The 5:2 diet is a form of intermittent fasting. In this diet (although it is not so much a diet but a way of eating), you can eat anything you want for 5 days and fast the rest 2 days.
Because the 5 days are easy to understood (you eat normally, as you have been eating so far, trying to avoid processed foods as much as possible), I will write about the other 2 days that matters the most in this diet. The 5:2 diet is about limiting the calories you will be consuming on 2 days of fasting and specifically about 25% of your daily need. For women this means ~500 calories and for men ~600 [1, 2].
Like intermittent fasting, this diet/fasting can also help your health by giving your body the opportunity to repair its cells, detoxify (as the liver does not have a workload with the foods as it normally does when you eat) and potentially prevent diseases such as Alzheimer's, heart disease, diabetes and cancer.
Those who have tried the 5:2 diet have noticed an improvement in their blood pressure and cholesterol, weight loss (fat) and - especially for women - an improvement in their hormones. If you are a woman, expect to lose about 1 pound (half a kg) per week, while a man can lose up to 2 pounds (1kg).
Note that by the term "fasting" does not mean that you will not eat anything all day. As already noted, during the 2 days of fasting, you should limit your calories up to 25% of your total daily needs. Because you do not want to get hungry during the day (believe me, the first times are the most difficult, as you will get hungry constantly, then it will become a habit and you will not feeling hungry anymore), you can cut those calories to 2 or 3 meals, that is eat something light at midday and something - in the afternoon or - in the evening. Some people spend these days without eating anything, but you do not have to do that.
As I said, your choices do not have to limit you, even when you fast. Just make each of your calories count, by eating healthy foods. For example, on fasting days you can have eggs, or small amounts of chicken or fish, vegetables, soups, salads, etc. You should avoid processed carbohydrates and sugar on these two-days. The rest of the days do not need to be limited, you can eat anything you want. But this is not an excuse to eat like a pig.
These two days of fasting, doesn't have to be in a row, but in the middle of the week. For example, you can fast on Tuesdays and Fridays and the rest of you eat normally, without counting calories and without being anxious about your choices.
However, I have to say it one more time, the remaining 5 days are no an excuse to eat any garbage or eat like a pig, and keep telling to yourself that you will "fast" the other two days and you'll lose what you eat. The goal is to reduce processed foods as much as possible, but you do not have to starve. For example, you can have pizza for one day, pasta, biscuits or chocolates the other day etc. But do not eat all these processed food in one day, because in the end you will gain weight.
For those who want to drink any beverage, you should avoid sugar and any other sweeteners on fasting days. If you can not drink them without sugar, you can put half a teaspoon of honey (raw and not processed) into each beverage (or a piece of cinnamon in the tea) to avoid elevating insulin levels and stopping the benefits of the fast.
If you exercising on the fasting days, you should leave some calories as your post workout meal. You can prepare milk, a little protein (about 10-15 grams of whey) and oats to give your body the necessary ingredients it needs. For the rest days and after your workouts, you can eat what you normally eat.
Now if you get to the weight you want, you can keep eating like this forever or until you get bored. Many people, when they reach the weight they want, change the diet to 6:1, that is fasting only one day a week.
Eat, Fast & Live Longer
In the following video, Michael Mosley explores 5:2 diet and the science behind fasting to find out if you lose weight, you live better in this way of eating. Michael is really experiencing fasting and he's showing us his results.