain working optimally by increasing neurogenesis and BDNF (which protects your existing neurons).