The 5:2 diet, or fast diet, is a fad diet which stipulates calorie restriction for two days a week and unconstrained eating the other five days. A form of intermittent fasting, it originated and became popular in the UK, then spread to the rest of Europe and to the USA.
Proponents of the diet claim it causes weight loss and has some beneficial effects on health; however as with other fad diets these claims are not supported by high-quality evidence.
5:2 diet Wikipedia
The diet specifies a low calorie consumption (sometimes described as "fasting") for two days a week but allows unmoderated eating for the other five days. Men may eat 600 calories (2,500 kJ) on fasting days, and women 500 calories (2,100 kJ).
Proponents say that fasting for only two days a week may be easier for dieters to comply with than daily calorie restriction.
In general there is a lack of research evidence on intermittent fasting, and there is only limited evidence of the 5:2 diet's safety and effectiveness.
According to NHS Choices, people considering the diet should first consult a physician, as fasting can sometimes be unsafe.
In the UK, the tabloid press has reported on research claiming the 5:2 diet could reduce the risk of breast cancer; however according to the NHS the evidence being considered formed an inadequate basis for making such statements.
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.
The diet has enjoyed media attention and celebrity endorsement, but skeptics, dieticians and the UK National Health Service have categorized it as a fad diet.