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The 5:2 diet, or fast diet, is a diet which involves calorie restriction for two non-consecutive days a week and unconstrained eating the other five days. The diet originated and became popular in the UK, and spread in Europe and to the USA. It is a form of intermittent fasting.
The diet specifies a low calorie consumption (sometimes described as "fasting") for two days a week, which should not be consecutive, but allows unmoderated 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.
Proponents say that fasting for only two days a week may be easier for dieters to comply with than daily calorie restriction.
There is limited evidence of the diet's safety and effectiveness.
The diet became popular in the UK after the BBC2 television Horizon documentary Eat, Fast and Live Longer  written and presented by Michael Mosley was broadcast on 6 August 2012 and many books on the diet quickly became bestsellers, soon after.