The 5:2 diet, also written as 5/2 diet, is a fad diet which involves severe calorie restriction for two non-consecutive days a week and normal eating the other five days, which originated and became popular in the UK, and spread in Europe and to the USA. It is a form of intermittent fasting. The diet is claimed to promote weight loss and to have several beneficial effects on health. According to the UK National Health Service there is limited evidence on the safety and effectiveness of the diet; they advise people considering it to consult their doctor.
The diet specifies a low calorie consumption (sometimes described as "fasting") for two days a week, which should not be consecutive, but allows normal eating for the other five days. Men may eat 600 calories on fasting days, and women 500. A typical fasting day may include a breakfast of 300 calories, such as two scrambled eggs with ham, water, green tea, or black coffee, and a lunch or dinner of grilled fish or meat with vegetables, amounting to 300 calories. The daily 500 or 600 calorie limit requires small portions.
There is some evidence that 5:2 diet can achieve short-term weight loss similar to a calorie controlled diet, in females. However, such results are fairly common for fad diets like this and there is no data on the long term safety or healthiness, even of short term use, nor how good it is for long term weight loss. The NHS says that "Compared to other types of weight loss programmes the evidence base of the safety and effectiveness of the 5:2 diet is limited. If you are considering it then you should first talk to your GP to see if it is suitable for you. Not everyone can safely fast."
Mosley's book, The Fast Diet (co-written with Mimi Spencer), became the top selling book in the United Kingdom in February 2013. The Complete 5:2 Diet App (by Papertrell and HarperCollins) became the top selling diet app in the United Kingdom in September 2013. Portions of content adapted from Wikipedia's article on 5:2 diet which is released under the CC BY-SA 3.0.