I got interested in the idea of calorie restriction after picking up a copy of "The Longevity Diet" essentially a book about calorie restriction and the perceived effect it has on increasing your lifespan and overall health.
I digested the book and sat on the idea for a couple of years whilst trying to get my weight down a bit before embarking on a CR lifestyle, more as an experiment with myself more than anything, I just kind of left it there on the back burner as something I would come back to.
Another book crossed my path "Fasting and Eating for health" by Dr Joel Furhman if there is one book I suggest you read, this is the one. As I was busy leading a healthier lifestyle, less work, more gym, more exercise generally (and reaping the benefits of that) I was looking around for something (anything) that would give me that final push to the kind of general well being that I (and I suspect everyone) would like.
I read the book and determined to give fasting a go, as suggested in the book the short and long term health benefits of performing a prolonged fast (we are talking here of 7 days +) far outweigh the downsides that are primarily imposed by our modern "I want it now" lifestyle. You can also add a sense of accomplishment, being willing and able to eschew the temptation of putting what you like into your mouth is probably a corner stone of getting your health/weight/lifestyle under control.
So last year I embarked on the plan of a 21 day fast, this was preceded by a month of healthful eating as suggested by Dr Furhman, less protein, more leafy greens, more fruit and veg generally (and this from someone who already ate a comparatively good diet). The first 7 days I was joined by my 20 year old son, which helped immensely, I had decided that each 7th day of the 21 day fast would be a feed day, consuming approx 1000 cals of nutrient dense foods, leafy greens again, nuts and seeds and other such goodies.
I was delighted with the results, which I wish I had diarised at the time, but the net effect was clearer skin, clearer eyes, greater mental acuity, a surprising amount of energy whilst fasting and still plenty of it afterwards and also the fact that I managed to reduce my requirement for addiction foods, chocolate, coffee & empty carbs like bread etc. I had stepped away from those habits and taken control again, eating them when I chose, rather than because my blood sugars were asking for a kick.
Whilst I was fasting BBC2's Horizon programme featured an episode fronted by Dr Michael Mosley where he looked into calorie restriction and fasting, he put himself through a 6 week challenge of following 5:2 fasting, eating 5 days a week normally and then calorie restricting down to 600 cals a day on the other 2 days. His blood markers, weight and general health afterwards indicated that you could get the same benefit from 5:2 as you could from following a harsher CR lifestyle, the net effect being the same, you are dropping 2-3000 calories a week either way, but with 5:2 you get to eat pretty much what you want on your non fasting days (that is if you eat within your daily calorie allowance!)
So there I was, hooked on fasting, over the last 8 months since finishing the 21 day fast I have followed 5:2 fasting on and off, there have been occasions where I dropped the fast altogether (training for the Paris marathon) but I do feel that long term it is a plan I will stick with, possibly for the rest of my life, with an occasional longer fast thrown in for good measure both to detoxify, reset my habits and to give myself a challenge, which for me is what life is all about.
Hopefully that gives a little bit of insight into the why.
