Hi. I have been doing this for three weeks now. I generally fast on a Monday and Thursday as I find it easiest. It appeals to me because I don't have to do any drastic cutting out of food groups, which is how I normally fail as I am a foodie! It has reduced my appetite on the days I don't fast, a good thing. I just watch my portions. There is a whole thread on the 5:2 diet and a MyFitnessPal thread. I use MFP for the support and because it works out your calories and nutrients.
PORRIDGEANYONE, you're right, it's 2 days of 500 kcal and 5 days of normal eating. But unless you're five foot nothing with a desk job and who's idea of exercise is lifting your arm to point the remote at the tele then 1500 kcal is not a normal amount of food to eat for the day. Govt recommendations are 2000 a day for women, but for most of us this is a little generous. For example, I am 5'3.5", 10 stone 8 and pretty much sedentary so my daily calorie needs are around 1750.
I've been doing 5:2 for over 5 months now and have not counted one single calorie on my normal eating days. No need, after the novelty of being able to stuff my face on normal days wore off (after a week or two) I found my appetite diminished on normal days. I now eat a sensible amount of the right kind of foods because the fast days seem to have acted as a kind of reset button. I've lost almost 2 stone, some on-going health problems are improved and I'm really enjoying it.
No, it's not dangerous. Those with serious medical conditions should consult with their doctor first, particularly diabetics. It's probably not suitable for insulin dependant diabetics. If you do the maths it's actually less extreme than some of the traditional calorie restriction methods. Traditional recommended weight loss rate is 1-2lbs per day. To acheive this by daily calorie restriction you need to eat 500-1000 calories less than your body needs each day. Total weekly calorie deficit of 3500-7000. Doing 5:2, for 5 days a week you don't eat less than you need, and on the other 2 days you eat 1500 (for an 'average' woman) to 1900 (for an 'average' man) kcal less than you need. Total weekly calorie deficit: 3000-3800. In reality, the total weekly deficit on 5:2 is probably not going to be this high because the numbers are based on recommended daily calorie intake, which is usually a bit too generous.
This is not a crash diet and it doesn't guarantee rapid weightloss. It is a long term sustainable way of losing and maintaining body weight. There are some studies which suggest it also has health benefits in addition to weight loss. The reason it's such a great thing for me is that it's so ridiculously easy to stick to so even if the weight loss is slow, it will eventually come off.
Do come and join us on the main thread. There is a lot of info linked to in the OP. Please do have a read of those links.