Answer:
There is little in the way of research done into the long-term effects of this diet; whether the five days off, two days on is the most effective ratio; or even if this diet works in terms of sustainable weight loss – of course, if your overall calorie intake goes down, you will lose weight anyway.

There are some animal studies that suggest restricting calories in this way may increase lifespan and protect against dementia. There is also the question of a gene called SIRT1 AKA “the skinny gene” which may trigger the body to start burning its fat reserves. It is thought by some that this gene, which may also suppress tumours offering some protection against cancer, might be activated by following this kind of diet. However, animal studies are self-limiting in terms of applying results to a human population.

If you are planning to try this diet, there are a few things you should remember. The five days you are not fasting are not an excuse to eat whatever you like. If you eat too much of the wrong foods, this diet is not going to work! Also on the fasting days it is important that you still get a balance of nutrients. Fruit and vegetables, whole grains and protein with some healthy fats will need to be squeezed into your 500kcal limit. It’s also not sensible to have two consecutive fasting days.

But, if the thought of restricting calories every day leaves you miserable, then this could be a diet for you to try. If followed properly there is unlikely to be any negative health consequences – although some people have complained of lethargy on the fasting days and it is not sensible to diet when pregnant. As with any diet (although probably not what a dieter wants to hear) gradual weight loss is more sustainable than losing lots of weight quickly so if you want to keep the weight off, take it slowly and aim for half to one kilo a week. And remember, if you stop the diet, going back to the way you were eating before is likely to cause weight gain in the same way it did before so making changes to your diet should be life-long to break the diet-followed-by-rebound-weight-gain cycle.