However, the researchers found that the approach, which involves two days of heavily restricting calories (500 calories for women, 600 calories for men) and five days of sensible eating, was rated more highly by the obese people in the study because it was easy to follow.
Doctors may want to consider including the 5:2 diet as part of their standard weight management advice to patients, she said.
The study involved 300 obese people in Tower Hamlets, an inner city area of high deprivation in London. The participants either followed the 5:2 regimen or a more conventional approach to losing weight that stressed eating more vegetables and whole-grain foods, cutting out foods high in sugar and fat, eating smaller portions and exercise.
The results of both approaches were very similar and “modest,” the study said.
At six months, those using the 5:2 diet had lost, on average, 1.8 kilograms (4 pounds) compared to 1.7 kilograms (3.7 pounds) on the standard diet advice. At 12 months, those figures were 1.9 kilograms (4.2 pounds) and 1.8 kilograms (4 pounds), respectively.
Some 18% of 5:2 dieters had lost at least 5% of their body weight after one year compared to 15% using the conventional approach.
Of the group following the 5:2 diet, half attended six group support sessions for the first six weeks after the initial information session. However, its impact of the group support diminished over time, the study found.
Participants were positive about the different weight loss approaches, but those on the 5:2 diet were more likely to recommend it to others and said they were more likely to continue with the approach.
The study was a randomized control trial, regarded as the most rigorous kind of research, and while the number of participants was larger than most previous studies of intermittent fasting, the authors said “some findings of borderline significance could have become clearer if the sample size was larger.”
The people following the conventional weight loss guidance were also more likely to try other strategies such as Weight Watchers, Slimming World or other diets. This factor could have masked the effects, but it would not have been ethical or practical to stop participants trying alternative approaches, the study authors said.
Some experts think that alternating between fasting and eating can improve cellular health by triggering metabolic switching.
The method is not appropriate for everyone, however, particularly pregnant women and those with medical conditions such as diabetes or eating disorders.